Dear all,
An appeal from a fellow Europa owner, Europa Classic Mono 5Y-LRY in Kenya, East
Africa.
I would be truly grateful to anyone in UK with an aviation Garmin, especially
Garmin
96, 196, 295/6, 396 or 495/6 who can help me. Do you live by the sea?
Can you walk on your beach (when convenient!), turn on your GPS, let it settle,
and tell me what altitude/ elevation above / below mean sea level (MSL) that
your unit reads? It would be best if, beforehand, you can set the coordinates
to latitude/ longitude, datum to WGS84, turn off WAAS or EGNOS augmentation
Generally speaking, GPS units measure altitude as a distance to the 'reference
ellipsoid' of WGS84, this being the squashed-ball representation of the earth.
But much research has shown that the gravity of the earth is lumpy, and that
MSL is therefore lumpy. This lumpiness is incorporated into the 'earth
geopotential
model' ('geoid') EGM96 (now updated to EGM2008), generated by NASA from
satellites measuring gravity. Some GPS units also incorporate EGM96 into their
altitude corrections, but, despite my constant queries to them, Garmin has
been absolutely tight-lipped about whether they do this for their aviation
units,
and there is no reliable information online.
The 'height' difference between EGM96 and the reference ellipsoid WGS84 is
highest
in northern Europe (something like 70m), hence my question to UK Garmin owners
(or in France or Scandinavia)! In East Africa this difference is just 15m.
Please note this is not a question about the general precision / accuracy of
elevation
measurements using GPS units, which is much researched, but about geoid
offsets that might be hardwired in to GPS units.
This information is vital for our research into use of GPS elevation logs in
aerial
wildlife surveys in Africa (my field of work).
So... earth physics lecture over, just need your altitude above/ below MSL!!
Thanks, your help very much appreciated!
Richard,
Europa Classic 5Y-LRY, Mono, 912UL, Kenya
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=482735#482735
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