grids-graticulesrasterstatistics
I have a raster image (.tif) that is multi bands (35 band), floating point data type that does not have attribute table, and a grid shapefile of 107 polygon created by the fishnet function in ArcGIS. Now, I need to overlay them to get the mean value of each polygon of that grid from underlying raster image values.
How can I do that using ArcGIS or ENVI?
I tried to use zonal statistics (tabulate) but nothing would be computed as there is no attribute table for the image!
I was struggling to do exactly the same thing, but for various reasons I'm committed to using QGIS. I tried using v.rast.stats using the GRASS plugin and also via the Sextante plugin. The latter approach failed, because it seems to attach the stats to a temporary vector layer which it then deletes. The GRASS plugin worked, but it doesn't deal with overlapping polygons.
After doing some digging around (in the source of the promising-sounding ZonalStats plugin), I found that QGIS actually has zonal statistics methods built into the API, and these also have Python bindings. So as long as you are only looking for count, sum and mean statistics for your polygon features, the Python Console (Plugins > Python Console) is currently the easiest way to attach the stats to the polygon attribute table.
Select your raster layer in the TOC, and type the following into the console (it grabs the source file name of your raster layer)
>>> rasterfile = qgis.utils.iface.mapCanvas().currentLayer().source()
Select your vector layer, and execute the following command in the console (it grabs the vector layer itself)
>>> vectorlayer = qgis.utils.iface.mapCanvas().currentLayer()
Execute the following three commands in the console (they pass the vector layer and raster file to QGIS' built-in zonal stats calculator)
>>> import qgis.analysis
>>> zonalstats = qgis.analysis.QgsZonalStatistics(vectorlayer,rasterfile)
>>> zonalstats.calculateStatistics(None)
The results will be appended as extra fields in the polygon layer.
Note that if you want to append a prefix to the new fields (rather than just having them as sum, mean and count), then you would replace the first command in step 3 with the following (assuming you wanted to use 'zonal_' as the prefix:
>>> zonalstats = qgis.analysis.QgsZonalStatistics(vectorlayer,rasterfile,"zonal_")
what I would recommend is to avoid zonal statistics when your zones are smaller than the pixel size, especially in your case where you have a regular grid. Instead, you should get the centroids of your polygons, then use the extract multiple value to point. There is an option for the interpolation.
Best Answer
You do not need vat's for your 35 band stack. It sounds like the problem is that your zonal raster does not have an attribute table. I would check what bit type the zonal raster is. If it floating point you cannot build a vat and will need to recreate it.
Once you have ascertained that the zonal raster is not float, you can build a vat for it using the ArcGIS Build Raster Attribute Table tool.
I am not sure if the zonal functions in ArcGIS will operate on a raster stack. If not you will need to bring in one raster at a time. When adding data you can double click on a raster and it will display the bands in the stack and you can then add a single raster from the stack.