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nadar

different WGS84 implementations ?

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WGS84 is used by most GPS, and became a de-facto standard, but it is not always easy to use:

 

As far as I'm informed, different versions of WGS84 datum exist.

Originally (1986 ?), the elevation reference was an ellipsoid (defined by a few mathematical parameters such as the length of 3 axes, the position of the center, etc.) . Accordingly, important differences existed between elevations read on the GPS (reference level = ellipsoid) and actual elevation (read on a topo map or seen on a coast line or a boat)  (reference level = mean sea level).

 

Apparently, later implementations of WGS84 added some correction to better fit to the actual world. AFAIK, these corrections were implemented in the form of a look-up table defining discrete Z-corrections values according to longitude and latitude position on a grid. (contrary to an ellipsoid, a a geoid cannot be described by a few math parameters).

 

Apparently multiple implementations of this correction exist. Unfortunately, most (basic) GPSes still document it as "WGS84".

Comparing measurements doneon the same place with different GPSes can give very different results (for elevation). In my place; up to 40m.

 

My questions to the community:

 

1) can you confirm my interpretation ?

2) how can you determine which WGS84 flavour you are using

3) is there an easy solution to correct this ?

 

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dark. has just uploaded a great ebook which discusses the projections in details.. I recommend you read it.

I saw that. Seems a little bit complicated for me ;-(

I don't need to understand the maths behind projections and datums. Just need to use a simple GPS and get about 0 elevation reading when I'm sailing on a boat, not +40m...

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Using a GPS, a Garmin 62sc on a boat near the shore: the error was less than 6 m.

Error in theory, because they are other factors also: tide, the error of GPS system itself......

Place: Africa, 2 degrees south from equator 

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Using a GPS, a Garmin 62sc on a boat near the shore: the error was less than 6 m.

Error in theory, because they are other factors also: tide, the error of GPS system itself......

Place: Africa, 2 degrees south from equator 

I think Garmin GPSes use a modified WGS84 datum, to correct for the difference between the ellipsoid and the geoid. My problem is to know the parameters of this correction.

On the GPS of my Nexus 7 tablet, you can select either ellipsoid or geoid elevation, and playing with this option highlights the difference between both system. 

The problem is related to the fact that this correction is 1) not documented, 2) not the same for all brand/model. 

In my view, when data are documented as "WGS84" referenced, it should implicitely be "WGS84 Elliposoid". If a correction is added, then it's no more "WGS84", but somethink like "WGS84 bis", "WGS84 corrected" or anything similar.

Edited by nadar

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Forget. A lot of Garmins are using also a barometer sensor. The 72 I think, it has a menu for skydiving......

But on a tablet is different of course

But barometric elevation readings are also given in a specific reference system, WGS84 elliposid, WGS84 corrected or anything else. 

If your gps is set on WGS ellipsoid, you will read "Z+40m" when actually standing on the ground. Not a good new if you are skydiving...

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