GPS Trip Mileage

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Posted by LeRoy Johnson on March 12, 2003 at 15:27:41:

I ran a test today to help answer the question of GPS trip mileages. I have a Magellan, Meridian Platinum, which I like. Jean drove me from home to the top of Swall Meadows. The odometer on the car recorded 5.35 miles (both ways). I turned the GPS on at home and it recorded the ?Trip Odometer? mileage as 5.20 miles.

I called Magellan and they said my machine enters data?while on a trip?every second so long as you are moving over 2 mph. GPS machined DO NOT measure slope distances, only horizontal distances. Thus, every second the machine calculates how many ?crow miles? (actually portions there of) I traveled and keeps a running total, which it calls ?Trip Odometer.? Keeping the machine on all the time uses batteries (Two days ago Jean and I hiked 12 hours from Upper Emigrant Spring to nearly the summit of Pinto Peak and I had to replace the batteries once.)

At Swall Meadows I got out and hiked home (with a 60 pound pack) via the road?the same route we drove up. The GPS again recorded 5.20 miles (Trip Odometer).

I am convinced my Platinum gives me accurate trip mileages when I hike. Again, remember, a GPS records crow miles and not slope miles. However, because the Platinum calculates trip mileage about every second, the cumulative total of thousands of crow miles will reasonably approximate slope distance.

A word about elevation: It appears my machine does not give me elevation readings when I am below sea level. However, I have found it extremely accurate as I travel across the Great Basin. On several trips I have kept the machine on the dash (connected to the car battery) and at every pass the elevation has been correct to between three and ten feet. In the Great Basin I often have six to eight satellites.

HINT: When hiking with a GPS bring along a divider or drafting compass. Take a waypoint at a known spot (like Upper Emigrant Spring) and if (and this is a BIG IF) you know the canyon or ridge you are on take another waypoint. Measure the GO TO crow miles on the map scale with the divider and cut an arc from your known point. Where the arc intersects the canyon or the ridge is your location.


p.s. I always carry a Ranger Silva compass and a barometric altimiter.

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