I'm trying to analyze nighttime lights of a specific area with VNP46A1 data. I got my data from this website, but I could find only few tutorial about this product, and one of them was VIIRS_SR_Tutorial. When I listed VNP46A1 Metadata, I found my first challenge.
from h5py import File data_path = 'path-to-my-h5-file' file = File(data_path) fileMetadata = file['HDFEOS INFORMATION/StructMetadata.0'][()].split() fileMetadata = [m.decode('utf-8') for m in fileMetadata] print(fileMetadata[:11]) >>>'GROUP=SwathStructure', 'END_GROUP=SwathStructure', 'GROUP=GridStructure', 'GROUP=GRID_1', 'GridName="VNP_Grid_DNB"', 'XDim=2400', 'YDim=2400', 'UpperLeftPointMtrs=(20000000.000000,50000000.000000)', 'LowerRightMtrs=(30000000.000000,40000000.000000)', 'Projection=HE5_GCTP_GEO', 'GridOrigin=HE5_HDFE_GD_UL'
In this page, It says "VIIRS/NPP Daily Gridded Day Night Band 500m Linear Lat Lon Grid Night" but in Production Information section it says the data has 750m spatial resolution. However, when I checked this value by dividing sides by pixel counts I found this
(30000000.000000 - 20000000.000000) / 2400 >>> 4166.666666666667 (50000000.000000 - 40000000.000000) / 2400 >>> 4166.666666666667
what is the real spatial resolution of my data ?
Also another challenge for me is was projection types. In the tutorial, the data was clearly projected Sinusoidal coordinate system,that can be seen in metadata of VNP09GA
and the author defined it like this
prj = 'PROJCS["unnamed",\ GEOGCS["Unknown datum based upon the custom spheroid", \ DATUM["Not specified (based on custom spheroid)", \ SPHEROID["Custom spheroid",6371007.181,0]], \ PRIMEM["Greenwich",0],\ UNIT["degree",0.0174532925199433]],\ PROJECTION["Sinusoidal"], \ PARAMETER["longitude_of_center",0], \ PARAMETER["false_easting",0], \ PARAMETER["false_northing",0], \ UNIT["Meter",1]]'
But in my data, it says the projection of VNP46A1 is
In this Manual, section 8.3.1 (GCTP Projection Codes), I found out what HE5_GCTP_GEO stands for, which is Geographic. But still don't know how to define it.
If the the projection, used to define my satellite data, was Geographic, are my projection coordinates and latitude-longitude values are the same? In what coordinate system my corner points are defined?
I need to georeference my data but with all these unknowns I cannot do that