# What is the method for finding the percentage of common cells among 2 or more DTMs for the same area?

I have a study area and 6 different DTMs for that area from different sources and I would like to know the percentage of pixels with the same value. I suppose I will have to extract some statistic and make some calculations in Excel, but I don't know what to extract and how to do it. If somebody knows some methods or something similar please give some indications.

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One way: create an indicator grid showing where the local range is zero. You can then read the answer off the (two) records in the attribute table. I would like to suggest, though, that this might not be a very useful way to compare DTMs for most purposes: in principle, if they have been independently generated, they should differ among each other almost everywhere--but only by a little bit. Consider using local statistics to get better insight into how they differ. – whuber Jul 17 '12 at 20:07

One problem you will have is that the grids won't be lined up properly.

6 different grids with 6 different cell size, extent and numebr of cells, means that you not only have data differences, but also you have problems with the interpolation required to make them coincident.

But, once they're coincident, you could work our the standard deviation by using some map algebra, then do some summary statistics to work out how many have an SD > 0.

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They already have the same cell size, same projection, but the number of cell is slightly different. I will try to fix that:).Thank you – Frantiuc Alexandru Jul 19 '12 at 1:14
I also thought on making subtraction between DTMs, which in a way would be usefull, but the percentage ofcommon cells would be more precise for my paper, because I could make some graphs from the results. – Frantiuc Alexandru Jul 19 '12 at 1:23
I think using whuber's suggestion or finding a standard deviation by cell (or per area?) is best. Using subtraction only works conparing two items. You would need to compare all of them to all the others to use subtraction. Which is basically what a standard deviation is! Just use statistics. Work out the SD for each cell, then work out the mean and SD of SDs. This gives you nice comparison numbers. Alternately, you can hold one fixed, and subtract all the others from that. I think the above is better. – Alex Leith Jul 25 '12 at 4:40