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#1614473
Good morning, help please! Can anyone tell me if the entry-level Garmin family of handheld units, eg Garmin 95,196, 296 read MSL above the EGM96 geoid model (rather than the WGS84 ellipsoid). ie that the EGM96 is hardwired into them. I cannot get an answer to this anywhere, from Garmin or online, and certainly not in any Garmin manuals. A couple of paragliding websites suggest that EGM96 is built in to aviation GPS units that have no barometric correction. Any definitive reference will help. Thanks!
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By Paul_Sengupta
FLYER Club Member  FLYER Club Member
#1614522
I don't know about Garmin, but on my old Lowrance GPS I believe you could set which you wanted. I think default was WGS84.

http://www.mikeg.net/hobbies/aviation/avionics/operation/Lowrance%20300%20Manual.pdf

P62.

All datums are named. The GPS system is based on the WGS-84 datum, which covers the entire world. Other datums may also cover the entire world, or just a small portion. By default, the AirMap 300 shows your position on the map using the WGS-84 datum. However, it can show your position using one of 99 different datums.

Sorry, doesn't answer the question regarding the Garmin.
#1615037
Thanks Paul and Riverrock. The geoid is the geo-potential model of the earths surface, rather than a map datum. Based on where land masses are, it models where MSL would be at that point if there is no land in the way. The global standard is EGM96, although it was refined further as EGM2008. There are fascinating maps online showing both. WGS84 is both a map datum, and an ellipsoid, meaning that it models MSL as a perfect ellipsoid, which it is not. Many GPS units measure height above the WGS ellipsoid. If you did an approach to an airfield that is technically at sea-level, using 'GPS Altitude' on a GPS set to WGS84 ellipsoid, you might connect with the ground earlier or later than expected, based on where you are. Unfortunately Garmin don't give out information about whether the EGM96 is built into aviation units. But thanks for your help, and Riverrock the situation in northern Europe would confirm some sort of geoid in use. I fly in East Africa, where the difference between EGM96 and the WGS84 ellipsoid is just 15 m, around 50 feet.
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By Paul_Sengupta
FLYER Club Member  FLYER Club Member
#1615038
I would have thought that the older units, not having WAAS/EGNOS, would not worry too much about altitude, as they're not accurate in altitude anyway. With WAAS/EGNOS they tended to introduce "pseudo-ILS" type features, and for this, I would have thought that they'd use an accurate datum for the airfields in the database.