# [Proj] Can I convert the ellipsoids using Proj?

Noel Zinn (cc) ndzinn at comcast.net
Tue Aug 19 19:32:28 EST 2014

```Cleo,

That’s right.  You’re looking primarily at a 70cm difference in height, but it varies a few centimeters as a function of latitude.  Jose’s instructions should nail it exactly, but I’m not a proj4 expert, so I leave you in the hands of others more knowledgeable.

Noel

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

From: Cleo Drakos
Sent: Tuesday, August 19, 2014 6:29 PM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Can I convert the ellipsoids using Proj?

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> 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 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.

Thanks for the link.

Noel

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

From: Cleo Drakos
Sent: Tuesday, August 19, 2014 3:39 AM
To: PROJ.4 and general Projections Discussions
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> 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>:

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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