[GRASS5] Re: Mars planet location

[Alessandro Frigeri]

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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