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Author Topic: Coordinate conversion from ellipsoid to geoid  (Read 7154 times)

saskiag

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Coordinate conversion from ellipsoid to geoid
« on: June 01, 2015, 02:04:43 PM »
Hi,

I imported the image coordinates in WGS84 (EPSG::4326) and converted them in CH1903/LV03 (EPSG::21781).
Can you please tell me what kind of transformation you (Agisoft) used to convert them from the ellipsoid to the geoid? (WGS 1984 Geoid? or  EGM96 Geoid ?)

Cheers,
saskiag

Majaa

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Re: Coordinate conversion from ellipsoid to geoid
« Reply #1 on: June 01, 2015, 03:10:48 PM »
Maybe its Helmert transformation. I think It is the best transformation in geodesy. isnt it?

Sascha

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Re: Coordinate conversion from ellipsoid to geoid
« Reply #2 on: June 02, 2015, 03:23:13 PM »
Helmert tranformation is the easiest way for 7-parameter transformation.
So i assume its helmert.

EGM 96 is part of the WGS84 definition, so the geoid model u use when selecting WGS must be EGM96.

That was my contribution to the solution :).

Now my question (i am quite new to agisoft):

How did u convert the project into another coordinate system?EDIT:  FOUND IT
« Last Edit: June 02, 2015, 03:29:32 PM by Sascha »

MPB

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Re: Coordinate conversion from ellipsoid to geoid
« Reply #3 on: June 04, 2015, 01:58:39 AM »
If I understand the question: Converting ellipsoid to geoid isn't really a full transformation - it's just a matter of the geoid height being subtracted from the ellipsoid height for each node/xy position, to produce 'orthometric heights.' Each ellipsoid has an associated geoid. EGM08 is the current best for IGS08 (i.e., WGS84) ellipsoid/reference frame as I recall, and GEOID 12B for NAD83. Geoids come in various resolutions, for example EGM08 on a 2.5' and an 1' (~ 1 GB file) are common. Hope this helps.

saskiag

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Re: Coordinate conversion from ellipsoid to geoid
« Reply #4 on: September 15, 2015, 07:28:25 PM »
Thank for your inputs and maybe I should have added that the Swiss Coordinates CH1903/LV03 (EPSG::21781) are a projected coordinate system (and WGS84 a geographic coordinate system). Therefore one need to re-project the coordinates  and therefore choose an ellipsoïd for the vertical correction.

In this case, I dunno if Agisoft chose the WGS 1984 Geoïd, the current Swiss Geoïd CHGeo2004, or a European one?
I'd like to know which one and what are the resulting elevation (Orthometric height, Ellipsoïd, Geoïd?)

Any thoughts?