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posted by: ChrisO
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Posted Thursday, Feb 21 at 4:27 AM
Thu, Feb 21 at 04:27 AM
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| Hey, I've been using my Garmin GPSmap 60Cx for a while now. Pretty happy with it's ability to map in steep valleys, and under dense trees, but I've noticed a bit of a weird thing. When I'm in a valley, the track log it produces is quite often one or two hundred yards up the hill. Has anyone else ever noticed this kind of thing, with this GPS or any others? |
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There are 5 replies to this message.
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Park2 email this member | | Posted Tuesday, Feb 26 at 7:50 PM
Tue, Feb 26 at 07:50 PM | Yes, the extreme sensitivity of the SiRF receivers has an unfortunate side effect. They are much more susceptible to multi-path. The sat signal can hit you while coming straight down but can also bounce off a canyon wall and get to you a second time. (Exactly like the ghosts on old TVs with rabbit ears).
That extra delay of the second signal calculates out to being some random number, usually in the neighborhood of 50 - 250m away.
Really wierd is when it sees you here, then a second later 200m away & shows your speed as 6000kmh. But I think newer software designs have filtered that out. |
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ChrisO email this member | | Posted Tuesday, Feb 26 at 9:36 PM
Tue, Feb 26 at 09:36 PM | | This isn't quite like that though. It's not jumping around. I've noticed this a couple of times hiking and ski touring where the access is along a road in a valley along a creek. It puts my tracklog consistently up the hill pretty much the whole way, parallel to the road. |
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terryjw email this member | | Posted Thursday, Mar 13 at 6:28 PM
Thu, Mar 13 at 06:28 PM | | Check to make sure that your GPS is set to the same datum. If your GPS is set to WGS84 and the track is being plotted on a map with NAD27 datum then it's likely to be out by as much as 200M. That sounds suspiciously close to the error you're getting. |
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trailpeak email this member | | Posted Friday, Mar 28 at 8:40 PM
Fri, Mar 28 at 08:40 PM | | How many satellites are you tracking. What I've found in narrow canyons (even urban ones) on the older GPS's, if signal reception was marginal (e.g. only 3 satellites), that a consistent error would appear. Also, with all satellites directly overhead, the triangulation is not as good as picking up satellites from very different positions on the horizon. Just like compass triangulation, if you pick three distant points close together to triangulate with, you'll have greater error. If you are on Mt. Rainer, look out, the reception and accuracy will be incredible because the horizon to choose satellites from is so wide. |
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rexvirtue ( rex virtue ) email this member | | Posted Monday, May 26 at 6:45 PM
Mon, May 26 at 06:45 PM | | i think it also depends a little on your base map. I have noticed when importing GPS to Google Earth, I frequently cross rivers but it looks like i missed the bridge. not usually by 200m, but sometimes 10 or 15 meters maybe. |
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