Simple calculation for multiple rasters

Question

Hello, I am working on a project that chronicles the snow covered area in the Clearwater Basin in Idaho.  I have hundreds of rasters which all have pixels that are classified as either snow or no snow.  I want to calculate the snow covered area for each raster, which is the number of pixels in the raster classified as snow divided by the total number of pixels in the raster.  Obviously it would be a huge pain to have to do this manually for each raster so I was hoping that there would be a way to write a program to do this.  Let me know if you have any suggestions.  Thanks very much!

StephenFricke · Answer

Thanks alot JSkinn, I really appreciate it!

jskinner · Answer

Hi Peter,

It appears that if the raster has any rotation, the columns and rows will include the 'NoDATA' areas which will give you more pixels when multiplying the columns and rows together using the Describe function.  You can sum the COUNT field using a Search Cursor and appending the values to another list.  The below should work:import arcpy
from arcpy import env
from arcpy.sa import *

arcpy.CheckOutExtension("Spatial")

env.workspace = r"C:	emp\pythonasters"
env.overwriteOutput = True

lstRasters = arcpy.ListRasters("*")
for raster in lstRasters:
    outTimes = Times(raster, 100)
    outTimes.save(r"C:	emp\python\outTimes.tif")
    intRas = Int(r"C:	emp\python\outTimes.tif")
    intRas.save(r"C:	emp\python\intRas.tif")
    arcpy.BuildRasterAttributeTable_management(r"C:	emp\python\intRas.tif", "Overwrite")
    list2 = []
    rows = arcpy.SearchCursor(r"C:	emp\python\intRas.tif")
    for row in rows:
        list2.append(row.getValue("COUNT"))
    totalpixels = sum(list2)

list = []
    pixValue = 3.25 * 100

rows = arcpy.SearchCursor(intRas, "VALUE > " + str(pixValue))
    for row in rows:
        list.append(row.Count)

print float(sum(list)) / float(totalpixels)

del row, rows
arcpy.Delete_management(r"C:	emp\python\intRas.tif")
arcpy.Delete_management(r"C:	emp\python\outTimes.tif")

StephenFricke · Answer

JSkinn-
Thank you so much!  That script is very helpful.  The only problem is I am not getting the total pixels correctly.  Each raster has an attribute table that looks like this....

Rowid VALUE COUNT
0        1        16456
1        2        50594

The values for no snow are 1 and snow is 2.  I am able to get the sum(list) of snow values correctly, but unable to calculate the total pixels correctly (which is simply the two values in the count field added together).  How would I make the "totalpixels" value equal to the sum of the COUNT field.  Thanks again!