Hi all I just thought I'd share a wee observation of GPS use which I found interesting while paddling a couple of weeks ago. I was out for a paddle on a very familiar bit of coast by myself, and planned on heading south down the coast a couple of miles to some skerries, then off shore east a few kilometres to a buoy, then back to the put in. As I headed down the coast I noted I was fairly shifting - there was an appreciable tide running south. There isn't usually much in the way of a tidal stream here on the East coast (of Scotland), tide sets south turns sets north, mostly at less than a knot if that, but it was close to a reasonably large spring tide - which was great as around the skerries I was heading for is one of the few spots on this coast where the tide accelerates, and there are reasonable eddies to play in. There were a lot of seals around the skerries, so as I set south towards them I sat watching them (many were sleeping in the water which I rarely see, not upright but lying horizontally just submerged, eyes closed, big zzz's. As I approached in the tidal flow I realised the seals, although not moving a muscle were stationary in the stream! As I pondered how they do this (they hold their rear flippers at quite an acute angle and look like they are 'flying' down) I switched on the GPS to work out the speed of the tide - it was setting south at 3.1 knots around the skerries. After a while I ferried out to the buoy, up and across the stream. Glorious day, glassy calm, no wind. Good visibility to the red can now north east of me. The tide eased off away from the headland and shallows around the skerries and was running at less than 2 knots at the buoy three kilometers offshore. I sat in the eddy behind it and had a coffee, then thought I'll head back to the skerries and play about a bit more. I'd logged the outer skerry as a waypoint as I'd gone past watching the seals, and just for entertainment (visibility was good) set the GPS on a 'go-to' back to the way point and left it on the (garmin etrex) compass screen. The buoy was up stream from the skerries, and as I paddled back into the tide I lazily paddled across. I then realised that the GPS was pointing me about 30 or so degrees off course to the skerries. Hmm, checked the waypoint with the chart, checked the bearing to it with my deck compass, all OK. But the GPS go-to pointer was 30 odd degrees off. It then myopically dawned on me that the GPS was lined up with my boat which was pointing more or less due west, but my direction of travel was considerably south of that because I was in the tide stream setting south at 2-3 knots. While the GPS knew where it was and was giving my the right bearing to the skerries, the go-to pointer is relative to the direction of the movement of the unit, which because of the set of the tide wasn't the direction my boat and the unit on deck was pointing. Thus the error in indicated direction! Paddling across a tide stream where your direction of travel is 'sideways' is the extreme example, but it follows that if you are making any leeway or are in an unknown tide stream or eddy, then unless you know which way you are travelling and line the GPS up with that (unlikely if no transits are visible) then the garmin compass screen / navigation pointer is going to be less than useful. The moral of the story - do not rely on a gps without a steering compass!! Cheers Colin 57º19'N 2º10'W *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Hi Colin. I think that your problem with the use of the Garmin compass screen stems from a misunderstanding regarding the way it works (which may in turn be due to Garmin's poor documentation). If I understand what you are saying below, you seem to think that the compass pointer is supposed to point to the waypoint. It does not, and is not designed to do so. I could see why one might be confused, since the summary of this feature in the Garmin manuals describes the pointer as "pointer to waypoint". This is very misleading. If you dig a bit deeper into the function of the pointer (by reading the documentation for the fully featured Garmin receivers, not the "dumbed-down" eTrex) you will find that the pointer does NOT point to the waypoint, but rather is provides information that is much more useful than this. So what is the function of the pointer, if not to point to the waypoint? The pointer on the compass page is a course-correction pointer--it shows the discrepancy between your heading (actual direction of travel, or "track" angle to use Garmin's nomenclature), and the bearing to the waypoint. For example, if you are holding a 30-degree ferry angle and this is the correct ferry angle to take you to your waypoint, the pointer will point straight ahead. This shows that you are holding the correct ferry angle, and should keep paddling in the same direction. (Do not correct your angle.) On the other hand, if your ferry angle is not correct (so that you are not heading directly towards your waypoint) the pointer will point left or right (indicating the direction and angle to correct your course so as to track directly to the waypoint). This becomes obvious if you are using a more fully-featured GPS (such as a Garmin 12), since on the compass page it will tell you simultaneously both your track angle and the bearing to the waypoint. The arrow is simply a graphical representation of the difference between these angles. I hope that this clears things up. I certainly agree with you that one should carry a deck compass in addition to the GPS, but not for the reason that you state. Happy paddling! Dan Hagen *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
A correction to my last post (in which I misused the term "heading"): I wrote: > The pointer on the compass page is a course-correction pointer--it > shows the discrepancy between your heading (actual direction of > travel, or "track" angle to use Garmin's nomenclature), and the > bearing to the waypoint. I should have written: "The pointer on the compass page is a course-correction pointer--it shows the discrepancy between your actual direction of travel (or "track" angle to use Garmin's nomenclature), and the bearing to the waypoint." (The heading is the direction the boat is pointed, and has nothing to do with the compass display. The rest of the description is correct.) Dan *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
At 09:15 AM 9/5/2000 -0700, Dan Hagen wrote: he pointer on the compass page is a course-correction pointer--it shows the discrepancy between your heading (actual direction of travel, or "track" angle to use Garmin's nomenclature), and the bearing to the waypoint. For example, if you are holding a 30-degree ferry angle and this is the correct ferry angle to take you to your I am not familiar with the Garmin units, but on the Magellan units I have (ColorTrak and the Map 330X (New and not on dealer shelf yet) they provide "CTS" = Course To Steer. This is the corrected course for drift, etc. to take you to your destination vs. the direct bearing from your current position. Another area that should be considered when following the GPS Compass Rose is: * It only works when you are moving above xx mph/kph speed * It can be set to True North or Magnetic North If you set it to "Magnetic North" does it correct for Magnetic Variation automatically or just show magnetic headings like your compass that must be corrected for Declination and Variation. Interesting, I will do some more research on the last item. Fred California Kayaker *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
>> If you set it to "Magnetic North" does it correct for Magnetic Variation automatically or just show magnetic headings like your compass that must be corrected for Declination and Variation. >> The Garmin GPS 12XL, and probably most, if not all, other Garmin models, can correct automatically for declination/variation. You can even find out what your current variation is by going to the Setup Menu and selecting Navigation. If you have Auto Heading selected, it shows your variation in degrees east or west. Chuck Holst *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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