RE: [RC] GPS Distance Accuracy with Hill - Anna Larson
I use a GPS on a daily basis and have found that the ONLY really descrepancy
is the actual elevation. I use both comercial quality and regual personal
(garmin) GPS. I hike on a daily basis about 3-7 miles over varied terrain
and have downloaded the data and even differental corrected it. I have used
both GPS's at the same time to compare milages. I have even compared speed
accuracy with my car speedometer. I basically live with my GPS (work
related stuff) and other then the actual elevation numbers, the only other
reasons for poor accuracy are
1. poor/no satellite reception
2. using up the entire track log and having the first points over written by
the newest ones. (way to avoid this is to take specific way points along
the way so you can trace back when you down load the data)
3. low batteries/turning unit off and on while trying to track
4. operator head space
I do agree though that a topo maping program is a great help especially when
you run out of track log points.
Mules and Other Critters
About Barefooted Horses
Subject: [RC] GPS Distance Accuracy with Hills (long)
After it was brought up that my GPS probably wasn't accurate in the
hills, I tended to agree. So my husband and I decided it was time to buy
a topographic map program. We got Topo USA 4.0 by Delarme. There are
better ones out there (read more expensive), but we figured this one
would do the job. When Daniel got home with it, we immediately installed
it on our computer and began to play with the features.
I loaded up my track from Sunday's 26.4 mile ride with Tanna. I did a
profile of the terrain we were working on. It was quite interesting to
examine all the statistics presented. What I was really interested in,
after all the discussion about my Sunday training ride, was the linear
distance (just straight as the crow flys) compared to the terrain
distance (with elevation calculations included). There it was in black
and brown (brown background).
Linear distance: 26.4 miles
Terrain distance: 26.4 miles
What?!? There must be something wrong with the program, I was sure of
it. So I moved over to the Grand Canyon. There are some longer, steeper
trails there. The differences in terrain distance vs linear distance was
also minimal there, but more pronounced than my ride on Sunday. I moved
back to more familiar territory. Pigeon Mountain, GA, where I did my
first LD ride many moons ago. Grades of 8-13. Minimal differences. Like
only hundredths of a mile.
So I asked a couple of friends what the deal was. Here are exerpts of
"The grade of the trail needs to be very steep before you get a
significant difference between horizontal and terrain distances. For
example, the steeper of the two main trails out of the Grand Canyon has
a grade of about 13% (one mile climb over about a 7.5 mile distance). In
that case, where you are climbing rather steeply the whole way, the
horizontal distance would be 7.43 miles vs. terrain distance of 7.50 miles.
SQRT(7.5^2 - 1^2)
Most trails are less steep and have even smaller differences."
"I was shocked by a similar experience. I couldn't figure out how the 12
linear mile trail I hiked in rugged country was only longer by 0.1 mile
terrain-wise. It just was. Most trail grades are kept within a certain
percentage in order for people to semi-comfortably walk, pedal, or ride
on. The whole rise-over-run thing doesn't really add up to much unless
you are going up (or down) a REALLY steep angle for a good distance."
So it looks like even though the terrian is more difficult for the horse
when there are hills, it doesn't significantly affect the milage unless
you're riding say hundred milers up and down steep grades (even then, it
might only add on a mile or two depending on the grade). Which I'm sure
some of you do! I don't at this point.
I'm open to contradiction; these are just my observations and those of a
couple of friends. I would love to find out that my 26 mile training
ride was more like 30 or 32. But so far, those numbers just aren't
If you're still with me and still interested, I would like to say that
the orignal 26 mile figure I had was not directly from the GPS. My GPS
trip computer said we went about 21 or 22 miles. But since I had found
the GPS turned off a couple of times (hazard of all that joggling it
gets sometimes in a canter or a trot), I knew that that wasn't accurate.
So I loaded up my track onto Mapsource and combined all the tracks into
one track that gave me my distance and overall speed. The distance with
that method is still somewhat an estimate. What the program does is
connect the dot from the previous track with the dot from the next track
and draws a straight line, losing any curviness you might have
encountered. But still is usually very close to the actual milage.
Especially, when you catch the GPS off and turn it back on quickly.
Ridecamp is a service of Endurance Net, http://www.endurance.net.
Information, Policy, Disclaimer: http://www.endurance.net/Ridecamp
If you are an AERC member - PLEASE VOTE in the upcoming By-Laws
Election!!!! (it takes 2/3rds to tango!!)