Marc Ramsey wrote:
Eric Greenwell wrote:
I don't think it's a good thing for student pilots, but how about the
licensed pilot flying cross-country? Would outlandings be safer if
pilots used the AGL information? I already do so to some extent, when
I use the "make waypoint here" feature over a good outlanding field,
then use that waypoint as my "target" for the glide computer.

I've been working with digital terrain elevation data since the mid-80s.
There are noticeable errors in some of the publicly available source
data. Fitting data for a usable area into the memory available in a PDA
requires a reduction in resolution. One also needs to consider the
possibility of significant GPS altitude errors. This data is fine for
drawing maps and getting a general idea of the height of the terrain,
but, in my opinion, depending on it for flying a pattern into an
unfamiliar field would be a mistake...

Marc is quite correct, and I should have mentioned I usually add 500
feet to my reserve to mitigate the problems he mentions; also, I
normally use it over relatively level terrain where the 2D resolution
wouldn't be important, and it's been accurate enough. I don't exactly
use it to fly a pattern, but just to get me back to the field high
enough to set up a pattern in the usual fashion - "that looks about right".

Now I'm curious about the accuracy of the terrain data used in my pda,
which runs SeeYou Mobile. I know SeeYou itself uses "relatively
accurrate" data now after the change (last year?) to data downloaded
from the Internet as needed, and SeeYou's terrain data is used to
produce the terrain data for the pda.

--
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Eric Greenwell - Washington State, USA

www.motorglider.org - Download "A Guide to Self-launching Sailplane
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