Re: [Celestia-developers] Re: Long/Lat

[Hank R. <hr...@qw...>]

On Wednesday, June 18, 2003, at 04:52  PM, Grant Hutchison wrote:

>
>> So why did the USGS institude the east longitude system for Mars?
> It dates to 2002 and MOLA. I can imagine a very large amount of data 
> being
> compiled in an office at the USGS before anyone noticed the mismatch. 
> It's
> just another "are these foot-pounds or newtons, oops where's the 
> satellite?"
> sort of problem.
>
> Grant
>

My impression is that the preference for planetocentric coordinates has 
more to do with the latitude definition than with the longitude 
direction. Technically, planetocentric coordinates define latitude as 
the angle between the equatorial plane and a line thru the center of 
the body, whereas planetographic coordinates define latitude as the 
angle between the equatorial plane and a line normal to the ellipsoid. 
The maximum difference between the two types of latitude on Mars is 
about 0.3 degree.

Here's a link to a discussion of the issue in a letter from Randy Kirk 
of the USGS Astrogeology Team:

     http://www.msss.com/mgcwg/mgm/kirk_letter.html#a

And here's a link to a dissenting opinion from Mike Malin, Principal 
Investigator for the Mars Orbiter Camera (MOC) on the Mars Global 
Surveyor (MGS) spacecraft:

    http://www.msss.com/mgcwg/mgm/letter.txt

Also, note that planetographic coordinates use west longitude only for 
objects with direct rotation. East longitude is used for objects with 
retrograde rotation.

- Hank