raster information/Fragstats

[ShannonChapin]

Hi all,

I am trying to extract a portion of a statewide grid for analysis in Fragstats.  My original grid (5 m resolution) has an uncompressed size of 7.54 GB, which is too large to be processed (Fragstats error message: Cannot allocate memory).  I've "extracted by mask" to a few different boundaries, but the smaller grids still hold on to the original grid's information, including the uncompressed size, and therefore still produce a Fragstats error.   I have been able to run Fragstats using the same grid, resampled to 30 m resolution (214 MB), but am not happy with losing some of the detail the 5 m grid provides.  Has anyone run into this before?  I'm new to Fragstats and I might be missing something obvious, but it just seems odd that the extracted grids "raster information" section is not getting updated.  I appreciate your consideration - thanks in advance!

Cheers

[JeffreyEvans · Accepted solution]

Fragstats looks at the array size based on the number of rows and columns. My guess is that even though you are extracting a specific region based on a mask that you are not redefining the extent of your raster. This in effect turns all the data outside of your mask to NoData but does not change the size of the actual array. To Fragstats the new raster is no different in size then the original one.  

Try this as an experiment. Instead of using the extract by mask tool, in ArcMap add your state-level raster and zoom into an area that is similar in extent to your mask. Go to Geoprocessing > Environments and under Processing Extent change the Extent to Same as Display. Then open ArcToolbox Spatial Analyst Tools > Map Algebra > Raster Calculator. Specify an output raster and just double click on your current raster in the upper left and hit OK. This will create a subset of your original raster that is consistent with your display window. This will change the dimensions of the raster as well. Keep in mind that a 5m raster is quite high resolution and Fragstats may still run into processing limitations on what you perceive to be a much smaller raster. I believe that in Fragstats help there are guidelines on row/col, N class limitations of a raster. The software is not 64-bit so you are stuck with 32-bit addressing space. Ignore the uncompressed size of the raster, in this regard it is meaningless.

[ShannonChapin]

You're right on both accounts - I did not redefine my extent, and even when I did (through the process you described) my raster was too big for Fragstats to process.  Thanks for troubleshooting that so quickly for me.  I plan on using your method instead of 'Extracting by Mask' to subset, and will just have to work with smaller sections at one time.  Thanks again!

[JeffreyEvans]

Depending on the goal of your analysis I may have a solution that does not involve subsetting your raster. Contact me so I can get some additional details.
Email: jeffrey_evans@tnc.org