[X] My Assistant

[Bill Harris]

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

[Bill Harris]

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

[mcaplinger]

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.