Re: Garmin GPS 12 - TRIP accuracy?

Dan Anderson <danderson2001@xxxxxxxxxxx> wrote:

Terry Pinnell wrote:
I've learned to disregard the Trip reading at the end of my walks, as
it always reads significantly low. If there are any other
walkers/hikers here, I'd be interested in knowing what error they
typically get.

Today's walk, shown in both Memory-Map and GPS Utility as 15.8 miles,
was 13.6 according to my GPS 12. So the actual was 16% higher than
reported, and that's fairly typical of my walks.

I'm still not clear *why* the error arises. The track seems
continuous. If PC programs can get it right, why can't the Garmin?

The trip odometer probably accumulates distance with its own filter,
otherwise it would be depend on the track log setting. The early
Garmin models seem to filter too much. I get very close values in
the GPSmap 76 and GPSmap 76S. Those values are always (unless
there are glitches) a bit higher than the GPS 76 when hiking. When
I did a hike with a friend who had a 60Cx, its trip odometer came in
significantly lower than the 76S and lower than its own track log.
Track log lengths were closer - I don't remember how much or how
many points per track there were.

Based on what I've heard and my friend's 60Cx, I'd say Garmin
increased the trip odometer's filter to remove shorter "zigs".

I probably got higher trip computer distances than you (Terry)
with a 12XL because I would stop at bends in the trail and "force"
the receiver to record a track point. This makes for a more
detailed track log and I'm guessing that it could also
effect the trip computer distance - accumulate shorter length
zip-zags instead of a straight line approximation.

I didn't post a comparison of the trip odometers and track
log distances when driving in the following link but I
remember the 12XL being close to the Venture and sometimes
higher. Since roads are straighter than trails, the longer
"filter distance" in the 12XL doesn't matter.

I posted some hike distances years ago at:

http://www.gpsmap.net/Distances.html

Many thanks, Dan, very interesting.

I did a quick summary of your 4 trips, focusing only on the 12XL
(which I'm guessing uses identical algorithms to my 12). For
simplicity, I took an average of your 3 comparative measurements
[Computed Distance (2D), Computed Distance (3D) and Tracklog Distance
from Mapsource] and called it 'Actual'.

12XL Trip Actual Error (Low)
--------- ------ -----------
Trip 1 (464 points) 6.66 7.28 8.5%
Trip 2 (454 points) 6.60 7.11 7.2%
Trip 3 (589 points) 8.30 9.23 10.0%
Trip 4 (558 points) 7.40 8.21 9.9%

That average error of around 9% low is very similar to those I've been
getting with my GPS 12, over 6 years. Yesterday's walk (650 points)
gave an unusually high error of 14% (13.6 versus 15.8).

Before seeing your post and rebel's, I replied to Ted, repeating my
'central point'. I'd appreciate your comments on that please. I
suspect I have some basic misunderstanding about the way the GPS 12
calculates 'Trip' distance. I am simplistically assuming it
recalculates it constantly, basically from a set of x,y co-ords. (The
GPS 12 doesn't record altitude - one of the main reasons I really
ought to upgrade soon!) But if it did, then the final value reported
should IMO be 'correct', i.e. the sum of the straight line distances
between all track points, just as my PC programs apparently calculate
it. Speed, stops, curves - none of these should enter the equation!

--
Terry, West Sussex, UK

.

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