Another Dumb Question

[granthutchison]

Tell me, how does one KNOW where the Central Meridian is in a planet map?

As I say, a responsible and sensible texture creator will tell you in the documentation. Sure enough, Bj?rn tells us on his web site that zero longitude is at the edge of the Callisto map - so all you need to do is to swap the left and right halves of the texture, and you'll have the prime meridian centrally placed for Celestia.

Grant

[Bob Hegwood]

Grant,

I understand that Bj?rn J?nsson's works are all *very* well documented, and I've even downloaded and converted Europa and Callisto from his site now. I've *successfully* converted them to images which are using a 0 longitude Central Meridian located at the center of the image.

(See how far I've come within the last three days? )

This has been a real educational experience, and I'd like to thank you all VERY much for the advice.

I STILL don't know how to determine where the Central Meridian is located on some of the other maps I've downloaded though. If the author doesn't tell you this information, you just have to *guess* at it don't you?

In other words, you already have to have an image in which you *do* know the location of the Central Meridian, before you can determine where it's at in the new image. Is this correct?

Or, is there a NASA location (or some such) from which one can derive this information?

Thanks again, Bob

[granthutchison]

I STILL don't know how to determine where the Central Meridian is located on some of the other maps I've downloaded though. If the author doesn't tell you this information, you just have to *guess* at it don't you?

Pretty much so. Guided by the knowledge that 180 degrees is very common, other multiples of 90 less so, and everything else vanishingly rare. (To my embarrassment, my first correction to Celestia's old Callisto texture involved a rotation of 150 degrees - something Fridger referred to as a "phantasy", as I recall ... but only after I'd realized my error for myself and corrected it to a full 180-degree twist ).
Referring to maps with known central meridians helps you judge which is the likely longitude shift from the above options - all the current Celestia distribution textures for bodies with a solid surface are aligned with a zero central meridian. Finding maps with longitude grids or named features you can use for reference is also helpful. In your image editor you should be able to read off the cursor coordinates when you place the cursor over a given feature - from that you should be able to work out how many degrees from the left margin a given prominent feature is, and compare that with the position of the same feature in a correctly orientated texture. (Angular offset from left margin = x/width*360, where x is the horizontal coordinate of your cursor position, and width is the image width in pixels.) The final check is to set up your new adjusted texture as an AltSurface and toggle back and forth between the Celestia standard and the new texture to make sure the features align.

Grant

[selden]

Bob,

The USGS maps of the Galilean satellites (Io, Ganymede, Europa and Callisto) and of some of the Saturnian satellites are at
http://astrogeology.usgs.gov/Projects/JupiterSatellites/
and
http://astrogeology.usgs.gov/Projects/SaturnSatellites/

They all have 0 in the center.

[Bob Hegwood]

(To my embarrassment, my first correction to Celestia's old Callisto texture involved a rotation of 150 degrees - something Fridger referred to as a "phantasy", as I recall ... but only after I'd realized my error for myself and corrected it to a full 180-degree twist ).

Well, speaking of Fridger, you and HE should know that his current TexFoundry version of Europa (The 2k Europa.png file) also has problems. This version of the moon's texture map also has the Central Meridian located at the left edge of the map. Using your excellent advice, I managed to correct the image so that its Central Meridian is now located in the center of the image.

I presume that this kind of error didn't matter much in earlier versions of Celestia, but it certainly does if you're trying to relate location files to an object.

I'd be happy to put up some pictures if that would help.

Thanks very much again, Grant... You DA MAN!

Take care, Bob

[Bob Hegwood]

Bob,

The USGS maps of the Galilean satellites (Io, Ganymede, Europa and Callisto) and of some of the Saturnian satellites are at
http://astrogeology.usgs.gov/Projects/JupiterSatellites/
and
http://astrogeology.usgs.gov/Projects/SaturnSatellites/

They all have 0 in the center.

Thanks very much Selden, but I've already been there. Your supreme Celestia Resources page got me to those websites somehow in my quest to understand this aspect of planetary map rendering.

By the way, THANKS for the website link on your page. Can't imagine why you'd list my little area of the web though.

I'm slowly getting up to speed on all this stuff, however, so stay tuned.

Take care, Bob

[granthutchison]

Well, speaking of Fridger, you and HE should know that his current TexFoundry version of Europa (The 2k Europa.png file) also has problems.

Ah, we know about that one, and some others, like Mars. Fridger produced his original textures to match the Celestia distribution textures of the time (not unreasonably), and then I came along and messed with the distribution textures to standardize them. Since then, Fridger has had difficulty finding the bandwidth to revise his larger textures to match.

Grant

[Bob Hegwood]

Well, speaking of Fridger, you and HE should know that his current TexFoundry version of Europa (The 2k Europa.png file) also has problems.

Ah, we know about that one, and some others, like Mars.

Well, if that's the case, might I suggest that Fridger place some kind of NOTE in his README file to relate this information? You have NO idea how much confusion this caused me when trying to understand this Central Meridian Concept.

If an unsuspecting newbie downloads these files, THEY'RE not going to know that anything is wrong until they plug in a location file.

Just a thought for you Fridger...

By the way, if you'd care to simply E-mail me the images, I would be more than happy to correct them for you.

Piece of cake now Grant thanks to YOUR information.

Thanks, Bob