A specific example:

In the log file in question, my take-off roll begins at 1:24:15pm, when the
latitude/longitute fixes start to move.

Calculating the instantaneous speed as the distance from the previous fix to
the current fix divided by the time between those two fixes, I have these
speeds:
+15 seconds: 43mph
+20 seconds: 59mph
+25 seconds: 72mph
+30 seconds: 81mph

But loading the same file in SeeYou and looking at the data that pops up
next to the mouse pointer and checking the speeds at the same points:
+15 seconds: 13mph
+20 seconds: 22mph
+25 seconds: 33mph
+30 seconds: 43mph

Those are some rather drastic speed differences. The tow plane was one of
Turf's Pawnees, which in the warmest weather don't have me sitting at 35
knots a full 30 seconds into the take-off roll.

The distance algorithm I'm using is the classic Great Circle:

distance_in_miles = 3956.0775034473 * arccos( SIN(lat1) * SIN(lat2) +
COS(lat1)*COS(lat2) * COS(lng1-lng2) )

Any other hackers out there played with this stuff?

-tw