>>However, we did receive feedback that GPS could be quite badly in >>error when satellites were low on the horizon and cliffs affected the >>signal. I'ld be interested to know if anyone has experienced this? >> >> > >GPS normally displays its range of precision, which depends on the number of >satellites that can provide a good signal at given conditions, i.e. >geographical position, obstructions etc. I don't remember what is the >lowest precision of my Garmin Legend (when it still gives some location), >but must be better than 900m error of dead reckoning. I've heard of >possible fog-induced distortion of signal, but haven't had a chance to check >it personally, so don't know whether it is just "honest" weakening of signal >(= honest lower displayed precision), or really distortion of data beyond >stated range of precision (this would be much worse). > I've used my GPS receiver on a daily basis for the last 6 years and have had a chance to see a number of anomalous indications - none of which made themselves known through exceptionally high EPE (estimated position error) or accuracy indications. These included my all-time high indicated speed of 4032.3 mph achieved while pedaling my bike in a canyon area near a medical complex. The tracklog indicated a sudden jump in position shortly followed by a loss of signal lock. I believe the hills were blocking most of the satellites and I was probably getting a momentary multi-path reflection off the medical buildings which the unit interpreted as my having rapidly moved to a new position. There was also the 5-minute period when the GPS indicated that I was at an elevation of 1500' over the north Pacific approaching Seattle at 734 mph while my other navigational tools (eyes) indicated that I was on a residential bike path in the SF Bay area and heading south at about 15 mph. The satellite page showed a lock on just 3 satellites (i.e. a 2D lock) and I suspect that this was a case where with only 3 signals the unit couldn't properly resolve between possible solutions of the pvt equations and picked the wrong one. Turning it off and back on quickly resolved the problem. But despite these and a few other momentary glitches, I've found GPS to be a highly reliable tool, especially if care is taken to check the indications on the satellite page showing the number and positions of the satellites being tracked. I have never had a problem with reception due to weather conditions and that includes thick fog and intense thunderstorms. But there can be a problem due to any film of water forming over the antenna - so condensation on the unit as a result of fog should be wiped off. Although I always have my GPS turned on when kayaking, I rarely use it for navigation. The main uses are to keep a record of the trip, incl. speeds and distances traveled, give an immediate indication of actual speed and track including the effects of wind and currents, and to show me where on shore appropriate restaurants, parks, restrooms, etc. may be located to help in deciding on landing spots. It's rarely used for navigation since I normally know where I am just by looking around. But under unusual conditions, such as the fog coming in unexpectedly, the GPS can be a valuable addition to other navigational aids. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I've had saved GPS waypoints be off as much as several miles. I suspect I had a 3 satellite (2D) fix at the time. I've never had such errors with 4 or more satellites, but it certainly is wise to cross check GPS against other means of navigation. I'm a pilot as well as paddler, and I never trust any single means of navigation in the air. That said, GPS is wonderful for a pilot. It let's me go directly to my destination with a lot less work. Bruce Flagstaff, AZ *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
G'Day, Thought it would be worth summarising those posts in the GPS thread which discussed GPS and coastal navigation errors. This is not to argue against GPS which most contributors considered a very useful tool. Nevertheless the comments seemed to be useful in describing how occasionally errors might occur. Hope I don't misinterpret people: - 1. Peter Rathmann, during six years daily experience of using a GPS in a variety of activities, occasionally observed errors so large as to be obviously wrong, even though they did not show up as exceptionally high estimated position error or accuracy indications. He ascribed these either to momentary multi-path reflections off buildings or to the GPS receiving signal from only three satellites. 2. Mike Daly reported that his GPS frequently shows an EPE of less that 15m and he would ignore errors at that low level though larger errors he would take notice of. Mike also mentioned that if only three satellites were visible, then the receiver would assume the current altitude based on the most recent determination. This gave rise to a discussion on how changes in altitude could give rise to errors when moving from land to sea or paddling in lakes. 6. Bruce Grubbs reported errors of several miles in saved way points when he had a three point fix only (Bruce is a pilot and didn't say whether the error was while flying or paddling). 3. My own experience with a GPS during a coastal navigation exercise comparing a GPS against dead reckoning and non GPS pilotage. Almost certainly the GPS error was negligible, and our non GPS measurements had an average 10% error i.e 900metres over about 10km. 4. A second hand report from a yacht in Jervis Bay of a GPS reading that computed their position to be on a reef, which they were nowhere near. The explanation given was a possibility of interference by a cliff in the signal transmission path because the satelites were near the horizon. 5. A report back channel that if the chart co-ordinates were supplied in a different datum to the GPS then significant errors could occur. I don't completely understand this but it seems as if it could easily be checked before going on a trip. The most common source of errors seemed to be when only three satellites could be received. Those who reported occasional errors nevertheless felt the GPS to be a very useful tool. Hope this is helpful, All the best, PeterO *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 2 Sep 2004 at 19:12, PeterO wrote: > 2. Mike Daly reported that his GPS frequently shows an EPE of less > that 15m and he would ignore errors at that low level though larger > errors he would take notice of. Just to clarify. A low EPE means that the GPS thinks everything is fine even though those estimates are less than what is possible with the technology. It isn't the errors that I ignore, it's the EPE. Higher EPE means that there are real errors creeping into the position readings and I make sure that I consider that when looking at the reported position. Even at SA levels of error (<100m, 95% of the time), that's still a lot better than what you can do with a sextant, so error interpretation is relative. Mike *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Michael Daly wrote: > > Even at SA levels of error (<100m, 95% of the time), that's still a > lot better than what you can do with a sextant, so error > interpretation is relative. When I was practicing daily and using a $1000 sextant and artificial horizon I could get to about 1/4 mile radius. Using normal gear on a sailboat I was more than happy if my 'triangle' was only 2 miles across. These days, with GPS in use I am more concerned about errors on my charts as most have not been updated in many years and when they were done much of the better position fixing systems did not exist. I was on RV Hero in 1984-85 and we found Chiloe Island to be almost 3 miles from the charted position. The charts of the Antarctic Peninsula were really interesting. Lots of hand drawn updates and corrections, mostly from our own observations. Remember, that there at least two positions that are important, yours, and that of land and obstacles on the chart. Both are subject to error. michael (still more of a sailor) *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 2 Sep 2004 at 12:11, Michael Neverdosky wrote: > When I was practicing daily and using a $1000 sextant and artificial > horizon I could get to about 1/4 mile radius. That's twice as accurate as what most mariners consider very good. I assume that's on land. > Using normal gear on a sailboat I was more than happy if my 'triangle' > was only 2 miles across. That's a little more like it :-). GPS users who complain about errors or insufficient accuracy are just whiners IMNSHO. When you sail and are glad you can find a bay, worrying about a few meters seems excessive. Mike *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Another observation about GPS: it can serve as a backup for a compass! I'm usually careful about storing metal items near my deck compass, but at the beginning of our last trip, my wife handed me a food bag that I put into the bow hatch without thinking. As it happened, it contained a can of chicken meat. We had not been on the water long when I noticed a discrepancy between my compass heading and the map. The compass said I was heading SSE. Observation and the map said I was heading east. My wife's compass also said we were heading east, so after that I ignored the compass and just used the map and the GPS until we made camp. We should have compared compasses when we put in, but we were so busy I forgot. Chuck Holst *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
This thread about errors using navigation instruments reminded me of something I was reading yesterday. On a long train ride home yesterday I was reading Bill Brysons books "A Short History of Nearly Everything" and he was writing about carbon dating. Apparently a couple of years after carbon dating was first used to determine the age of historical objects it was discovered that the numbers used for the half-life of carbon-14 were a not completely accurate. When the corrected number was used it resulted in a 3% difference in the age of an object (younger) than when the original half-life value was used. However, because the method had been used for a couple of years and a lot of object had been carbon dated, rather than recalculate the age of all of them, all future calculations are made with the original incorrect value for carbon-14 half life. As a result, any object that has been carbon dated is actually 3% older than published. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
It is very interesting, mostly academically since 3% in thousands and millions of years, still is thousands and millions of years. Error there is of no significance, while 3% on a moon landing could be catastrophic. Best Regards, Rafael Mexico *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
GPS frequently shows an EPE of less > > that 15m and he would ignore errors at that low level though larger > > errors he would take notice of. > > Just to clarify. A low EPE means that the GPS thinks everything is > fine even though those estimates are less than what is possible with > the technology. I think (have to turn it on, and wait for a while) that my Garmin can show 15 ft accuracy, which is 4.5m. There are additional (or different) satellites to achieve such increased accuracy, called WAAS. But not in all areas. I've seen such accuracy displayed quite often, and didn't notice it to be incorrect, though such accuracy exceeded that of the built-in map :-). And, yes - GPS can be used as a subsitute for compass (but it should be a supplement, - there is no excuse not to carry at least $5 around-the-neck compass). Even GPS without a compass can display a bearing to some map point or waypoint. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 2 Sep 2004 at 11:19, alex wrote: > I think (have to turn it on, and wait for a while) that my Garmin can > show 15 ft accuracy, which is 4.5m. There are additional (or > different) satellites to achieve such increased accuracy, called WAAS. > But not in all areas. I've seen such accuracy displayed quite often, > and didn't notice it to be incorrect, though such accuracy exceeded > that of the built-in map :-). Sorry, my GPS is pre-WAAS so I never consider those things. You're correct - with extras like WAAS or DGPS, you can get increased accuracy. You also have to consider that some GPS units will do averaging of a position over time - taking lots of readings to cancel out the errors. My comments should be considered in terms of basic GPS functionality when the unit is just turned on. Under those conditions, the basic position error is <15m, 95% of the time. The latter pretty much describes how I use my GPS. It's off almost all the time, but I may turn it on, take a reading and turn it back off. In fact, I only bring my GPS for a long trip or in an unfamiliar area. > And, yes - GPS can be used as a subsitute for compass (but it should > be a supplement, - there is no excuse not to carry at least $5 > around-the-neck compass). Even GPS without a compass can display a > bearing to some map point or waypoint. If you're picking up a GPS for the first time, make sure you know the difference between a GPS with an electronic compass and a GPS that figures out direction based on motion. If you don't have an electronic compass, the compass direction indicated by the GPS relies on the GPS moving at a significant speed to interpret compass directions based on relative locations of adjacent position readings. If you're sitting still or moving very slowly, the apparent compass orientation may be meaningless. A lot of paddlers I know stop paddling to confirm locations with map and compass. Mike *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
> If you're picking up a GPS for the first time, make sure you know the > difference between a GPS with an electronic compass and a GPS that > figures out direction based on motion. If you don't have an > electronic compass, the compass direction indicated by the GPS relies > on the GPS moving at a significant speed to interpret compass > directions based on relative locations of adjacent position readings. > If you're sitting still or moving very slowly, the apparent compass > orientation may be meaningless. A lot of paddlers I know stop > paddling to confirm locations with map and compass. This expalins why I sometimes noticed a significant discrepancy between the direction shown by my non-compass GPS and actual direction to waypoint when paddling. I thought GPS calculated the direction using calculated location of the kayak and known location of the waypoint. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 2 Sep 2004 at 21:04, alex wrote: > This expalins why I sometimes noticed a significant discrepancy > between the direction shown by my non-compass GPS and actual direction > to waypoint when paddling. I thought GPS calculated the direction > using calculated location of the kayak and known location of the > waypoint. The GPS does calculate that way and that bearing's accuracy is directly related to the accuracy of the kayak position measurement. However, it has to calculate the direction to north (N) from the motion of the kayak. Since slow speed means short distances between position determinations and each position can be in error by up to 15m, the line between two positions can point all over the place. x1 <--- 10m ---> y1 y2 <--6m--> x2 For example, assume your kayak moves from point x1 to point x2. If the error in determination of the first point is right 10m giving y1 and the error for the second is left 6m giving y2, you can see that the apparent direction of motion y1->y2 is very different than true direction x1->x2. If this angular error is used to determine N, you'll be way off. If you're moving very quickly, the distance from point 1 to point 2 is larger and the angular error is reduced even if the linear error is the same. Note that the GPS is reading every second or so, so averaging out tends to reduce the errors. The error in estimating N is still velocity dependent as is the estimated speed (in the above diagram, estimated distance y1-y2 is somewhat larger than true distance x1-x2, so the speed will be overestimated). Sitting still is the best way to ensure that the estimated position of N is bad. If I turn on the GPS and don't move, the direction that my GPS reports it's traveling is usually way off. The reported speed also wanders a lot, rarely stating 0. Mike PS - above assumes my usual worst case limits - no WAAS or DGPS and GPS turned on briefly. With WAAS and leaving the GPS running, things improve due to smaller errors in position determination and more averaging. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Just a note- the position averaging feature works only when the GPS receiver is not moving. Bruce Flagstaff, AZ *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Alex's point is well-taken: If the chart indeed was not accurate enough to check the 15 ft "accuracy" then there is no way to establish the accuracy limit is that size or smaller. I have noticed errors in placement of waypoints (established in the field, not from a chart), on a subsequent revisit of a marker (not a buoy), of about 200 feet in one case and 400 feet in another case ... which I took as an indication of the limits of the system --- and as a piece of knowledge important if/when I need to REALLY trust the dang GPS. -- Dave Kruger Astoria, OR ----- Original Message ----- From: "alex" <al.m_at_3web.net> > I think (have to turn it on, and wait for a while) that my Garmin can show > 15 ft accuracy, which is 4.5m. There are additional (or different) > satellites to achieve such increased accuracy, called WAAS. But not in all > areas. I've seen such accuracy displayed quite often, and didn't notice it > to be incorrect, though such accuracy exceeded that of the built-in map :-). *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 2 Sep 2004 at 12:24, Dave Kruger wrote: > Alex's point is well-taken: If the chart indeed was not accurate > enough to check the 15 ft "accuracy" then there is no way to establish > the accuracy limit is that size or smaller. That's a good point. Canadian 1:50k topo maps are not stated as being more accurate than 25 or 50m (can't remember which). GPS is better than that most of the time. Mike *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 2 Sep 2004 at 17:44, Michael Daly wrote: > Canadian 1:50k topo maps are not stated as being > more accurate than 25 or 50m (can't remember which). In English, that's "stated as not being more accurate...". Sorry. Mike *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
"This expalins why I sometimes noticed a significant discrepancy between the direction shown by my non-compass GPS and actual direction to waypoint when paddling. I thought GPS calculated the direction using calculated location of the kayak and known location of the waypoint." Alex You must read the manual. Some GPS (Magellan e.g.) gives you a choice between a magnetic compas and a satelitecomputed compas. But the precision of the magnetic measurement is limited, recalibration (easy to do when on land) necessary and battery consumption increases a bit. Jens Viggo *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Michael Neverdosky wrote: >Using normal gear on a sailboat I was more than >happy if my 'triangle'was only 2 miles across. Michael Daly replied: >That's a little more like it :-). G'Day, Well if thats what is achievable with a $1000 sextant it makes our student efforts with deck compass, thumbs or sticks and string look quite cost effective. (900m error over a 10km distance). But I'm guessing the two Mike's error estimates were for navigation rather than pilotage, i.e not using landmarks by Mike's earlier explanation. Would that be the case? In fact I would have thought a kayaker more experienced in pilotage would have made less than half the error we did. By the way has anyone actually tried to use a sextant in a kayak? I tried once with a very cheap plastic sextant. My friends refused to paddle with me until I put it away. They made a number of extremely impolite remarks:~) All the best, PeterO *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
PeterO wrote: > > Michael Neverdosky wrote: > >Using normal gear on a sailboat I was more than > >happy if my 'triangle'was only 2 miles across. > > Michael Daly replied: > >That's a little more like it :-). > > G'Day, > > Well if thats what is achievable with a $1000 sextant it makes our student > efforts with deck compass, thumbs or sticks and string look quite cost > effective. (900m error over a 10km distance). But I'm guessing the two > Mike's error estimates were for navigation rather than pilotage, i.e not > using landmarks by Mike's earlier explanation. Would that be the case? In > fact I would have thought a kayaker more experienced in pilotage would have > made less than half the error we did. If you want to compare using a GPS to using compass bearings on landmarks then you need to use a DGPS. Try using your compass, thumbs and sticks and string to fix your position on a large sheet of ice with no identifiable features. The big reason for pilotage when along a coast is because it references to the LAND and that is what you care about. When you are out of sight of land you use celestial nav because you need to. In celestial you have to be concerned about the position error of the charts because you are fixing your position relative to the sun and stars. In coastal pilotage you have removed the chart position error because your reference is to features on land that are shown on the chart. > > By the way has anyone actually tried to use a sextant in a kayak? I tried > once with a very cheap plastic sextant. My friends refused to paddle with me > until I put it away. They made a number of extremely impolite remarks:~) > There are a number of problems with using a sextant on a kayak. First is staying upright while taking the sight. Second is the height of eye, as it is very low and keeps changing. Third is the difficulty of plotting the LOPs found onto your chart. The smallest boat that I *used* celestial nav on was a 24 foot monohull sailboat and it was not very easy. Doing celestial on my 28 foot trimaran was downright easy because of the stability. The GPS is always more accurate and easier than celestial as long as long as it is working. OTOH celestial only requires your tables and watch with a sextant. michael *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Michael wrote: >If you want to compare using a GPS to using compass >bearings on landmarks then you need to use a DGPS. SNIP >Try using your compass, thumbs and sticks and string >to fix your position on a large sheet of ice with no >identifiable features. G'Day Thanks for that Michael. I realised from reading your post: that the error in "pilotage" would vary enormously depending on amongst other things the quality of landmark; that it would probably not be possible to have a general idea of error for pilotage as compared with the range of error you and Mike Daly were able to describe for celestial navigation. Three questions if you have time. i) What is GPSD compared with GPS ii) Is it possible to determine longitude without a time reference (Greenwich) using celestial navigation. iii) With your measurements on the trimaran is the sextant gimballed in any way to help with stability or is that a ludicrous concept? By the way I still have a problem using the word pilotage or piloting for finding ones way along the coast in a kayak. My friends in the Australian Navy tell me its a bit inappropriate considering the sophistication and skill required to bring a modern large ship into harbour. Is the phrase "coastal navigation" generally considered an acceptable alternative these days? All the best, PeterO *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 4 Sep 2004 at 17:45, PeterO wrote: > Three questions if you have time. i) What is GPSD compared with GPS Differential GPS (DGPS) uses a special second receiver to pick up a radio signal from a ground station (usually obtained by paying a fee to the operator of the ground station). This signal transmits information that indicates accurate position information (IIRC, the info is more precise satellite positions). It can be used to produce a position estimate with much higher accuracy. > ii) Is it possible to determine longitude without a time reference > (Greenwich) using celestial navigation. Determining longitude depends on two things - finding your local time and knowing Greenwich time. The difference in these two times, taking into account that the Earth rotates once every 23h56m (24h sidereal time), allows you to determine longitude. Local time can be determined from observing time of transit of the sun (hence the importance in the days of sailing ships of measuring noon), the moon, planets or stars. With a sextant, you can determine this indirectly without resort to actually observing a transit. (Transit is when a celestial body passes your meridian or is at it's highest point in the sky.) Regardless of how the maths are done, the process is equivalent to the above. Hence, the clock is critical. > iii) With your measurements on > the trimaran is the sextant gimballed in any way to help with > stability or is that a ludicrous concept? If you swing a sextant side to side, the apparent angle changes and the minimum angle is the correct one. The quality of the navigator is indicated by their ability to get the angle correct regardless of how the ship is moving. At one time, there were experiments with gimballed chairs to steady the observer. They were largely futile. This was in the days of the lunar distance method. The latter was a process of measuring the angular distance between the moon and several stars to detemine the moon's true position. Once this is known, the time at Greenwich can be determined. It was an unweildy method and was largly ignored once marine chronometers were perfected. Lunar distance tables were still published up to the first world war, however (aah, naval tradition!). What made the method unpopular was not the measurement, but the many hours of computation required to produce the time. It was the lunar distance method that gave us the sextant. The octant was sufficient for measuring the altitude of stars, but too narrow in range to measure the lunar distances. Hence the larger sextant was developed. There are many excellent books that cover the history of this stuff - notably Dava Sobel's "Longitude". It was also made into a superb made-for-TV movie starring Jeremy Irons and Michael Gambon. Check it out of the video store! (In the film, Irons plays the role of Rupert T. Gould, an amateur chronometer enthusiast who rebuilt the original Harrison chronometers and got them working. Gould's book "The Marine Chronometer" is out of print (both editions) and is quite valuable. I spend untold numbers of hours in used bookstores when traveling to other cities in the hopes of finding a copy - it's on Amie's list of things to look for at garage sales). Another book I like is David Landes' "Revolution in Time", but it focusses almost exclusively on the development of the chronometer rather than the other aspects of navigation. The root of modern astronomy and much of physical science arises from the problem of determining longitude. All of the significant observatories were set up to determine the star positions with great accuracy. All the great scientists were involved and many theories and discoveries in physical science resulted. Galileo proposed using the moons of Jupiter as an accurate timepiece (they were used to establish longitude on land for many years). Robert Hooke's experiments on springs (to power timepieces) resulted in the foundations of the theory of elasticity. Newton invented the Calculus of Variations to explain the characteristics of a perfect pendulum to govern a clock. The concept of open scientific publication was also debatably enhanced as a result of the constant communication between seventeenth and eighteenth century scientists trying to solve the problem - all this in spite of frequent wars. Economists can point to the collapse of the guild system and the development of the free market in Britain as a reason why the technology of navigation blossomed in Britain and was stifled in the rest of Europe. Fascinating stuff! But I digress... Mike *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Michael Daly wrote: - >Determining longitude depends on two things finding >your local time and knowing Greenwich time. SNIP Local time can be determined from observing time of transit of the sun (hence the importance in the days of sailing ships of measuring noon), the moon, planets or stars. With a sextant, you can determine this indirectly without resort to actually observing a transit. (Transit is when a celestial body passes your meridian or is at it's highest point in the sky.) Regardless of how the maths are done, the process is equivalent to the above. Hence, the clock is critical. SNIP >but I digress. G'Day Mike, Wonderful digression, very much enjoyed it, as far as I'm concerned you can digress as much as you like! The book "Longitude" was the source of my question on measuring longitude without a Greenwich time reference. I realise navigators need a means of measuring time at their location but still wonder if there are ways of calculating longitude without the need for a time reference at another point on the earths surface. I'm thinking in particular of the lunar distance method. A sub plot in the Longitude book is the dispute between the Astronomer Royal, Nevil Maskelyne and John Harrison, the inspired clockmaker. Maskelyne claimed to be able to measure longitude using the "Lunar Distance" method, which I think precluded the need for measuring the difference in time between two points on the earth's surface, however the method required such careful and prolongued calculation as to be considered impractical. Thats if it worked at all? The Nautical Almanac, which Maskelyne first wrote, continues to be updated and published to this day, hence my question. If the lunar distance method did work, albeit clumsily, then I wondered if nowadays it would be more practical given the availability of more accurate celestial tables and computers. By the way Dava Sobel and William Andrews wrote a second version of Longitude called "The Illustrated Longitude" which includes a parallel story of the development of 17th century navigational instruments in notes attached to each picture. William Andrews has also written a book "The Quest for Longitude". which is top of my Christmas list:~) All the best, PeterO *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
G'Day Michael, Sorry I see now that you have already covered the Lunar Method, I must start wearing spectacles :~). All the best, PeterO *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 5 Sep 2004 at 17:14, PeterO wrote: > A sub plot in the Longitude book is the dispute between the Astronomer > Royal, Nevil Maskelyne and John Harrison, the inspired clockmaker. All due to his overly strong belief in Newton, who'd claimed that if the problem of longitude was to be solved, it would be by the astronomers and natural philosophers, not the mechanics (clockmakers). Maskelyne wasn't the only one, which indicates the god-like status that Newton had attained. > Maskelyne claimed to be able to measure longitude using the "Lunar > Distance" method, which I think precluded the need for measuring the > difference in time between two points on the earth's surface The lunar distance method determines time, not longitude. It's an astronomical clock, like Jupiter's moons. Since the motion of the moon was well understood, its position among the stars could be predicted with high accuracy. Tables listing position versus GMT were calculated and printed. If the navigator used the sextant and tedious calculations to determine the moon's position, the time could then be looked up in the table. > By the way Dava Sobel and William Andrews wrote a second version of > Longitude called "The Illustrated Longitude" which includes a parallel > story of the development of 17th century navigational instruments in > notes attached to each picture. This past spring at our kayak club's Spring Rendezvous, I gave a talk on the development of navigational instruments from ancient times to the sextant. I made the instruments out of foamcore (cardboard- laminated stiff foam) and held them together with spring clips. That way I could show how if you move this and change that, the instrument changes from, say, a quadrant to a backstaff or a backstaff to a reflecting quadrant and thence an octant. I also pointed out things like a kamal and a cross staff are two different forms of the same instrument. > William Andrews has also written a > book "The Quest for Longitude". which is top of my Christmas list:~) Something to look for! Thanks, Peter. Mike *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I use GPS primarily for checking milage. For navigation I use the old fashion map and compass. Three reasons: First, Map and compass do not rely on batteries. Second maps are like a book to me that tell a story and give me a lot more info than the tiny GPS screen does. Third I can get lost just as easy with a GPS as I can a map. ("I have never been lost...totally bewildered for a couple days but never lost" Davie Crocket) My problem with GPS (a Garmin 12) is the altitude reads are inaccurate. I went down to the boat harbor when the tide level was at zero, held the GPS a couple inches abobe the water and got a reading of 32 +feet. (No wonder tourists ask how far we are above sea level!) Bob Petersburg, Alaska N 56 48 636 W132 57 265 *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Bob, Could be your GPS uses a different datum for its altitude determination than what is used for the tide book's "sea level." The tide book uses MLLW (= Mean Lower Low Water) as its zero point, while USGS topos use a National Geodetic Vertical Datum, usually the 1929 datum, which uses MHHW, I think, as "zero altitude." Around here, the difference is about 7 ft (2.1 meters). If the tide range where you are is 20 some-odd feet, that would account for most of the discrepancy. -- Dave Kruger Astoria, OR ----- Original Message ----- From: "Bob Carter" <revkayak_at_aptalaska.net> > My problem with GPS (a Garmin 12) is the altitude reads are inaccurate. I > went down to the boat harbor when the tide level was at zero, held the GPS a > couple inches abobe the water and got a reading of 32 +feet. (No wonder > tourists ask how far we are above sea level!) *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On 4 Sep 2004 at 12:41, Dave Kruger wrote: > The tide book uses MLLW (= Mean Lower Low Water) as its zero point, > while USGS topos use a National Geodetic Vertical Datum, usually the > 1929 datum, which uses MHHW, I think, as "zero altitude." Further to that, remember that elevation errors in GPS are roughly 50% greater than horizontal errors. A good spread of satellites for horizontal determination is not necessarily a good spread for vertical. For horizontal you can get satellites all around you, but for vertical, you never get satellites below you. Also remember that as with vertical, there are several horizontal datum standards that can fool folks when they compare GPS to maps. If the GPS uses a different standard than the map, it won't show the same location as the map. GPS usage is not as simple as just turning it on. Not everyone realizes that. You don't just read the numbers, you have to interpret them sometimes. Mike *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
>From: "Dave Kruger" <kdruger_at_pacifier.com> > Could be your GPS uses a different datum for its altitude determination than > what is used for the tide book's "sea level." > > The tide book uses MLLW (= Mean Lower Low Water) as its zero point, while > USGS topos use a National Geodetic Vertical Datum, usually the 1929 datum, > which uses MHHW, I think, as "zero altitude." Thanks, mystery solved. Bob *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
>From: "Dave Kruger" <kdruger_at_pacifier.com> > Could be your GPS uses a different datum for its altitude determination than > what is used for the tide book's "sea level." > > The tide book uses MLLW (= Mean Lower Low Water) as its zero point, while > USGS topos use a National Geodetic Vertical Datum, usually the 1929 datum, > which uses MHHW, I think, as "zero altitude." Makes sense when you stop to think about it. Nautical types divide things into "Water/Aint water" and land types divide things into "Land/Ain't land" so that strip of stuff that spends some time as both land and water would be considered out of play by both groups. Thanks Dave! Carey *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
PeterO wrote: > Three questions if you have time. i) What is GPSD compared with GPS Differential GPS uses a second signal from a land based reference to correct for some of the errors in the signal. It makes a very accurate system much more accurate. The WAAS system is another version of the same idea. > Is > it possible to determine longitude without a time reference (Greenwich) > using celestial navigation. Yes it is possible but not easy. The lunar distance method was mentioned and has been used by a few people. Joshua Slocum was one who used lunar distance, he had a clock but it had no minute or second hands. :-) At least so goes the legends. > With your measurements on the trimaran is > the sextant gimballed in any way to help with stability or is that a > ludicrous concept? The only boat I know of that used a mounted sextant is a submarine. They have the sextant function built into the periscope. On a trimaran the boat rolls so much less than a monohull that it is far easier to get a good sight and it is faster so that a navigator might shoot three rounds of stars in the time it would take to do one round on a mono. The better accuracy of each sight and the greater number of them to show you when one of them is not good you can be much more certain of the fix. In general, on a ocean crossing you don't need to be very accurate at all. You only need to be good enough to be sure of not missing the destination land mass. In many cases this can be pretty rough. Most navigators make it a matter of pride to refine their ability to produce an accurate fix and make a landfall at a desired spot at a predicted time. > By the way I still have a problem using the word pilotage or piloting for > finding ones way along the coast in a kayak. My friends in the Australian > Navy tell me its a bit inappropriate considering the sophistication and > skill required to bring a modern large ship into harbour. Is the phrase > "coastal navigation" generally considered an acceptable alternative these > days? Yes, today most people, especially in the small boat world (rather than ships) would call it coastal navigation. Just to note a "Pilot" is also a term used to refer to a book of information used to navigate and operate a vessel in a particular part of the world. Much information is gathered together and kept for reference, some of it dates back centuries. These books make interesting reading. > All the best, PeterO Thanks, I'm rambling again so will stop here. michael *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
That might explain why my GPS track from my last trip in Canada (Silver Islet to Rossport) doesn't always correlate with what the map shows. Most of the time it is a little south and east of where I remember us being, and so I thought it might be a matter of map calibration in OziExplorer -- except that in some places on the same map it is north of where I remember us being. 25 to 50 m mapping error would cover that discrepancy nicely. Chuck Holst -----Original Message----- From: Michael Daly [SMTP:mikedaly_at_magma.ca] That's a good point. Canadian 1:50k topo maps are not stated as being more accurate than 25 or 50m (can't remember which). GPS is better than that most of the time. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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