Questions Concerning Venus

Bob Hegwood · Post #1by **Bob Hegwood** » 16.05.2004, 01:59

Brain-Dead Bob again...

Just curious to know if anyone knows where I can find a 2k or larger texture
map for the surface of Venus?

And, secondly... Why does Venus rotate itself upside down when you specify a
location via cel script and the gotolonglat command? None of the other planetary
objects I've dealt with do this, so I'm just curious to know what is different
about Venus. I know that it features a retrograde rotation, but this shouldn't
cause this strange behaviour. Any ideas?

Thanks, Bob

maxim · Post #2by **maxim** » 16.05.2004, 06:34

As I remember longitude and latitude values are inversed on retrograde rotators. So you have to change the sign(+/-) on your values too.

maxim

Post #3by **granthutchison** » 16.05.2004, 07:01

It's the retrograde rotation - the rotational north pole of Venus points south of the ecliptic. I'd guess your script command is rotating the viewpoint to place Celestia north at the top of the screen.

Grant

Bob Hegwood · Post #4by **Bob Hegwood** » 16.05.2004, 15:26

Maxim, Grant...

Thanks very much for the information. I didn't realize that the rotation would
make a difference, but I see now that it does. Interesting...

So Venus' North Pole is actually located where the South Pole should be? Just
because it rotates backwards? Curiouser and curiouser. :roll:

Thanks a lot, Bob

Post #5by **granthutchison** » 16.05.2004, 16:52

Bob Hegwood wrote:So Venus' North Pole is actually located where the South Pole should be? Just because it rotates backwards?

The problem is how you define "north". There are two systems in use:
1) If you hover above the pole and look down on a rotating body, the north pole is the pole around which it is seen to rotate in the same direction as the Earth does around its north pole - counterclockwise. (Hold your right hand with the fingers curled and the thumb extended - if your fingers are pointing in the direction the body rotates, your thumb is pointing towards its north pole.)
2) The north pole is the rotation pole that points north of the ecliptic plane.

Definitions 1 and 2 are the same for objects that rotate in the same direction as the Earth does, and opposite for retrograde rotators (like Venus).

Celestia uses definition 1; asteroid scientists seem to use 1 or 2 as the fancy takes them; the USGS, NASA and JPL use definition 2, which is why you'll find that most Venus maps on the web need to be rotated through 180 degrees to work in Celestia, and why the stated east longitude and north latitude of feature on Venus need to be multiplied by -1 to appear correctly in Celestia.

Why does Celestia use definition 1? Because it's independent of the solar system - it would be crazy to have the north pole of a planet around another star defined in terms of the plane of the Earth's orbit around the Sun. It's also completely unambiguous: I know of one asteroid with a rotation axis that lies (to the limit of accuracy with which it can currently be measured) exactly in the plane of the ecliptic - the USGS system can't handle that; it's also possible to come up with scenarios in which the rotation axis of an object precesses alternately above and below the ecliptic plane - the USGS method would require the maps to be flipped every time the axis orientation changed!

Grant

Bob Hegwood · Post #6by **Bob Hegwood** » 17.05.2004, 02:36

Thanks Grant...

I appreciate the explanation. It just didn't occur to me that the planet's
rotation would have an effect on which pole was which. I was thinking that
the North Pole would be the same one which is "UP" in relation to the plane
of the Earth's ecliptic. Now you explain it though, I can see why this wouldn't
make much sense outside of the Solar System, or even inside the Solar System
for objects like Uranus.

Another educational experience for Hegwood... :wink:

Take care, Bob

Bob Hegwood · Post #7by **Bob Hegwood** » 18.05.2004, 04:44

Maxim?

Did you forget about me? :wink:

Would still like the 4k Venus radar image texture if you have it.

Thanks, Bob

maxim · Post #8by **maxim** » 18.05.2004, 17:13

No, I haven't :wink:

.
Just had a heavy start of the week. Tomorrow I think it'll be more calm, and I'll get some time for uploading.

maxim

Bob Hegwood · Post #9by **Bob Hegwood** » 19.05.2004, 05:16

Okay, no hurry...

Thanks very much for your effort. I've had an interesting first part of the
week too. Whew! :lol:

Thanks also for your Germanese translations in the
Users Forum. :wink:

Take care, Bob

danielj · Post #10by **danielj** » 21.05.2004, 16:53

It reminds me that we don?t have any very high resolution map from Venus.We have 4k ones and a 16k,but it is not finished,and it is in black and white.Why do you skip 8k textures?Other thing : the 4k map of Venus I have have lines crossing the surface(the slices taken by Magellan).This is so unnatural.Is there any map without these artifacts?

Bob Hegwood wrote:Maxim?

Did you forget about me?
Would still like the 4k Venus radar image texture if you have it.

Thanks, Bob

Bob Hegwood · Post #11by **Bob Hegwood** » 21.05.2004, 17:35

danielj wrote:It reminds me that we don?t have any very high resolution map from Venus.We have 4k ones and a 16k,but it is not finished,and it is in black and white.Why do you skip 8k textures?Other thing : the 4k map of Venus I have have lines crossing the surface(the slices taken by Magellan).This is so unnatural.Is there any map without these artifacts?

Has it *occurred* to you that a 1k, 2k, 4k and 16k texture can all be made from
a single 16k image? All you have to do is reduce (re-sample) the larger textures
to get the smaller ones. By the way, some of those "lines" you're talking about are
actually features on the surface of Venus. If you don't believe me, go look at
some of the NASA images. The artifacts that *do* exist in the textures
supplied by Celestia users are *minimal* to say the least. Has it occurred to you that
the entire Venus texture is in itself "unnatural?" This isn't a good view of the planet's
surface, it's a radar-image. What do you want? Care to fund a mission to Venus
that will withstand the atmospheric pressure, sulpheric acid rain and take good
pictures of the surface for you?

Sheesh!

danielj · Post #12by **danielj** » 22.05.2004, 00:15

Forgive my ignorance,Bob.I was thinking that radar images was the same as optical images.I didn?t know radar image limitations

danielj · Post #13by **danielj** » 22.05.2004, 00:15

Forgive my ignorance,Bob.I was thinking that radar images was the same as optical images.I didn?t know radar image limitations

Evil Dr Ganymede · Post #14by **Evil Dr Ganymede** » 22.05.2004, 04:18

By the way, some of those "lines" you're talking about are
actually features on the surface of Venus. If you don't believe me, go look at some of the NASA images.

Er, I think the lines that Daniel is referring to are image artifacts - they're either the pale strips made by joining the individual orbital passes together (as seen here going from angled toward the bottom right) or they're the black strips of missing data as seen here

I think both sorts of strips have been removed by post-processing in individual images, but I don't know where you can get a global map that has them removed.

There's a colourised map (it's fake colour though) here but you can still see strips on it.

This isn't a good view of the planet's surface, it's a radar-image. What do you want? Care to fund a mission to Venus that will withstand the atmospheric pressure, sulpheric acid rain and take good pictures of the surface for you?

Snippy, aren't we? :roll:

It's a bloody good view of Venus' surface actually, even if it is a radar image - most of it is at about 75 metres per pixel resolution in the original Magellan images. It's one of the best datasets we have of any planet other than Earth (and possibly Mars now), I think.

If you want to translate radar into visual, the bright areas are either coarsely textured (probably covered in rocks) terrain and the dark areas are smooth terrain. Unless you're looking at bright mountain tops, in which case it's more likely that you're looking at reflective metallic frosts deposited there.

Venus has very nifty geology

.

Bob Hegwood · Post #15by **Bob Hegwood** » 22.05.2004, 05:21

Evil Dr Ganymede wrote:Snippy, aren't we?

No... We're just damned tired of nothing but negative comments from certain
of the Celestia users. If that offends you, then I'm sorry. It offends the HELL out of
me when all someone can do is gripe about this great - and free - resource.
Constructive criticism is one thing... A never-ending stream of crap about how the
program isn't worth a damn just pisses me off.

Sue me...

As for the lines which are being discussed, I have *never* seen either of the features
your post directed me to. The only Venus textures I've ever seen feature
*none* of those artifacts, thus my answer above.

Take care, Bob

Evil Dr Ganymede · Post #16by **Evil Dr Ganymede** » 22.05.2004, 05:57

No... We're just damned tired of nothing but negative comments from certain of the Celestia users. If that offends you, then I'm sorry. It offends the HELL out of me when all someone can do is gripe about this great - and free - resource.

First, you're damn tired of it. Don't claim to speak for everyone here.

Second, bite your metaphorical tongue and ignore those comments if you find them so offensive. If you're incapable of replying civilly to questions - no matter how stupid you may think they are - then it's much better if you don't reply to them.

Constructive criticism is one thing... A never-ending stream of crap about how the program isn't worth a damn just pisses me off.

I don't think Daniel's done anything like that at all. You're over-reacting, and you're hardly giving anyone the impression that people here are helpful. How would you like it if people replied to you when you ask your questions in the same way you've replied to Daniel here?

As for the lines which are being discussed, I have *never* seen either of the features your post directed me to.

Well, you have now. I'm pretty sure I've seen a global Venus radar map that has those stripe artifacts still on it too (I used to work on Magellan data).

The only Venus textures I've ever seen feature *none* of those artifacts, thus my answer above.

Then perhaps Daniel is using a texture that you haven't seen.

Bob Hegwood · Post #17by **Bob Hegwood** » 22.05.2004, 06:06

I did reply civilly... You didn't hear what I wanted to say. :lol:

Sorry... Mis-communication I guess. I'll keep my mouth shut if he will. How's that?

Love, Bob

Evil Dr Ganymede · Post #18by **Evil Dr Ganymede** » 22.05.2004, 06:26

Bob Hegwood wrote:I'll keep my mouth shut if he will. How's that?

How about you keep your mouth shut if he asks any more questions? Go amuse yourself on other threads that you enjoy instead, instead of getting pissed off at people that you think are stupid. Let other people with clearly more patience than you have answer them, and then everyone can enjoy Celestia and have a generally nicer experience on the forums.

Bob Hegwood · Post #19by **Bob Hegwood** » 22.05.2004, 06:33

I never said that he was stupid. I'm the one who's Brain-Dead, remember? :wink:

This was the name I got for myself by asking exactly the same sort of questions
that Daniel is asking. However, I didn't continually gripe about the lack of features,
slowness of releases, or the quality of the program.

Perhaps you're correct though... I'll just keep my mouth shut. Sorry, I didn't
realize that he had relatives encouraging him to criticize the Celestia package.

Good Night to you, Sir...