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Details needed for LROC NAC mosaic .IMG file


ayushman
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Hello all, 

I have downloaded an LRO NAC mosaic of the Vikram Landing site(November image) from the following link -

http://wms.lroc.asu.edu/lroc/view_rdr_product/NAC_ROI_VIKRAM__LOC_P708S0237_5M

I am trying to read the image(.IMG file) on MATLAB and need the following information for subsequent analysis- 

  • I couldn't find any label file for the .IMG file and was facing difficulty in reading the data. If it exists, can someone guide me to the link.
  • Is there any way in which I can find out the actual latitude and longitude coordinate of each pixel in the mosaic. The link above for the site only tells the Lat/Long coordinates for the extremes. Also, does the westernmost longitude correspond to the left side of the mosaic and the maximum latitude correspond to the top of mosaic ?

Thanks 

Ayushman

 

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Hi Ayushman,

This LROC IMG data has an attached label. You can view the label content with UltraEdit or a Nodepad tool. From the link you gave above, you can also download the Geotiff image from Extra Downloads box. When adding this Geotiff in Arcmap and moving you mouse on the image, you can see the coordinates of each pixel. Because this product is in Polar Stereographic projection, the default coordinates display is in meters. You can change the map setting to view the coordinates in degrees. Since this data including NoData, the red part as shown below, the westernmost longitude is not  the left side of the mosaic and the maximum latitude doesn't correspond to the top of mosaic.

Please let me know if you need more help. Thanks,

image.thumb.png.68311c9de3d431b9806b81b566e743af.png

June

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Hi Ayushman,

To get the lat&lon for each pixel of the LROC map projected RDRs, please read the ‘DSMAP.CAT’ at https://pds.lroc.asu.edu/data/LRO-L-LROC-5-RDR-V1.0/LROLRC_2001/CATALOG/, which gives the equations and steps on how to convert image array coordinates (Sample, Line) to map coordinates (x, y), and then from map coordinates (x, y) to planetocentric coordinates (Lat, Lon) for different kind of projections. All the parameters needed for the calculation could be find in the attached label of this product. Please let me know if you need more information. Thanks,

June

 

 

 

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Thanks June for your reply.

I downloaded the DSMAP.CAT file and also was able to access the label of the file. However, I still am not getting proper Latitude values and need certain clarifications with regard to the equations mentioned for obtaining Lat,Long coordinates.

  1. In polar stereo graphic projection section it is mentioned that
    • P = SQRT(x^2 + y^2) ; 
    • C = 2 * ARCTAN(P / 2 * Rp)
    • Just wanted to confirm if C = 2* ARCTAN[(P*Rp)/2] and not C = 2* ARCTAN[P/(2*Rp)]
  2. Secondly, it is mentioned that 

   where LonP is the central longitude, LatP is the latitude of true scale and is always 90 or -90. However in the same section we have 

 LatP  defined to be equal to the CENTER_LATITUDE in the PDS Label. When I looked up the label, I found out that the CENTER_LATITUDE mentioned is not -90 but -70.8. 

Which of the 2 definitions should one take for calculation ??

 

Ayushman

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Hi Ayushman,

Question 1-> It looks to me C = 2* ARCTAN[P/(2*Rp)] is correct, when compared this with the equation (21-15) in book page 159 of the book “Snyder, J. P. (1987). Map Projections: A Working Manual. U.S. Geological Survey Professional Paper 1395. Washington, DC: United States Government Printing Office.” Please let me know if this is not correct to your test.

Question 2-> Please use the value in the attached label. The sentence written in the DSMAP.CAT is more like a general case.

Thanks,

June

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Hi June,

Thanks a lot for resolving the queries. 

However, on going through the chapter on stereo graphic projections in the reference book   “Snyder, J. P. (1987). Map Projections: A Working Manual",  I have some more queries related to the value of Lat P and the expression to be used for Latitude/Longitude calculation.

For LatP not equal to + or - 90 degrees , longitude is to be calculated as given  in equation number => 20-15.

image.png.e507af3dfb561baa208b9866ac105389.png

  1. However, an equation similar to the above equation can't even be found in the DSMAP.CAT file.
  2. Secondly equation 20-17 which is found in the DSMAP.CAT file to calculate the longitude can only be used for the case where LatP is -90 degrees
  •  image.png.d3a7e738f9ded198e478e1b39196da1a.pngimage.png.ecd2de1d7bac9c97a2117d04cd2831e0.png

Does the above suggest that for Polar Stereographic projection in moon's southern hemisphere we always have to take LatP = -90 degrees which was mentioned in the DSMAP.CAT file earlier and not the one to be used in the label file.

I have tried with different combinations of Lat P and Lat/ Long expressions. The results obtained are attached in the excel file. I have not been able to replicate the results for Lat/Long ranges which are mentioned in the label file.Results are calculated using C = 2* ARCTAN[P/(2*Rp)] as pointed by you in the previous post.

  • (Case 4) LatP = -90 degrees gives the best result. However the Latitude ranges are quite different from expected while Longitude range is much closer to the expected values but still different.
  • Surprisingly (Case 2, LatP = -90 degrees)  also gives decent result even though the expression used for calculating longitude is one which is reserved for case of LatP not equal to + or - 90 degrees.
  • In any case LatP = -70.8 degrees (value mentioned in the label) doesn't give appropriate result.

Kindly clarify. Please let me know of any discrepancies that may exist in the equations for calculating the Latitude and Longitude. Let me know if you need any other details from my side to resolve the issues.

Thanks a lot for your patience.

Ayushman  

 

 

 

 

 

 

 

 

 

 

results_calculated_using_different_LatP_and_Longitude_expressions.xlsx

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Hi Ayushman,  

Attached is the screen capture of the LROC RDR (orange color, NAC_ROI_VIKRAM__LOC_P708S0237_5M.TIF) overlain on a WAC mosaic near the South Pole area together with its spatial reference information. The feature matches well. If everything is correct in the book, it looks to me equation 20-15 is a better fit for this case. Have you checked other parameters or steps in your calculation? If you can send me your code and your original value input for the calculation, I can take a further look. Thanks,

June

Capture_20201009.JPG

Capture_20201009_spatial.JPG

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Hi Ayushman,

Two things here. In your code, you looped around all the pixels. Please don’t include the nodata area in your calculation.

The LINE_PROJECTION_OFFSET value in the label doesn’t match with the value of CENTER_LATITUDE. Need to check with the data provider to see what the value used in their processing pipelines.

Thanks,

June

 

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