You've got answers on the bike computer side...
GE will squish and stretch the photos to form one image. This will lead to errors, some obvious like objects at the edges having two shadows.
I have found GPS coordinates of single points I've marked in the field to be fairly close in North-South direction, and to be off by up to 50 meters or so in East-West. Don't know if that's a local effect, or generally so.
Thomas Prufer
Thanks Thomas - that's an interesting reply. The distance that I am using is predominantly east-west so it will be interesting, once I come to terms with getting my gps to work (!), to see if the distance is correct as re-setting the calibration figure now gives a reasonable distance agreement.
Rob
The points are off, then they aren't I think the pictures are stretched nonlinearly to join them at the seams, and presumably somehow matched up to known lat/lon positions. So -- and I'll wave my hands a bit here -- the error could be anything from a few tens of meters at each end, to a few tens of meters at each point of the path, at the very worst...
I've usually done the "vertical valve stem to vertical valve stem" measurement. Found it to be different enough from the value given in the table to be worth doing.
Thomas Prufer
Assuming GE uses the same overlays that Google Maps does, I've never experienced anywhere near the deviations that you seem to be getting when comparing the photos with the reality on the ground. To keep in touch with my inner child, I'm a "geocacher", so using a GPSr to find reasonably precise spots is a bit of a hobby of mine. Sometimes when I don't have my GPSr with me I'll locate the place where something has been hidden simply by looking at the aerial photo instead, and invariably the location (e.g. a specific tree, stile, etc) is a 100% match for the coordinates (to within a metre or so).
It could simply be that the maps / photos are much better aligned around here than they are in your area. I was going to wonder whether your GPSr is at fault, as obviously the distance between E/W points varies dependent on your N/S, but I can't conceive of a GPSr not taking this into account.
Matt
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