[GIS] How to calculate a standard deviation across multiple raster layers

qgisraster-calculator

I have 15 raster layers. Each one shows accumulated precipitation (rainfalls) for the month of April for the same US state during a given year. So, I cover the period from 1997 to 2012. I would like to have an idea of the variability of precipitation during the last 15 years.

I would like for example to calculate the standard deviation at each pixel across the 15 layers. I am guessing I can do it through the raster calculator using sums, subtractions and divisions. But the final formula would pretty long.
Is there a more efficient way to achieve my goal? (So far, I am only a QGis user and I don't know Python…but I can start learning if necessary).

Best Answer

You could use the GRASS module r.series with method=stddev. For example, if you have rasters named apr_precip_97, apr_precip_98, and so on then in a GRASS terminal you would run:

r.series in=`g.mlist rast pat=apr_precip* sep=,` out=apr_precip_stddev method=stddev

Related Solutions

[GIS] Creating Series of Rasters that displays Standard Deviation from Mean using ArcGIS Spatial Analyst

For each cell in your 100 rasters you know:

Current cell value (total rainfall per year?)
Mean rainfall per year at that cell

And you also know:

Standard deviation based on 100 years (population)

I would think the simplest approach would be to use the raster calculator in conjunction with a ModelBuilder iterator. You can write a short equation to place your current raster cell value about the mean and then assign the cell the appropriate value for standard deviation.

[GIS] Iterate through rasters in subfolders using Model Builder

When faced with a problem like this I use os.walk() which returns all files and then see if the files match what I want by extension, here's an example for shapefiles:

import sys, os, arcpy
InFolder = sys.argv[1]

for (path, dirs, files) in os.walk(InFolder):
    for ThisFile in files:
        fName,fExt = os.path.splitext(ThisFile)
        if fExt.upper() == ".SHP":
            fc = path + "\\" + ThisFile

Change fExt.upper() == ".SHP": to fExt.upper() == ".ASC": and then operate as normal with fc being the full path to the dataset.

The other option is to put all the rasters in manually... or semi manually:

Open a command prompt in your base folder and type:

DIR *.ASC /B/S

Which gives a list of all the ASCII files in all subfolders... if there's too many to select then try piping to a text file:

DIR *.ASC /B/S > ASC_File_List.txt
START NOTEPAD ASC_File_List.txt

Then select all, copy, and paste into the tool.

Best Answer

Related Solutions

[GIS] Creating Series of Rasters that displays Standard Deviation from Mean using ArcGIS Spatial Analyst

[GIS] Iterate through rasters in subfolders using Model Builder

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