Book Milage vs. GPS Milage - Which to believe?

[DougPaul]

That's exactly what I thought, but some of them seem totally convinced it is otherwise.

Many (most?) users don't know much about how GPS works...

Based on my rudimentary knowledge of how a GPS works-triangulation in a vertical plane (up to the satellites) as opposed to a horizontal plane, like using landmarks to triangulate where you are. I couldn't see how it would work any other way. Using a simple right triangle, as Doug pointed out, is how you would figure actual distance traveled.

It uses trilateralization, not triangulation. (The GPS directly measures the time-of-arrival of the signals and their Doppler shifts from four or more satellites from which it computes the distance and velocities relative to the satellites. The math then coverts this to PVT (position (3D), velocity (3D), and time (1D)). The position and velocity are initially computed in Cartesian coordinates (XYZ) which are then converted into polar coordinates: lat, lon, and altitude (above the geoid).

Here is a list of GPS tutorials if you want more details: http://www.edu-observatory.org/gps/tutorials.html

Although, if for example I was in a plane at 6K feet, got a GPS reading of where I was, landed then stood directly under that spot so that my position on the ground was the same longitude and latitude, how does the GPS account for the difference in altitude when it makes the calculation? I suspect that because the satellites are so far away, a few thousand feet in altitude will make a very small, virtually insignificant difference in the angle to them.

The GPS has full 3D information about your present and past locations and can compute anything from it the designer desires--such statistics as the distance traveled are 2D by convention.

As for altitude affecting distance, perhaps the local altitude is used, perhaps the average altitude of the track is used, or perhaps an altitude is chosen by convention (eg mean-sea-level). (Don't know--I presume there is a standard convention.) In any case, the radius of the earth is ~4000mi so an altitude of 20000 ft (~4mi) would only add .1% to the length of a mean sea level track.

As for a track that includes changes in the altitude, I know the convention is to compute the 2D horizontal, but as stated in the previous paragraph I don't know exactly which altitude is used in the computation, but the difference is too small to affect a hiker. (Off hand, I would guess than MSL is used for land navigation.)

Doug