analyzing seasonal changes with functions | arcgis resources
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Analyzing Seasonal Changes with Functions | ArcGIS Resources
http://blogs.esri.com/esri/arcgis/2013/09/06/analyzing-seasonal-changes-with-functions/[9/26/2013 6:18:57 PM]
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by kevin_butler on September 6, 2013
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Often we difference two images to see where change has occurred, and usually that’s a pretty decentapproach. I came across this paper recently that had a simple, but effective approach for measuring thechange in vegetation over a season. By differencing a late summer and spring image, converting that toz-scores, and then differencing the first year’s z-scores from the second, the researchers were able toanalyze seasonal changes. They were working on Landsat data in the southwestern U.S. For the sakeof being different, I’m going to show you how to do this using the MODIS EVI product for central China. Irecommend renaming the rasters from the MODIS naming convention into something meaningful for you.To start, I have 4 raster datasets, one for the fall and spring of 2000 and 2013. The equation forcalculating z-scores for each year is:
(Fall2013 – Spring 2013) – µ(Fall2013 – Spring2013)σ(Fall2013 – Spring2013)
Because everything is parentheses is the same, another way to think of this is:
A – µAσA
Analyzing Seasonal Changes with Functions | ArcGIS Resources
http://blogs.esri.com/esri/arcgis/2013/09/06/analyzing-seasonal-changes-with-functions/[9/26/2013 6:18:57 PM]
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Step 1: Year 2 minus Year 1
I can do all of the workflow in the Image Analysis Window, using functions. When working with theFunction Editor, you have to work from the top, and move down, and the first step you implement is thelast step in your analysis, so in this case it will be a minus operation because I will be subtracting theyear 2000 z-scores from the 2013. Start with one of the rasters (it doesn’t matter which one, but keeptrack of which one it is) and right click to insert an arithmetic function. The operation should be minus. Ilike to keep my workflows clean, so I tend to rename every step. I called this y2-y1. This will be yourmain branch. From here on out, every operation you implement in the first branch will be mirrored in theother. The only difference will be that the functions in the second branch will run on different datasets.
Analyzing Seasonal Changes with Functions | ArcGIS Resources
http://blogs.esri.com/esri/arcgis/2013/09/06/analyzing-seasonal-changes-with-functions/[9/26/2013 6:18:57 PM]
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Step 2: Numerator/Denominator
The second operation is a division, this will take care of dividing the denominator from the numerator.Right click on the raster and insert an arithmetic function, and choose division as the operator.
Step 3: A – µA
Next, insert another arithmetic function—this time, minus—on the raster. Finally, right click on the rasterand insert another arithmetic function with a minus operator. This time, change Input Rasters to be thefall image in your later year (Input Raster 2) and the spring image in your later year (Input Raster 1).Your function chain should look like this:
Right-click on the raster that is outside of the Arithmetic Function operation you just implemented andadd a Statistics Function, with mean as the operator. The cell size should be 4800 x 4800 becausethat’s how many pixels are in this tile, and we just need the mean value of all the pixels. Then, right clickon the raster under the statistics function and add an Arithmetic Funciton, with minus as the operator.
Analyzing Seasonal Changes with Functions | ArcGIS Resources
http://blogs.esri.com/esri/arcgis/2013/09/06/analyzing-seasonal-changes-with-functions/[9/26/2013 6:18:57 PM]
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Make sure the Input Rasters are set up so that the Spring is subtracted from the Fall.
Step 4: σA
To calculate the standard deviation, repeat the same steps from the previous paragraph, but choosestandard deviation instead of mean as your operator. It should look like this:
Step 5: Year 1
So, you’ve set up the function chain for the second year. To subtract year 1 from this, simply repeat theprocess outlined above on the remaining raster that doesn’t have any functions applied to it.
Click OK to process your imagery, and the results should render quickly. When you’re done, right clickon the .afr file (just below the Function Chain header) and select export as template and then save it.This will save your function chain so that you can apply it other layers. The paper that I referenced wentinto much greater depths in terms of its analysis—and they did some things differently in terms ofdifferencing—but hopefully this gives you an idea of how to work with arithmetic functions.
Analyzing Seasonal Changes with Functions | ArcGIS Resources
http://blogs.esri.com/esri/arcgis/2013/09/06/analyzing-seasonal-changes-with-functions/[9/26/2013 6:18:57 PM]
This entry was posted in Imagery and tagged evi, function chain, function editor, Imagery, MODIS, Raster. Bookmark the permalink.
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Analyzing Seasonal Changes with Functions | ArcGIS Resources
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Analyzing Seasonal Changes with Functions | ArcGIS Resources
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