GPSMAP 60Cx and shortchanging distance on hikes

[PortlandHawk]

When I hike (usually through fairly heavy cover) with my Garmin 60Cx, it is not very accurate in terms of distance traveled. For example, if I go on a 5 mile roundtrip hike, the unit only says 4 miles traveled. But then when I download the track into Mapsource, it gives the correct 5 mile distance there. It seems the unit is calculating distance differently than mapsource. Is this just a limitation of the unit, or is there a setting I have wrong? I'm somewhat new to this, so I could just be missing something simple.

[+user13371]

Though some will claim that the GPS they own doesn't have this problem, I have seen it to some degree on every Magellan, Lowrance, and Garmin unit I have owned.

 

In short, it will underestimate speed and distance when I'm moving slowly (walking), and be more accurate when I'm going fast (cycling, driving, flying).

 

I've always assumed the GPS "rounds down" the distance it measures between each update, used to report your present speed (and add up your track distance). If you move a long way each second, and it rounds of a few centimetres, it's no big deal. If you're dawdling, then those few centimeetrs might be a more significant fraction of the distance travelled.

 

I bet Robert Lipe will chime in on this one

[Hiker2008]

I like that round-off theory, as long as it always rounds down. If it rounds off according to acceptable mathematical practices then rounding will be statistically irrelevant.

 

I think the problem here is poor reception causing track to cut corners.

 

Here is one of my theories. I believe my unit (etrex vista cx) overstates distance. As I walk along a trail gpsr will mark my postion with an error equal to some radial distance around me. Now, the radii pointing forward will cancel the ones pointing backward. But the ones pointing right and left will add to my distance as they bounce back and forth on each side of the trail.

 

In other words, what would normally be a straight line is really a zig-zag.

 

Of course this assumes I always have good reception and no corners are cut by track.

 

What do you think of that?

Edited June 5, 2007 by Hiker2008

[PortlandHawk]

Reception could partially be an issue, but I think it's more than that. I just find it odd that the GPS unit will say I've travelled 4 miles, but then when I upload the track to Mapsource it gives the correct 5 mile distance. These were fairly steep hikes, so I was moving rather slowly.

 

The rounding idea is interesting. That could also be a possibility. And I think speed is definitely a factor in distance accuracy.

 

Here's a thought that people with more knowledge of this might be able to address. It seems like the GPS may be calculating distance by taking average moving speed multiplied by time between track points. On the other hand, mapsource is calculating distance by simply measuring the actual distance between 2 track points. So when I'm moving very slowly up a hill, the GPS at times may actually think my speed is 0, when in fact it's not. So it underestimates my rate between points, and thus my distance traveled.

 

Maybe I just have to move faster.

Edited June 5, 2007 by PortlandHawk

[+geobc]

I think was discussed here before. IIRC the consensus was there is a distinct difference btwn what the Trip Computer says and what the track measures out to. When hiking, I've definitely found that the Trip Computer shows a distance that is definitely wrong. But the track log distance (as displayed in the GPS or on Mapsource) is quite accurate.

 

GeoBC

[+imajeep]

FWIW, our Garmin GPSmap 60Csx gives pretty good track distances, on both the handheld, in MapSource, and in Google Earth. We use it mainly on 3 to 6 mile hikes, in both desert and forest environments. It gets awesome reception under the trees!

[PortlandHawk]

I think was discussed here before. IIRC the consensus was there is a distinct difference btwn what the Trip Computer says and what the track measures out to. When hiking, I've definitely found that the Trip Computer shows a distance that is definitely wrong. But the track log distance (as displayed in the GPS or on Mapsource) is quite accurate.

 

GeoBC

This might be it. I've been looking at the distance on the trip page. I guess I didn't know that those distances would be different. Next time I'm out I'll monitor both that and the track log. Thanks.

[PortlandHawk]

I think was discussed here before. IIRC the consensus was there is a distinct difference btwn what the Trip Computer says and what the track measures out to. When hiking, I've definitely found that the Trip Computer shows a distance that is definitely wrong. But the track log distance (as displayed in the GPS or on Mapsource) is quite accurate.

 

GeoBC

 

**Update for those interested**

 

This was the problem. Took it out again today on a ~8.5 mile hike through heavy cover. During the hike the Trip Computer page was again well under the actual distance, while the track log distance was very accurate. In the end, the track log had the correct 8.5 mi distance while the trip computer said 7.0 miles. I guess I'll just be watching the track log from now on.

Thanks for the tip!

[+GOT GPS?]

Have had several Garmins and Magellans, and where the Magellans all under reported their Trip Distances on the Trip Odometers, the Garmins I owned all seemed about right on, except for the Map60Cx. I have found that the SiRF III receiver has been a problem in some sense, when you are going real slow at times, while hiking. I have a Map60C, and it was just about perfect, as compared to my Map60Cx. The "S" units with sensors, I think are a little more OFF with accuracy of the Trip Odometer readings. The most accurate "S" unit I have had, is the Map76S.

 

All my Magellans I have had except for my eXplorist XL, all reported a Trip Odometer distance of about 85 percent the true distances of any hike I have done.

 

To gain the best accuracy on a Map60Cx/CSx, you need to hold it just about vertical with the antenna getting a decent view overhead, and to be moving at a good walking speed.

 

On the GPS units, I wish that there was a calibration of the Trip Odometers against their tracklogs to improve the quality of the Trip Odometer.

[+EraSeek]

I have been playing with this for some time. My trip computer is usually more than my tracks. Hmm. I still have not figured it out bu will try the slow walk vs. fast thing.

 

BTW, mapsource will often trim your track (edit) when you load the track into mapsource. To fix this I believe it works to chose the fiter option and push the bar up to maximum points.

[+Miragee]

This is interesting. When I remember to Reset the Trip Computer on my Garmin Vista C , the Trip Computer is quite accurate compared to the Tracks I upload to Mapsource later. However, where I hike, I don't have to contend with tree cover.