I am using ArcGIS 9.3 and python 2.5. My code is shown at the bottom of this post.

I have a raster containing fire intensity information at 200m cell resolution. I run the script and the first count cell part returns the size of the fire. This works fine.

I then clip the raster using a polygon representing a forested catchment. The new raster represents the area of the catchment that is burnt. When I try to use the same code to count the number of cells of the new raster, it always gives me an answer of zero, even if there is definitely a raster created by the clip tool.

I have tried every combination of every tool I can think of.

my code...

    fire = str(ignitionID) + "_" + str(weatherID) + "_Intensity.asc"
    print "fire: " + fire

    # ---------------------------------------------------------------------------
    # this section calculates the size of the fire for model calibration purposes
    # ---------------------------------------------------------------------------

    # Process: Build Raster Attribute Table...
    the_fire = input_fires + fire
    shutil.copy(the_fire, "C:\\FIRESTORM\\Workspace") # copy the fire to workspace
    the_fire = "C:\\FIRESTORM\\Workspace\\" + fire

      gp.BuildRasterAttributeTable_management(the_fire, "NONE")
      resultf = gp.GetCount_management(the_fire)  # count num of rows in raster
      countf = int(resultf.GetOutput(0))
      fire_area = countf * 4  # each cell has an area of 4 ha
      print "the fire area = " + str(fire_area) + " ha"

    except:  # the attribute table cannot be created coz the fire is too large

      fire_area = 300000 # default value for large fires 300000 ha
      print "the fire area EXCEEDS 262,140 ha"

    # -----------------------------------------------------------------------------
    # this section calculates the area of the catchments that are burnt by the fire
    # -----------------------------------------------------------------------------

    # Local variables...
    catch_extent = input_catch + "WS_Catchment_region.shp"  # catchment boundary
    fire_raster = "C:\\FIRESTORM\\Workspace\\fire_raster"
    fire_ascii = the_fire
    Output_raster = "C:\\FIRESTORM\\Workspace\\Extract_ASCI1"
    Output_ASCII = "C:\\FIRESTORM\\Workspace\\rastert_extract1.asc"
    catch_burn_area = 0  # reset catchment burn area

    # Process: ASCII to Raster...
    gp.ASCIIToRaster_conversion(fire_ascii, fire_raster, "INTEGER")

    # Process: Define Projection... based on projection of catch_extent
    desc = gp.Describe(catch_extent)
    spatialRef = desc.SpatialReference
    gp.DefineProjection_management(fire_raster, spatialRef)

    # Process: Extract by Mask...
    gp.ExtractByMask_sa(fire_raster, catch_extent, Output_raster)

    # Convert raster to ascii
    gp.RasterToASCII_conversion(Output_raster, Output_ASCII)

    # Process: Build Raster Attribute Table...
    gp.BuildRasterAttributeTable_management(Output_ASCII, "OVERWRITE")

    # count number of rows in raster
    resulti = gp.GetCount_management(Output_ASCII)
    counti = int(resulti.GetOutput(0))
    catch_burn_area = counti * 4  # each cell has an area of 4 ha

    print "catchment area burnt = " + str(catch_burn_area) + " ha"
  • 1
    why not use zonal statistics ? – radouxju Jan 20 '14 at 10:32
  • 1
    At a first glance, I would remove the conversion to ascii. – radouxju Jan 20 '14 at 10:34
  • 1
    Is the raster that is extracted by the mask turning into a floating point raster? If so the Build Raster Attribute Table tool will not create an attribute table and presumably the subsequent GetCount call is returning a zero? – Hornbydd Jan 20 '14 at 11:48
  • 1
    A zonal sum (using the polygon as a zone) would likely be a better solution in all respects: speed, flexibility, accuracy, data storage. – whuber Jan 20 '14 at 15:54
  • 1
    @Jens: You are correct, however, because the value in each of my cells is a fire intensity from 1 to 2,000,000 there are very few rows with the same value. GetCount gives me 99.9% accuracy and, to be honest, i didt know how else to count the number of cells in a raster. I will attempt to use a zonal sum for both parts though, thank you for your interest. – McVegas Jan 21 '14 at 12:15

couple of options that occur to me without getting too deep in your code. Both rely on a zone mask, which you could make by pushing your raster, which is presumably a mix of values, through a Con (doc here) so output has only values of 1 where there is data. Can take that mask/zone grid and use it w/:

  1. zonalgeometry (doc here)
  2. @whuber's suggestion of zonalsum the 1s and multiply by the area.

Alternatively, you could use a cursor to sniff the one and only VAT row and extract the COUNT field (not sure of the syntax for this back in 9.3). Example:


csr = arcpy.SearchCursor(gridVAT)
row = csr.next()
min = row.getValue('COUNT')

Thinking COLUMNCOUNT + ROWCOUNT won't work on irregularly shaped (i.e., not square and oriented w/coord sys).

Agree w/@radouxju, don't really see the purpose of the ASCII representation. Seems wildly expensive, but can't see your whole workflow.

  • Thank you, this is really helpful. I have converted the raster into 1s and nodata using the Con function as you describe. At the expense of appearing really stupid, I use the zonalgeometry to calculate the raster area but the output of the ZonalGeometry function is another raster. I just want the Area as a number! Can you please tell me how to get a number out of the ZonalGeometry function? Thanks again for you help. – McVegas Feb 2 '14 at 2:06
  • Zonal stats will output a table if you ask it. BTW, McVegas, you now have enough reputation to upvote helpful answers like this one: doing so helps others in the future when they're searching for good solutions. – whuber Feb 3 '14 at 20:10

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