OPenDAP netCDF retrieval increases filesize considerably

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timothy.w.hilton
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OPenDAP netCDF retrieval increases filesize considerably

by timothy.w.hilton » Wed Dec 13, 2023 6:37 pm America/New_York

Hello,

I'm using OPenDAP to retrieve netCDF files for a subset of layers in MOD13Q1.061. I'm noticing that my netCDF files containing 3 layers are larger than the original HDF files containing 11 layers. I suspect this is because hyrax is converting HDF short types to netCDF float types. Is this expected, and is there any way around it?

For example:

This hdf file is 21MB:
https://e4ftl01.cr.usgs.gov/MOLT/MOD13Q1.061/2013.06.26/MOD13Q1.A2013177.h29v13.061.2021233101659.hdf

This OPenDAP-retrieved netCDF file is 173MB, despite containing only 3 of the 11 variables:

https://opendap.cr.usgs.gov/opendap/hyrax/MOD13Q1.061/h30v12.ncml.dap.nc4?dap4.ce=/Latitude%5B0:1:4799%5D%5B0:1:4799%5D;/Longitude%5B0:1:4799%5D%5B0:1:4799%5D;/YDim%5B0:1:4799%5D;/XDim%5B0:1:4799%5D;/_250m_16_days_VI_Quality%5B307:307%5D%5B0:1:4799%5D%5B0:1:4799%5D;/_250m_16_days_NIR_reflectance%5B307:307%5D%5B0:1:4799%5D%5B0:1:4799%5D;/_250m_16_days_EVI%5B307:307%5D%5B0:1:4799%5D%5B0:1:4799%5D;/time%5B307:307%5D

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timothy.w.hilton
Posts: 6
Joined: Sun Dec 10, 2023 9:59 pm America/New_York
Answers: 0

Re: OPenDAP netCDF retrieval increases filesize considerably

by timothy.w.hilton » Tue Dec 19, 2023 4:16 pm America/New_York

OK, on further consideration this makes more sense to me. I did not notice that the HDF file does not include the longitude/latitude coordinate values. The netCDF file includes lon and lat as 4800x4800 64-bit floating point netCDF variables, which comes out to 175 MB each just for lon and lat. If I assume that the netCDF library transparently applies some compression then the 173 MB netCDF file seems reasonable.

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