[GRASS5] Re: Mars planet location

[Paul Kelly]

Hello Markus

Markus Neteler wrote:
> 
> thinking again about the Mars coordinate system, I found that
> PROJ4 support such shifts (guess you know that already):
> 
> cs2cs +proj=latlong +to +proj=latlong +pm=-180
> 180W 0N
> 0dE     0dN 0.000
> 0E 0N
> 180dE   0dN 0.000
> 180E 0N
> 360dE   0dN 0.000
> 100E 0N
> 280dE   0dN 0.000
> 
> Could we make somewhat use of the prime meridian do support the Mars
> coordinate system (probably still changes are needed for the global
> wrap around)?

I thought a little bit about this and as far as I can see that will not
be any help as the main problem (apart from perhaps georeferenced data
i/o operations) is just presenting the range of longitudes to the user
as 0 to 360 instead of -180 to +180. Internally to GRASS the data could
still be stored as it is now. Perhaps this would be as simple as
changing the G_scan_{easting,northing} and G_format_{easting,northing}
to accept a parameter flag that would indicate if longitude values
should be interpreted in the 0 to 360 or -180 to +180 system. But it is
sure to turn up other places that would need to be changed (maybe bugs
that should use these functions but don't).

We would be best to look at as well how it is handled by other software.
It doesn't seem to be a very important problem to me as it doesn't
affect GRASS' analysis capabilities (it is only an aesthetic thing) but
I don't think I completely understand why it is important for Mars.

Paul