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Map Features Overlays on a Cylindrical Projection
scalbers
post Jan 28 2012, 07:25 PM
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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


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john_s
post Feb 8 2012, 05:21 PM
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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
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scalbers
post Feb 14 2012, 04:33 PM
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QUOTE (john_s @ Feb 8 2012, 05:21 PM) *
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


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john_s
post Feb 14 2012, 05:50 PM
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QUOTE (scalbers @ Feb 14 2012, 09:33 AM) *
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
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scalbers
post Feb 14 2012, 06:04 PM
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QUOTE (john_s @ Feb 14 2012, 05:50 PM) *
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).

Thanks much for the elucidation. Here is a link that looks to be consistent with this scenario:

http://planetarynames.wr.usgs.gov/TargetCoordinates

And here is the result when I refrain from doing the transformation:

Attached Image


Steve


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Posts in this topic
- scalbers   Map Features Overlays on a Cylindrical Projection   Jan 28 2012, 07:25 PM
- - scalbers   I have now a preliminary map of Mars with the feat...   Feb 3 2012, 06:25 PM
- - JohnVV   nice but a question on the location of 180 in the ...   Feb 3 2012, 10:49 PM
- - djellison   That's a debate that's been a part of plan...   Feb 3 2012, 11:07 PM
- - scalbers   John - I had used the 180 center longitude that cu...   Feb 3 2012, 11:10 PM
- - JohnVV   as djellison pointed out both are right i was just...   Feb 4 2012, 12:40 AM
- - scalbers   The funny thing is that the quasi-standard way of ...   Feb 4 2012, 12:55 AM
- - scalbers   Meanwhile, here's the latest with Enceladus. T...   Feb 4 2012, 02:22 AM
- - Phil Stooke   Don't get too hung up on the question of havin...   Feb 4 2012, 03:45 AM
- - scalbers   Here's Enceladus with the longitudes a little ...   Feb 4 2012, 05:09 PM
- - john_s   The longitude system on Enceladus and the other ic...   Feb 8 2012, 05:21 PM
|- - scalbers   QUOTE (john_s @ Feb 8 2012, 05:21 PM) The...   Feb 14 2012, 04:33 PM
|- - john_s   QUOTE (scalbers @ Feb 14 2012, 09:33 AM) ...   Feb 14 2012, 05:50 PM
|- - scalbers   QUOTE (john_s @ Feb 14 2012, 05:50 PM) Th...   Feb 14 2012, 06:04 PM
- - hendric   Someone should make a map with 0 Longitude going d...   Feb 9 2012, 09:20 PM
|- - scalbers   Hendric, yes that would be an interesting presenta...   Feb 14 2012, 04:28 PM
- - Phil Stooke   Re: Post 12. That is called a Transverse Cylindr...   Feb 14 2012, 04:37 PM
- - JohnVV   QUOTE Very confusing- it took me a long time to so...   Feb 14 2012, 05:56 PM
- - scalbers   Next one - here's what I have so far for Io sh...   Feb 15 2012, 07:37 PM
- - scalbers   And here is Europa with features larger than 40km....   Feb 15 2012, 08:47 PM
- - scalbers   And back at Saturn, here are the Mimas features (a...   Feb 15 2012, 09:18 PM
- - scalbers   Here's Iapetus, with a 5 degree shift that app...   Feb 18 2012, 08:12 PM
- - Phil Stooke   A very nice project, Steve - I'm enjoying thes...   Feb 29 2012, 02:00 PM


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