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I am a masters student, and trying to calculate the volume change for a glacier. I have 4 different DEMs, none of them are of the bedrock topography below the glacier. I have read that you can simply find the volume change by subtracting DEMs (older-newer) and then multiply by cell size to get the volume change. But the numbers I am getting does not give much sense to me at this moment, it basically tells me that the glacier is gaining volume, rather than losing it.

Anyone have any experience doing this type of analysis?

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    Welcome to GIS SE. As a new user, please take the Tour. Please Edit your question to specify the exact GIS software you are using and the exact commands you have issued. Detailing the units of the various data sets might be useful. – Vince Oct 31 '17 at 2:06
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    Also, please indicate what units you area using and the pixel resolution of the input DEMs. Consider using Cut Fill geoprocessing tool which provides you with a raster of gain, loss and unchanged areas (Assuming you are using ArcGIS and have access to the Spatial Analyst or 3D Analyst Extensions) – Jakub Sisak GeoGraphics Oct 31 '17 at 20:27
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The general process you've outlined is correct, but the order doesn't appear to be. You should subtract the newer DEM from the previous DEM starting from the oldest set.

If the volume is larger over time, it is possible that the images are not taken at the minimum snow depth for the year. If a glacier is thought to be receding, but the newest DEM was captured in mid winter, while the older DEM was captured in mid summer it would be entirely possible for the volume of the glacier to appear greater.

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