# [Proj] Changing Radius of the Earth?

Jose Gonçalves jagoncal at gmail.com
Tue Apr 10 11:14:23 EST 2012

```For Mars use its ellipsoid axis:  +a=3396200 +b=3376189
or the mean sphere: +R=3389508

Data from http://www.hnsky.org/iau-iag.htm

Regards

José Gonçalves

Em 10 de abril de 2012 16:16, Fischer, Robert P. (GISS-6110)[COLUMBIA
UNIVERSITY] <robert.p.fischer-1 at nasa.gov> escreveu:

> 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?
>
> Thanks,
> -- Bob
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