Why is there a flip in LROC Images?

[Shyam Mohan]

Consider an image downloaded  from the Lunar Orbital Data Explorer (http://ode.rsl.wustl.edu/moon/indexProductSearch.aspx).

If you filter the LROC-CDRNAC product by the below location then you will get the image with the product ID M1106080949LC.

Minimum Lattitude : 29 and Max Lattitude : 30.

Minimum Longitude : 334 and Max Longitude : 335.

However if you compare the above image with the LROC image in the  below link then you will realize that the images do not match and there is a flip along the vertical line.

http://quickmap.lroc.asu.edu/?layers=NrBsFYBoAYchGSJQBYF3gXSaUdZY4Ds%2BC2YAHKYWAJzXlGzNlJGIs0BMttVn5HrwaZBoLtSTAuoDhnIBmaF37ykKpSPLx4UAuhA60ApDpJyaS6HIKCK7Bqc0ng8BTdauFCx17wu3EgCFehcNCTD7CLVXFBZbdQpQUPlRJAUuaP0QDI8sNOAlENJoADooHOg4rXToBWN8cqkivVTFOp9OZuUU5nL250bwAY8y4dqMkv6ChXFJHNkprnHC0E6hxVQpldnWvuGZutV97tntgaCNmaIG7MKidb7ddq5bmFLn2uWpz8Llf0avyUXEuTwOiiIALuCiIe08MOO8KIvXhFHiqLyigoWVRj1Rb3y5Di6MQOWELmBeMsyhxhPSoFBBlWUPhoER3HsJM2KMsTF8MMx6SItNERNe82AKC4eLKCjFcNlYpZZVooqQKHAVKkGpF6vACum7XgPO60BZ1Kq-OgdSt1t84C4XMSa18KggrqSBLsuA9bJqwHAugwTMDXqQgcZNED7PIgZ5sa4o1KRATOLKKedkfayltcMJmCAA&extent=-28.872073427042995%2C26.705657147802196%2C-21.410826670490174%2C30.65888125804206&proj=16

I am assuming that the above link is giving the right image and the image from the Lunar Orbital Data Explorer is flipped. I would to like to know if this is true and if yes then why is there a flip in the image product from Lunar Orbital Data Explorer.

Thank You,

Shyam.

 

[June Wang]

Hi Shyam Mohan,

Both the LROC *.img data and browse images in Lunar ODE are the original data provided by the LROC team. There is nothing wrong with the data products you downloaded from ODE. It is the way how to interpret the image and the index tables. Please refer to the index.tab and index.lbl in the below link.
http://lroc.sese.asu.edu/data/LRO-L-LROC-3-CDR-V1.0/LROLRC_1013/INDEX/
In index.lbl, you will find below definitions for the columns 70 to 79 for the lat &lon coordinates of the center point and 4 corner points of the image. And you can find corresponding index records from index.tab for the data product M1106080949LC. If you fill those numbers for those 4 image corners, you will see the coordinates are correct and consistent with the LROC quickmap. 

OBJECT         = COLUMN
NAME           = CENTER_LATITUDE
COLUMN_NUMBER  = 70
DATA_TYPE      = ASCII_REAL
START_BYTE     = 798
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "The planetocentric latitude coordinate at the center of
                 the observation."
END_OBJECT     = COLUMN

OBJECT         = COLUMN
NAME           = CENTER_LONGITUDE
COLUMN_NUMBER  = 71
DATA_TYPE      = ASCII_REAL
START_BYTE     = 805
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "The planetocentric longitude coordinate at the center of
                 the observation."
END_OBJECT     = COLUMN

OBJECT         = COLUMN
NAME           = UPPER_RIGHT_LATITUDE
COLUMN_NUMBER  = 72
DATA_TYPE      = ASCII_REAL
START_BYTE     = 812
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "Upper right latitude."
END_OBJECT     = COLUMN

OBJECT         = COLUMN
NAME           = UPPER_RIGHT_LONGITUDE
COLUMN_NUMBER  = 73
DATA_TYPE      = ASCII_REAL
START_BYTE     = 819
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "Upper right longitude."
END_OBJECT     = COLUMN

OBJECT         = COLUMN
NAME           = LOWER_RIGHT_LATITUDE
COLUMN_NUMBER  = 74
DATA_TYPE      = ASCII_REAL
START_BYTE     = 826
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "Lower right latitude."
END_OBJECT     = COLUMN

OBJECT         = COLUMN
NAME           = LOWER_RIGHT_LONGITUDE
COLUMN_NUMBER  = 75
DATA_TYPE      = ASCII_REAL
START_BYTE     = 833
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "lower right longitude."
END_OBJECT     = COLUMN

OBJECT         = COLUMN
NAME           = LOWER_LEFT_LATITUDE
COLUMN_NUMBER  = 76
DATA_TYPE      = ASCII_REAL
START_BYTE     = 840
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "lower left latitude."
END_OBJECT     = COLUMN

OBJECT         = COLUMN
NAME           = LOWER_LEFT_LONGITUDE
COLUMN_NUMBER  = 77
DATA_TYPE      = ASCII_REAL
START_BYTE     = 847
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "lower left longitude."
END_OBJECT     = COLUMN

OBJECT         = COLUMN
NAME           = UPPER_LEFT_LATITUDE
COLUMN_NUMBER  = 78
DATA_TYPE      = ASCII_REAL
START_BYTE     = 854
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "Upper left latitude."
END_OBJECT     = COLUMN

OBJECT         = COLUMN
NAME           = UPPER_LEFT_LONGITUDE
COLUMN_NUMBER  = 79
DATA_TYPE      = ASCII_REAL
START_BYTE     = 861
BYTES          = 6
FORMAT         = "F6.2"
NOT_APPLICABLE_CONSTANT = 999.99
DESCRIPTION    = "Upper left longitude."
END_OBJECT     = COLUMN

The reality is the left and right of an image is not always the west and east of real direction as you noticed, and north of image will not always be the straight up direction. It totally depends on the flight direction of the satellite and the cameras onboard the satellite. That is why you will always need to look at the index table to find the corner coordinates of the LROC data to further interpret the images. Please let me know if you still have any questions about the LROC data set. Thanks,

June 

[Shyam Mohan]

Thank you for a quick response.

I understand that the left and right of an image is not always the west and east of real direction and it depends on the depends on the flight direction of the satellite.

I expect one image to be a180deg rotation of the other if the flight direction changes. For example if the flight direction is such that the camera moves from south to north then I might get a vertically long image with the up direction being the north,  down direction being the south, right direction being east and left direction being west. If the flight direction changes and the camera moves from north to south then the up direction of the vertically long image is south, down direction is north, right direction is west and left direction is east. Essentially if one image is flipped both left and right and up and down then I should get the other image.

However this was not observed in the images that I am referring to (cropped images are attached). If you compare the LROC quickmap with the product M1106080949LC then the up and down directions remain same without any flip. You will notice that there is a flip in the left and right directions only. This is strange unless the camera itself stores the image in a flipped manner along one direction only. Could you please help me in understanding this problem?

[Nick Estes]

Shyam,

Most of the time, whether it's a point and shoot, cellphone, etc, people are used to working with cameras that take the entire frame at once, but the NAC is a linescan camera which makes it a bit counter-intuitive at first.

 If we imagine LRO traveling from north to south, and pixel 1 is on the west side of LRO and pixel 5064 is on the east side, as LRO moves south, capturing 1 line at a time, we get an image with north at the top and sample 1 (left side of the image) is west.  If LRO performs it's yaw maneuver to keep the solar panel in the sun and is again traveling from north to south, now pixel 1 is on the east side and pixel 5064 is on the west side.  As LRO moves south, capturing 1 line at a time, the resulting image has north at the top due to the direction of travel and sample 1 is east due to the orientation, but sample 1 is still the left side of the image making it appear mirrored rather than rotated like a normal point-and-shoot style camera would be.  Line 1 is always the first line captured, whether moving from north to south or south to north, and sample 1 always maps to the first pixel on the CCD line array.

The EDR and CDR products are all in camera space, so it's the camera's point of view without any adjustment based on flight direction.  When projecting the image data using SPICE, the orientation kernels (CK) are aware of whether LRO is moving in the +X or -X direction relative to the spacecraft so the resulting map-projected products viewed with something like Quickmap or Lunaserv match the map coordinates (normally north up and west to the left, but can vary depending on the map projection and coordinate system used).  

The ISIS software package from the USGS is distributed with the appropriate LRO SPK and CK kernels and can take the EDR and CDR products and output a variety of map projected products.  Nearly all NACs are also available via WMS using our Lunaserv service, and can be used directly in GIS software such as QGIS, JMARS, or ArcMap without any downloading or processing steps required.  As June mentioned, the corner coordinates in the label can be helpful in interpreting the data in camera space, but keep in mind that those coordinates are calculated on a sphere.  The actual sample coordinates will vary based on terrain.  ISIS can take care of that as well.

I hope that explanation helps.

Nick Estes, LROC SOC Manager

[Shyam Mohan]

Thank you all. Your support is valuable.