MOC image data |
MOC image data |
Mar 29 2006, 04:43 PM
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#1
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Senior Member Group: Members Posts: 2998 Joined: 30-October 04 Member No.: 105 |
A bit OT, but this relates to MER matters.
I have some confusion on how to determine the Sun Azimuth from the MOC image data. This seems like it ought to be straightforward, but the "Explanation page" confounds me. The example: Image R1500822, map corrected. North Azimuth= 92.59 deg Sun Azimuth= 9.51 deg http://www.msss.com/moc_gallery/r10_r15/im...5/R1500822.html http://www.msss.com/moc_gallery/r10_r15/explain.html What should the compass direction of the sun be? --Bill -------------------- |
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Mar 29 2006, 05:01 PM
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#2
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Senior Member Group: Members Posts: 2511 Joined: 13-September 05 Member No.: 497 |
I have some confusion on how to determine the Sun Azimuth from the MOC image data. This seems like it ought to be straightforward, but the "Explanation page" confounds me. Seems pretty clear to me. "this is the angle in degrees clockwise from a line drawn from the center to the right edge of the image to the direction of the sun at the time the image was acquired." QUOTE Image R1500822, map corrected. North Azimuth= 92.59 deg Sun Azimuth= 9.51 deg North is approximately up. The sun is coming from slightly above right. Of course, the sun is very high (incidence angle is ~25 degrees) so it's hard to see where the sun is coming from just by looking at the image. -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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Mar 29 2006, 09:34 PM
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#3
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Senior Member Group: Members Posts: 2998 Joined: 30-October 04 Member No.: 105 |
OK:
"the angle in degrees clockwise from a line drawn from the center to the right edge of the image to the direction of the sun" "The sun is coming from slightly above right." Why not _slightly below right_ (ie, E 9.5deg S) since the angle is clockwise? Another example: R2200640, the next image over. North Azimuth= 92.96 deg Sun Azimuth= 41.85 deg Incidence Angle 45.18* Based on the explanation page, the Sun ought to be 42* below right, or at a bearing of E 41.85 deg S. Looking at the image, it is pretty clear that the Sun direction is to the Northwest and _not_ the Southeast. In other words, 180* out. The Local Solar Time is 14.5h, and it makes sense that the Sun will be in the NW quadrant. It seems to me that "a line drawn from the center to the right edge of the image" has it's origin in the center and it's endpoint at the right edge. Any angle clockwise from this line will rotate about the origin. I may be missing something, but I can't make the observed direction match the explained direction. Not to be argumentative, but this has puzzled me for a long while. --Bill -------------------- |
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Mar 30 2006, 12:19 AM
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#4
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Senior Member Group: Members Posts: 2511 Joined: 13-September 05 Member No.: 497 |
Why not _slightly below right_ (ie, E 9.5deg S) since the angle is clockwise? ... Not to be argumentative, but this has puzzled me for a long while. I'm sorry, you're right -- and it is confusing I think it's because the angles are relative to the original image's coordinate system, not the map-projected image's coordinate system. North is obviously straight up in the latter. Also, if you look at the full ancillary data table, there's this flag that isn't reflected on the website: OBJECT = COLUMN NAME = USAGE_NOTE COLUMN_NUMBER = 40 DATA_TYPE = CHARACTER START_BYTE = 346 BYTES = 1 FORMAT = "A1" DESCRIPTION = "Because of the MOC's line-scan nature, depending on the orientation and sense of rotation of the spacecraft and the optical inversion properties of the camera system involved, MOC images can either be normal or flipped left-for-right (independent of whether the image is north up or south up.) This flag will be 'F' if the image should be flipped left-to-right, and 'N' if it is already in normal form." END_OBJECT = COLUMN OBJECT = COLUMN NAME = NORTH_AZIMUTH COLUMN_NUMBER = 41 DATA_TYPE = ASCII_REAL START_BYTE = 349 BYTES = 6 FORMAT = "F6.2" NOT_APPLICABLE_CONSTANT = 0.0 DESCRIPTION = "The angle in degrees clockwise from the reference axis of the image (a line from the center to the right edge of the image) to the direction to the north pole of the target body. If the USAGE_NOTE described previously is 'F', the image should be flipped prior to applying this angle." END_OBJECT = COLUMN After 1/1/1998 they started flying MGS "backwards"; prior to that there would be another flip required. I'm pretty sure that all mapping NAs are "N" though. -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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Mar 30 2006, 01:43 PM
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#5
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Senior Member Group: Members Posts: 2998 Joined: 30-October 04 Member No.: 105 |
Good, I understand it better now. It will be helpful in interpreting these images to know sun Sun direction since a ripple can become a trough in an eyeblink.
Thanks... --Bill -------------------- |
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