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    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)
    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."
    "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.
    Nashville, TN
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