Ridecamp: [RC] GPS Distance Accuracy with Hills (long)

Hi, Ridecampers,

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 
their answers:

"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."

and

"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 
adding up.

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.

April
Nashville, TN
(Resident type A, controlling, metrics-loving endurance wannabe)


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