ZT: GPS vs Odometer Distance Measurement

Subject: GPS vs Odometer Distance Measurement Mon Mar 09, 2009 9:03 am

Have you ever noticed that GPSr mileage is always ~10% less than your odometer readings? Well, there is a reason that happens. I'll try to explain it.

The GPSr measures distance along it's referenced coordinate system, a geodedic spheroid. This is basically an oblated sphere, a purely mathematically described surface, that should(?) represent the average earth surface. The GPSr coordinates are locations on this surface. Some GPSr units also rocord a calculated elevation that may be used in your computer programs, or used to display an altitude profile of your tracks. The GPSr calculated elevation is much less accurate than the location data and should be only used with caution.

The odometer measures topographical distance, or distance along the earth's actual surface, up and down hills and valleys.

So one good thing about this is that you cannot say your GPSr, or your odometer, is wrong just becuase they don't agree. They cannot agree, so don't expect them to. It take a sophisticated computer program to convert one type data to the other, using well defined topographical elevation data in the process.

A note of caution... a consumer grade GPSr should never be used for land surveys, or other measurement situations where precise locations are required. GPSr insturmentation is available to do this but costs run into several thousand dollars and is very cost prohibitive for us casual users.

_________________
Dan
"If'n you ain't been throwed, you ain't been ridin."

ZT: GPS vs Odometer Distance Measurement

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