# [Proj] Changing Radius of the Earth?

Frank Warmerdam warmerdam at pobox.com
Tue Apr 10 10:52:09 EST 2012

```On Tue, Apr 10, 2012 at 8:16 AM, Fischer, Robert P.
(GISS-6110)[COLUMBIA UNIVERSITY] <robert.p.fischer-1 at nasa.gov> wrote:
> How do I change the radius of the (spherical) Earth in proj.4?  I'm using the proj.4 string:
>
>    " +proj=laea +lon_0=-40.000000 +lat_0=74.000000 +x_0=1000000 +y_0=1700000 +ellps=sphere"
>
> ..and have discovered that proj.4 assumes a radius of exactly 6370997m (from pj_ellps.c):
>
>    "sphere",   "a=6370997.0",  "b=6370997.0", "Normal Sphere (r=6370997)",
>
> We are running code on a spherical Earth, with a radius (slightly) different from that above.  For consistency sake, proj.4 will need to use the same Earth radius as the code that calls it.  How do I set up a projection on a spherical Earth with a custom radius?
>
> Also... if someone wanted to use proj.4 for mapping on Mars, how would they do that?

Bob,

You can supply explicit values for the semi minor and semi major axes
instead of using the symbolic "sphere" value.

eg.
" +proj=laea +lon_0=-40.000000 +lat_0=74.000000 +x_0=1000000
+y_0=1700000 +a=2000000 +b=200000"

Best regards,
--
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Software Developer
```