I am running through a big challenge to calculate Standard Precipitation Index (SPI) using raster images of precipitation(ftp://ftp.cpc.ncep.noaa.gov/fews/fewsdata/africa/rfe2/geotiff). Is someone have any idea about GIS or remote sensing software or freely available tool or some code in IDL which could calculate SPI on rater images? Please help me to solve this problem Thanks
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