# swath2grid

Recently I needed to convert swath data to gridded data. Most MODIS products come as gridded products which are properly geo-referenced / rectified. However, some low level products are provided as “swath” data which are the “raw” form when it comes to geo-referencing. Luckily most of these swath products do provide ground control point information to convert them from wobbly sensor output to a gridded geo-referenced image.

This procedure of converting from swath to gridded data is normally done with the MODIS Resampling Tool (MRT) software. Here, I provide a few lines of code which will do just the same using the community driven Geospatial Data Abstraction Library (GDAL). I would argue that the four lines of true code beat installing the MRT tool any day.

The code is a mashup of a stackexchange post and converts MODIS L1B data (or similar) to gridded data, requiring you to specify the file name and the requested scientific data subset (SDS). You can find the available SDS using the gdalinfo command, or using the product information sheet. The data is output as a geotiff.

#!/bin/bash
#
# swath to grid conversion for
# MOD04 data, but will work on other
# MODIS L1 / L2 data as well (I think)

# get the reprojection information, stuff it into
# a virtual file (.vrt)
gdal_translate -of VRT HDF4_EOS:EOS_SWATH:"$1":mod04:$2 modis.vrt

# delete the bad ground control points
sed '/X=\"-9.990000000000E+02\"/d' modis.vrt

# grab the filename without an extension
filename=$(basename "$1")
filename="${filename%.*}" # reproject the data using the cleaned up VRT file gdalwarp -overwrite -of GTIFF -tps modis.vrt$filename_\$2.tif