[Proj] Can I convert the ellipsoids using Proj?

Alain Orsoni Alain.Orsoni at ign.fr
Wed Aug 20 01:37:13 EST 2014

```Cleo,

If I understand correctly you have latitude, longitude, elevation (considered as heigth above ellipsoid) and geoid (considered as height above geoid).
If so, you have to input only latitude longitude and elevation in proj4. Heigth above geoid remains the same whatever the ellipsoid you use, it is just the height of the geoid above the ellipsoid which changes when you change ellipsoid.
The elevation above ellipsoid will change because of the change of ellipsoid dimensions.

Alain

Le 20 août 2014 à 01:29, Cleo Drakos <cleo21drakos at gmail.com<mailto:cleo21drakos at gmail.com>> a écrit :

Dear Noel

Thanks for your explanation. So it means that both ellipsoids have the same geocentric coordinates, and I can use Jose's instruction.
I have four columns (lats, lons, elevation, and geoid) in an ASCII file. I need to convert all these four parameters into WGS84 system.
How can I apply Jose's instruction? I mean how can I call cs2cs for different parameters? I did not know how to input these four parameters.

Sorry for this type of question, I hope you do not mind.

cleo

On Tue, Aug 19, 2014 at 8:44 PM, Noel Zinn (cc) <ndzinn at comcast.net<mailto:ndzinn at comcast.net>> wrote:
Cleo,

FAQ #7 under Altimetry products in the link you provide seems to answer the question you ask.  The FTP link in that paragraph leads you to  a program top2wgs.pro<http://top2wgs.pro/> that solves your problem.  The program assumes dX = dY = dZ = 0.  I’ve made the same assumption myself with TOPEX/Poseidon in a different context.  Then follow Jose’s cs2cs instructions.

Noel

Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472<tel:%2B1-832-539-1472> (office), +1-281-221-0051<tel:%2B1-281-221-0051> (cell)
noel.zinn at hydrometronics.com<mailto:noel.zinn at hydrometronics.com> (email)
http://www.hydrometronics.com<http://www.hydrometronics.com/> (website)

From: Cleo Drakos<mailto:cleo21drakos at gmail.com>
Sent: Tuesday, August 19, 2014 3:39 AM
To: PROJ.4 and general Projections Discussions<mailto:proj at lists.maptools.org>
Subject: Re: [Proj] Can I convert the ellipsoids using Proj?

Dear Jose

Thanks for your answer. Can you help me how can I make certain that both ellepsoids have the same geocentric coordinates.
Only the information I am having is below under the section of How does the GLAS ellipsoid compare with WGS 84?

http://nsidc.org/data/icesat/faq.html#3_all

cleo

On Tue, Aug 19, 2014 at 6:03 AM, Jose Gonçalves <jagoncal at gmail.com<mailto:jagoncal at gmail.com>> wrote:
Cleo

If cartesian geocentric coordinates are the same and you just want to express them in a different ellipsoid you can do:

cs2cs -f %.9f +a=6378136.30 +rf=298.257 +proj=latlong +towgs84=0,0,0 +to +a=6378137 +rf=298.25722356 +proj=latlong +towgs84=0,0,0

If there are shift and rotation you can express them in the +towgs84.

Regards

José

2014-08-18 15:13 GMT+01:00 Cleo Drakos <cleo21drakos at gmail.com<mailto:cleo21drakos at gmail.com>>:
Dear guys,

I have the data (latitudes, longitudes, and elevations) given in ICESat/GLAS ellipsoid. This equatorial radius and  reciprocal flattening (1/f) on this ellipsoid are 6378136.30 m and is 298.257 respectively.

Now, I want to convert my data into WGS84 ellepsoid with equatorial radius and reciprocal flattening (1/f) of 6378137.000000 and 298.25722356 respectively.

Can I convert the ellipsoids using Proj?

I hope some of you can help me.

cleo

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