Woody - Garmin was amazingly available by phone today. The tech rep said the tracking function does not work at speeds lower than about 3.3 knots. I pointed out that this means it wouldn't be useful for most kayakers, or for hikers or climbers. He agreed that is the case - have to use your bearing compass for that. The track function on my unit was still wildly inaccurate while paddling at a steady 4 kn speed, so I think the tech rep is being "disingenuous", as the diplomats say. Also, he pointed out that the map will only display the 12 nearest waypoints, so that's why my waypoint wouldn't display. In the case of my unit, I had to delete all but **six** of the waypoints before the new waypoint would appear on the map display. Nearby towns and villages apparently count as "near" waypoints, and they can't be deleted from the unit. A "near" waypoint, says the tech rep, is anything within 10 miles of your current position, and there is no way to adjust what the unit interprets as "near". Bill *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Bill wrote (regarding a Garmin GPS receiver): > > ...<snip>... The tech rep said the > tracking function does not work at speeds lower than about 3.3 knots. ... > The track function on my unit was still wildly inaccurate > while paddling at a steady 4 kn speed, so I think the tech rep is being > "disingenuous", as the diplomats say. I don't think that he was being disingenuous, but he was oversimplifying. There is no given speed at which the track function suddenly "works". The primary problem with tracking function accuracy at slow speeds is error introduced by "Selective Availability", which is the constantly varying error that the Department of Defense imposes to degrade the accuracy of non-military GPS receivers. This can cause apparent movement even when you are standing still. Whether and how much this throws off the tracking function depends on the direction and rate of the "false" movement relative to your actual movement. The faster you are traveling, the more accurately your receiver will be able to discern your actual direction of travel. Since the effects of SA vary, there are times when a 4-knot speed is sufficient to give you a reasonably accurate estimate of your actual direction of travel, and other times when it will not. The faster you are traveling, the more frequently the directional error will less than a given amount. As you have discovered, a way to work around this problem is to use the GPS to provide a compass bearing to your destination, and then use a deck compass to steer the course. Take an occasional glance at the GPS to see if the the bearing to your destination changes *persistently*, in which case you are drifting off course. The only way to "fix" this problem (i.e., to dramatically reduce the error at low speeds) is to add a differential GPS receiver. DGPS will significantly mitigate the effects of SA (and other errors). > > Also, he pointed out that the map will only display the 12 nearest waypoints... I have never found this limitation to be a problem so long as I am cognizant of this when I lay out waypoints. Of course you can still use the GOTO function and other functions with waypoints that do not show up on the screen (if for whatever reason the waypoint you are interested in is not one of the 12 nearest). Dan Hagen *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Since this is about a Garmin reciever, let me reveal a secret to getting good use out of the tracking feature for hiking and kayaking. Go into the setups screen, and change the tracking function to record one point ever 30 seconds, or 1 minute. This will track a whole days path, and dampens the effect that SA has on the recorded track, since the actual distance between the most recent few track points will be greater than with the default tracking mode. Using this mode, I have found that my recorded tracks almost always match up very nicely to my mapping software when downloaded. Tracks on rivers follow the river on the map. For all slow travel, a compass will give you a better idea of where you are pointed, but where you are pointed is not usually where you will end up going; you still have to compensate for wind and current. Using the GOTO function to get the desired course, and watch for the desired course to change as you paddle. If it stays near constant, then you are paddling in the right direction. If it is changing, then you need to compensate. So, use the GPS to tell you where you are and where something else is in relation to you. Use your compass to tell you which direction you are pointed in. Richard Walker Houston, TX http://www.neosoft.com/~rww/kayak_log.html *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
I agree that you can get adequate tracking by reducing the recording rater or using an averaging function for lower speeds, but normally you will not want to operate in the tracking mode. About the only time I use the tracking mode is to find that put in beach after dark that looks just like the last five miles of beach that you just paddled. Also once or twice in fog I have cheated with the GPS to zero in on a landing. Normally on a long crossing I only get a waypoint every hour or so and do a GO TO command to check both my pace and to correct course. The rest of the time I am on compass. I use a 12XL which can get a fix fairly quickly, and I usually can get by with a minute or two of operation every hour which keeps my batterys alive for days at a time. Running the GPS all the time only burns up the batterys for when you will really need it. Staring at an instrument on your deck will not only help you get seasick, but it is foolish to trust a gadget for your navigation. If you are staring at the GPS you are not paying enough attention to you chart, compass, and the landscape which are all needed for good piloting or dead reckoning. R. Walker wrote: > Since this is about a Garmin reciever, let me reveal a secret to > getting good use out of the tracking feature for hiking and kayaking. > Go into the setups screen, and change the tracking function to > record one point ever 30 seconds, or 1 minute. This will track a > whole days path, and dampens the effect that SA has on the > recorded track, since the actual distance between the most recent > few track points will be greater than with the default tracking mode. > Using this mode, I have found that my recorded tracks almost > always match up very nicely to my mapping software when > downloaded. Tracks on rivers follow the river on the map. > > For all slow travel, a compass will give you a better idea of where > you are pointed, but where you are pointed is not usually where > you will end up going; you still have to compensate for wind and > current. Using the GOTO function to get the desired course, and > watch for the desired course to change as you paddle. If it stays > near constant, then you are paddling in the right direction. If it is > changing, then you need to compensate. > > So, use the GPS to tell you where you are and where something > else is in relation to you. Use your compass to tell you which > direction you are pointed in. > > Richard Walker > Houston, TX > http://www.neosoft.com/~rww/kayak_log.html > *************************************************************************** > PaddleWise Paddling Mailing List > Submissions: paddlewise_at_lists.intelenet.net > Subscriptions: paddlewise-request_at_lists.intelenet.net > Website: http://www.gasp-seakayak.net/paddlewise/ > *************************************************************************** *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
The problem of the track function is caused by the basic error in the GPS system and by selective availability (SA). The problem is timing accuracy of the signals from the satelites and can't be fixed byanything in the receiver without receiving additional signals, i.e. using differential GPS (DGPS). Survey quality instruments get better accuracy by staying in one place during the fix and also by saving the data for additional processing later. This is a classic navigation problem, a time error translates into a position error of a certain distance. The faster you are going the smaller the error is compared to the actual movement. At walking and kayak speeds (also most sailing speeds) the basic errors of the system are large compared to the distance you are traveling in each update period (typically 1 second), so your speed and direction of travel displays are not very usefull. As for the display of waypoints, you are generally stuck with whatever the programmers thought was a good idea. If you can carry a computer then you can use a mapping/GPS display program to display things in any form you want. Unfortunately, this is not very usefull on a kayak underway. michael bhansen97_at_aol.com wrote: > > Woody - Garmin was amazingly available by phone today. The tech rep said the > tracking function does not work at speeds lower than about 3.3 knots. *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
>As for the display of waypoints, you are generally stuck with whatever the >programmers >thought was a good idea. If you can carry a computer then you can use a >mapping/GPS >display program to display things in any form you want. Unfortunately, >this is not >very usefull on a kayak underway. Sometimes I use that tracking/mapping option on my Garmin. It gives me a good visual reference so that I can locate myself on a map without measuring from grids, etc. It does not store every single waypoint -- couldn't, really -- probably not enough memory to do that. But I can enter waypoints while traveling for, say, good lunch stops. There is a price in that it eats batteries quickly. Mine has a continuous tracking mode that is particularly ravenous but at kayaking speeds 30 second intervals are more than sufficient. *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
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