Rounding off moons? |
Rounding off moons? |
Jan 4 2008, 09:20 AM
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#1
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Member Group: Members Posts: 715 Joined: 3-January 08 Member No.: 3995 |
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Jan 4 2008, 10:08 AM
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#2
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Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
It has to do with geometric reprojection of mosaic data. Cassini is somewhat notorious for the slow speed of taking images (typically on the order of a minute per frame for normal resolution images) and also the lack of a scan platform means the entire s/c has to slew to change the mosaic footprint. Both of those things take time and the spacecraft and the moon are moving (very rapidly in Mimas' case, it's low in Saturn's gravity well) and the perspective changes rapidly. Just mosaicking the footprints together causes problems with image seams due to the perspective shifts. Geometrically correct mosaics such as that one take into account the perspective and reproject actual image data on a shape model of the moon (say a triaxial ellipsoid at most) . This has a nasty sideeffect of giving a flat limb since the model doesn't have topography and some moons are very rough, but the end mosaic is scientifically more accurate. Fudging the mosaic footprints gives aesthetically more pleasing results, but they should not be regarded as scientifically accurate or used for serious mapping for example.
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Jan 4 2008, 11:12 AM
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#3
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Member Group: Members Posts: 715 Joined: 3-January 08 Member No.: 3995 |
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Jan 8 2008, 08:20 PM
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#4
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Member Group: Members Posts: 655 Joined: 22-January 06 Member No.: 655 |
Gladstoner - I'm with you - Give me mountains on Mimas. Can't wait for the extended mission sub 10,000km flyby
I think Gordan is right - the ciclops image you link to IS a mosaic, but only of two frames - shuttered in August 2005. My guess would be that it was easier for whoever spliced them, to assign a symmetrical limb during processing, so that both frames of the mosaic could be brightness-matched satisfactorily and quickly. It's aesthetic. But wonderfully, we all have the option of referencing the PDS (or the NASA raw image page) for the unabridged data whenever we wish. Once again for the record - fabulous outreach, congratulations to Linda S. and the team. |
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Jan 8 2008, 09:16 PM
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#5
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Senior Member Group: Moderator Posts: 3233 Joined: 11-February 04 From: Tucson, AZ Member No.: 23 |
That mosaic has four-frames actually. The speed of the flyby prevents just "adjusting" each frame so they fit. The best way fitting the frames together is to map project each frame into an orthographic map projection. The unfortunate side effect is that the limb is smoothed since the model used doesn't include all those little bumps on the limb. While annoying, it is the nature of the beast and it is better than the alternative in my opinion.
However, in my last few mosaics, I have combined two mosaics: my usual map projected mosaics and limb mosaics that use that original frames. All the seamlessness of the map projected versions with all of the limb-y goodness. -------------------- &@^^!% Jim! I'm a geologist, not a physicist!
The Gish Bar Times - A Blog all about Jupiter's Moon Io |
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Jan 8 2008, 10:10 PM
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#6
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Solar System Cartographer Group: Members Posts: 10190 Joined: 5-April 05 From: Canada Member No.: 227 |
All this is missing the point - the ultimate goal of the satellite imaging is to create global high resolution photomosaics of each moon. Regardless of the issues in any one flyby sequence, eventually we need to combine numerous images to make a global mosaic - such as Steve Albers has been doing - and then it can be projected onto a sphere for display from any orientation. Most Earth, Moon or Mars globes don't include relief (of course I know it wouldn't show up as much as on Mimas). There might be a chance later on for a high resolution global DEM to be used to create a more realistic global visualization complete with limb photography. But that's a long way off.
We cartographers don't like relief anyway - it distorts things. I won't be happy until all the craters have been filled in and all the mountains bulldozed, so Mimas is like a billiard ball. And it will be easier to map, too. Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PD: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Jan 9 2008, 10:00 AM
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#7
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Member Group: Members Posts: 655 Joined: 22-January 06 Member No.: 655 |
Phil, do I sense some exasperation relating to that pesky potato-shaped rock Eros you're working on??
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Jan 9 2008, 01:46 PM
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#8
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Solar System Cartographer Group: Members Posts: 10190 Joined: 5-April 05 From: Canada Member No.: 227 |
Blast all potatoes!!!
Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PD: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Jan 9 2008, 09:45 PM
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#9
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Interplanetary Dumpster Diver Group: Admin Posts: 4404 Joined: 17-February 04 From: Powell, TN Member No.: 33 |
We cartographers don't like relief anyway - it distorts things. I won't be happy until all the craters have been filled in and all the mountains bulldozed, so Mimas is like a billiard ball. And it will be easier to map, too. Phil Yes, but considering it doesn't have much in the way of albedo features, once you bulldoze it, who will want a map of the place? -------------------- |
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Jan 9 2008, 11:26 PM
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#10
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Founder Group: Chairman Posts: 14433 Joined: 8-February 04 Member No.: 1 |
Bulldozing contractor to write up the invoice
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Jan 12 2008, 04:54 AM
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#11
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Solar System Cartographer Group: Members Posts: 10190 Joined: 5-April 05 From: Canada Member No.: 227 |
Ted said "Yes, but considering it doesn't have much in the way of albedo features, once you bulldoze it, who will want a map of the place?"
That's a problem for the marketing department. I just make the maps. And no map could be more precise than a blank map of a featureless object. Besides, think of the money I'll save on printer ink. Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PD: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Jan 12 2008, 10:35 AM
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#12
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Senior Member Group: Members Posts: 1870 Joined: 20-February 05 Member No.: 174 |
"Blast all potatoes!!!"
Oh, Stop spuddering. |
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Jan 14 2008, 10:15 AM
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#13
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Member Group: Members Posts: 715 Joined: 3-January 08 Member No.: 3995 |
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