Difference at 4 decimals of geolocation files

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hanzhi.lin
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Difference at 4 decimals of geolocation files

by hanzhi.lin » Fri Dec 30, 2022 6:46 pm America/New_York

Hi ,

I found the geolocation files from OB and from LAADs that have the same timestamp, usually have slight difference since their 4 decimals. It seems this difference is within the allowed accuracy of 0.1 pixel, after I read the ATBD of geo-location.

But I'm not sure I understand it correctly, and how large the impact of this difference from two source of MODIS geolocation files will be for other MODIS products.

Wish you all a productive new year!

Hanzhi

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Re: Difference at 4 decimals of geolocation files

by LAADS_UserServices_M » Wed Jan 11, 2023 8:02 am America/New_York

Hi Hanzhi:
Sorry for the delay in response. Can you please explain your question "... impact of this difference from two source of MODIS geolocation files will be for other MODIS products.", which other MODIS products, from LAADS or from OB DAAC?
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hanzhi.lin
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Re: Difference at 4 decimals of geolocation files

by hanzhi.lin » Wed Jan 11, 2023 8:18 am America/New_York

Hi LAADS folks,

Thank you for replying my post. Sorry for the confusion. I mean other MODIS products in OB like L1A whose locations rely on these geolocation files.

For example, if I use geofiles from OB to select region of my interest in L1A from OB, can I get the same ranges of scan numbers and frame number when I use geofiles and L1As from LAADS?

Well, as in this case I suppose geofiles from LAADs be the reference for the comparison, maybe my dear folks from OB could have an answer to me.

Cheers,
Hanzhi

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Re: Difference at 4 decimals of geolocation files

by OB.DAAC - SeanBailey » Thu Jan 12, 2023 6:34 am America/New_York

The geolocation product produced by the OBPG is used to support processing through the l2gen program. l2gen performs an internal terrain correction, so we disable terrain correction in the generation of the geolocation files. This is the reason for the differences you are seeing. For the ocean at mean sea level this difference is on the order of 10m ...probably should be exact at zero elevation, but the Earth isn't a perfect sphere ;) If you are using the data for land processing (thus NOT using l2gen), you would want the geolocation files that LAADS distributes.

Regards,
Sean

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