Hello Paul and Markus,
The importance for using a 0-360 degrees system for Mars comes form international research institutions' recommendations from the 1970's.
The 'strange thing' is that there are two recommended systems, but now it seems that data will be produced according to one of them. Here you are the suggested coordinate systems for Mars formalized in the 1971 International Astronomical Union meeting:
Planetocentric Coordinates (east/'ocentric):
Longitude is measured positive eastward and latitude is planetocentric defined as the angle between equatorial plane and a line from the center of the body to a given point
Planetographic Coordinates (west/'ographic):
Longitude is measured positive westward (direction opposite to the planetary rotation) and latitude is defined as the angle between the equatorial plane and the normal to a spheroidal reference surface on the given point.
The Planetographic Coordinates were used for products from 1970's through 1990's.
Mars Global Surveyor's Mars Orbiter Laser Altimeter team adopted the right-handed Planetocentric system.
Also NASA's Odissey and ESA's Mars Express High Resolution Stereo Camera experiment are using Planetocentric system.
So it seems that while there are two 'official' systems, the east/'ocentric is beign adopted for latest data.
Grass longitude 'Terrestrial' system is also right-handled (if we think to it as -180 0 +180) but giving a full 0-360 extent will allow to skip coordinate tranlsation during import phase of data. Other 'Terrestrial' GIS used for planetology simply use -+180 long system. As far as I know, only specific Planetology (Spectrometer and Image Analysis) tools at the moment support the full 0-360 system.
Projected Mars data is furnished mainly in Simple Cylindrical (Plate Caree) projection with auxiliary files or headers providing the long system used and ellipsoidal parameters used in projection. There are several dataproducts that use different ellipsoidal parameter for data projection.
Another issue to take care of is, in my opinion, that if we use planetocentric coordinate system, latitude has to be considered 'geocentric' and not geographic/geodetic (see above). If we have a sphere the two systems coincide, but if we use an ellipsoid, we will get two different latitudes ('centric & 'graphic) for a given point (the maximum difference is at about 45 degrees). Anyway, I never seen Mars's dataproduct in planetocentric system projected on an flattened ellipsoid.
In my experience I've only processed data in planetocentric system projected in Simple Cylindrical proj over a Sphere (latest data) and data coming in Simple Cylindrical projection with planetographic system over a flattened ellipsoid (older data).
I hope this will be useful.
Cheers Alessandro
On 2003.08.24 21:09, Paul Kelly wrote:
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
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Alessandro Frigeri
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