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Appeal to European Europa owners, aviation Garmin question

 
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Richard Lamprey



Joined: 15 Jul 2011
Posts: 90
Location: Kenya

PostPosted: Mon Aug 27, 2018 10:44 pm    Post subject: Appeal to European Europa owners, aviation Garmin question Reply with quote

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


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Ivor



Joined: 23 Jul 2015
Posts: 39

PostPosted: Sat Sep 01, 2018 11:03 am    Post subject: Re: Appeal to European Europa owners, aviation Garmin questi Reply with quote

hi Ritchard not sure if this is relevant to your question but on Friday at
Lydd EGMD that is 10 feet above sea level my Garmin 496 was showing a GPS altitude of 25 feet


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Richard Lamprey



Joined: 15 Jul 2011
Posts: 90
Location: Kenya

PostPosted: Mon Sep 03, 2018 1:41 am    Post subject: Re: Appeal to European Europa owners, aviation Garmin questi Reply with quote

Ivor, thanks very much, this is very useful and relevant! If you have a chance, could you just confirm in your Units page, if your map datum is WGS84 (or the datum it is set to)? And in System Setup, also that WAAS and EGNOS are turned off?

After 6 months of prodding to Garmin UK Technical Department, still no clear answer on geoids, but will keep trying!
Thanks again
Best
Richard


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Ivor



Joined: 23 Jul 2015
Posts: 39

PostPosted: Mon Sep 03, 2018 2:13 am    Post subject: Re: Appeal to European Europa owners, aviation Garmin questi Reply with quote

Ritchard
i am not going to the airfield for a couple of weeks but will try and check for the information you require next time i do,

regards
Ivor


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Richard Lamprey



Joined: 15 Jul 2011
Posts: 90
Location: Kenya

PostPosted: Tue Sep 04, 2018 11:49 pm    Post subject: Re: Appeal to European Europa owners, aviation Garmin questi Reply with quote

Thanks Ivor, much appreciated! Best
Richard


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