Strange coords in metadata in VIIRS collections

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amacu
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Strange coords in metadata in VIIRS collections

by amacu » Fri Nov 24, 2023 1:38 am America/New_York

Hello!

I'm using a VJ109GA collection and note a strange thing. So, all data is provided with metada wich contains GRingLatitude/GRingLongitude with corners of image. Also there are corners specified as SouthBoundingCoord, NorthBoundingCoord and etc. The problem is that these sets of data are controvercial.

For instance, I consider this image: https://search.earthdata.nasa.gov/search/granules/granule-details?p=C2631841556-LPCLOUD&pg[0][v]=f&pg[0][id]=VNP09GA.A2023252.h20v01.002.2023253094431&pg[0][gsk]=-start_date&g=G2764169932-LPCLOUD&q=VNP09GA&tl=1686305417!3!!&lat=63.632084774238166&zoom=0

From metadata I've retrieved:
'GRingLatitude': '67.538726 79.533024 39.828005 68.383492 ',
'GRingLongitude': '44.055925 63.987494 -128.85675 95.657844 '

And also from metadata I've got
SouthBoundingCoord: [69.99999999]
NorthBoundingCoord: [79.99999999]
EastBoundingCoord: [172.81110410992767]
WestBoundingCoord: [58.47608798]

Also there are some granule information of earthdata (by the link above) and coordinates there are simillar with GRings. The thing is that GRing are wrong coordinates and dont match images georeferenced with official script: https://git.earthdata.nasa.gov/projects/LPDUR/repos/nasa-viirs/browse/scripts/VIIRS_HDF5toGeoTIFF.py.

Is that a mistake in metadata or there is some explanation of these effect?

Thank you.

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LP DAAC - lien
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Re: Strange coords in metadata in VIIRS collections

by LP DAAC - lien » Fri Nov 24, 2023 1:07 pm America/New_York

Hi,
The Gring coordinates are not wrong, they are exactly what they are supposed to be. Please see the full list of MODIS/VIIRS tile coordinates here: https://modis-land.gsfc.nasa.gov/pdf/sn_gring_10deg.txt
Gring (G polygon) coordinates are not the same as the actual tiles coordinates and are not designed to be. The two sets of coordinates you compare are not the same thing. The center of pixel coordinate of the upper left pixel (0,0) is:

vert tile 1 horiz tile 20 line 0.00 samp 0.00 => lat 79.997917 long 115.163659

The lower left:
vert tile 1 horiz tile 20 line 2399.00 samp 2399.00 => lat 70.002083 long 87.716804

The lower left:
vert tile 1 horiz tile 20 line 2399.00 samp 0.00 => lat 70.002083 long 58.488022

The upper right:
vert tile 1 horiz tile 20 line 0.00 samp 2399.00 => lat 79.997917 long 172.715501

These are from the MODLAND Tile calculator and are the center of the pixel: https://landweb.modaps.eosdis.nasa.gov/cgi-bin/developer/tilemap.cgi

In its native format and projection MODIS/VIIRS coordinates are the upper left of the pixel.
Please let us know if you have any questions.
Thanks,
Brett

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