Map Features Overlays on a Cylindrical Projection Options

[scalbers]

Greetings,

Thought I'd mention I'm starting a mini-project to make map feature overlays for planets and satellites. These are set up on a cylindrical projection so they can be displayed with Science On A Sphere, Celestia, and the like. The key is that text for feature names has to be "pre-distorted" so it looks correct when viewed on a sphere. This is especially important at high latitudes. The overlays are for now in the form of transparent PNG images.

So far I've started with Mars and Enceladus. The programming language is IDL, and the lat/lon info for the feature names is from the USGS Map-A-Planet Gazeteer. I'll try to post some results once they are a little better.

Enceladus is a special case, since the map I had put together is in planetocentric coordinates, and the feature lat/lons are in planetographic. So a conversion of one or the other would need to be done.

Steve

[john_s]

The longitude system on Enceladus and the other icy satellites was shifted around 2008- in the case of Enceladus, by about 3.5 degrees. This was because the longitude systems on the satellites are defined relative to geographical features, not relative to the Saturn-facing direction (though the definitions are designed so that zero longitude points approximately at Saturn). By 2008, Cassini mapping of the satellites was complete enough to better determine the locations of the craters used to define the longitude systems, which required shifting the longitude system so that those craters had the "correct" longitudes.

Perhaps this is the reason for the discrepancies in those label positions?

John

[scalbers]

The longitude system on Enceladus and the other icy satellites was shifted around 2008- in the case of Enceladus, by about 3.5 degrees. This was because the longitude systems on the satellites are defined relative to geographical features, not relative to the Saturn-facing direction (though the definitions are designed so that zero longitude points approximately at Saturn). By 2008, Cassini mapping of the satellites was complete enough to better determine the locations of the craters used to define the longitude systems, which required shifting the longitude system so that those craters had the "correct" longitudes.

Perhaps this is the reason for the discrepancies in those label positions?

John

Indeed John, the Enceladus labels in post #10 have a 3.5 degree shift compared with post #8 and that appears to make things somewhat better. I had thought I shifted the underlying map to account for this and I'm guessing the USGS Gazeteer might have the older coordinate system? We might check this by looking at the particular crater(s) that is the reference point.

There are a couple of other considerations as well. First is that I wrote a routine to convert the USGS label data in planetographic coordinates to my map that is in planetocentric. Hopefully that is working correctly. Still there might be on the order of a degree or two of error in the construction of the map. If there remains a source of systematic longitude shift this presents an opportunity to try and correct things.

Steve

[john_s]

There are a couple of other considerations as well. First is that I wrote a routine to convert the USGS label data in planetographic coordinates to my map that is in planetocentric.

Actually the USGS maps (and presumably, feature coordinates) are really planetocentric, in that the coordinates of features on non-spherical moons are defined by the orientation of the direction to the body center rather than the direction of the local surface normal. The fine print explains that they are "planetographic relative to a spherical surface" or something like that. What they mean by "planetographic" is not that surface normals are used to define coordinates (they aren't), but that west longitudes are used (planetocentric coordinate systems traditionally use east longitudes).

Very confusing- it took me a long time to sort this out myself.

John