Discrepancy in Line Projection offset value in PDS label for LRO NAC mosaic file

[ayushman]

I have downloaded an LRO NAC mosaic of Vikram landing site (November image, 5m resolution) image from the following link

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

I am trying to calculate and store the latitude and longitude coordinates for all the pixels in the mosaic into an array. 

As pointed in the 2nd point in the response below by Miss Wang, there appears to be a discrepancy in the LINE PROJECTION OFFSET value mentioned in the PDS label for the dataset.

https://geoweb.rsl.wustl.edu/community/index.php?/topic/2804-details-needed-for-lroc-nac-mosaic-img-file/&do=findComment&comment=4621"

"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"

 

 

Can you please let me know the correct LINE PROJECTION OFFSET value to be used for calculating Lat /Long values. 

 

Thanks 

Ayushman

[June Wang]

One more finding is if using the function of “y = (1 - L0 - Line) * Scale” in line 128 of ‘DSMAP.CAT’ file, y value will always be negative with current label value input, which doesn’t match with the real y value in the data. It looks to me this equation could be changed to “y = ( L0 – Line-1) * Scale”.

You can try LINE_PROJECTION_OFFSET = 4476. This value is just my guess, may not be correct.

I have contacted with the data provider. I will post the final solution here when I hear anything back from the data provider’s.

[ayushman]

Hi June, 

Hope you are fine. Thanks for the suggestions. Using

• y = ( L0 – Line-1) * Scale
• Central Latitude = -70.8 degree 
• Equation 20-15 for longitude calculation
• LINE_PROJECTION_OFFSET = 4476
• Also, ignored the no data region for calculation (that is took  sample = 1 for first data not in NODATA region) for each line.  Moreover line = 1 is the line corresponding to the first line which has a pixel that is not in NODATA region.

This has given good results for the latitude (obtained Lat range  -71.4527 to -70.0472)  but they still do not match exactly with expected latitude range of ( -70.06291 to -71. 47199). Longitude range obtained is 21.2084 to 25.46 (Against 21.38 to 26.011) .

Maybe, once you get proper line projection offset value from the data provider, the result would match exactly. 

Ayushman

[June Wang]

No-data area will not change the line and sample value of each pixel in the data. So the data will still start from (1,1) for the top-left corner. All you need is just ignore those no-data pixels in your calculation without changing the line and sample values.

[ayushman]

Thanks June. I made changes as suggested and now am getting results for both lat and long max/min accurate  upto 2-3 places after decimal. 

Is there a chance of further improvement in accuracy (say to 5-6 places after decimal) with updated line projection offset value provided by the data provider ?

[ayushman]

Hi June,

Have you received any update from the data provider regarding the line projection offset value ? 

Ayushman