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Downloading DEM from ALOS gives you a Median and an Average version. Which one is to be used for which purpose? In my case, I want to produce an R-Index map using QGis (Slope & Aspect algorithm + some Raster Calculation).

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The choice between median and average depends on the acquisition process, and of course on the intended use.

In the case of ALOS DSM, I would suggest to use the median, and I will try to explain why.

First of all, median and average of which values? The ALOS DSM has been produced by image auto-correlation, with multiple image overlapping each other from different views, and calculating the 3D position for every matching point in each group of images. Thus the stereo/auto-correlation process produces multiple z values for each horizontal position. The average and median are taken among these multiple z values.

Sometime the auto-correlation process can fail, producing a completely wrong value for z (called outlier).

The presence of outliers is more strongly affecting the average than the median; then in this case the median is a more reliable value for the DSM.

For further info on this subject, please see: http://math.tutorvista.com/statistics/median-vs-average.html

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