I would like to multiply all the values within a raster layer with one value, which is written in an *.csv table (it is the only value in this table).

I tried this code (changing Times tool to Minus tool)

>>> import arcpy
>>> from arcpy import env
>>> from arcpy.sa import* 
>>> arcpy.env.workspace = "D:/Bachelorthesis"
>>> dgm = ['dgm']
>>> gwf = ['Groundwatertable']
>>> path = 'd:/Bachelorthesis/gis_daten_footprint/Data(1)/GIS_data.mdb/gwf'
>>> value = [row[0] for row in arcpy.da.SearchCursor(path,gwf)]
>>> Gwt = set(value)
>>> print (Gwt)
>>> outMinus = Minus(dgm,Gwt) 

But then this error appears:

 Runtime error 
 Traceback (most recent call last):
   File "<string>", line 1, in <module>
   File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\sa\Functions.py", line 3840, in Minus
   File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\sa\Utils.py", line 47, in swapper
     result = wrapper(*args, **kwargs)
   File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\sa\Functions.py", line 3837, in wrapper
     return _wrapLocalFunctionRaster(u"Minus_sa", ["Minus", in_raster_or_constant1, in_raster_or_constant2])
TypeError: expected a raster or layer name

But 'dgm' is definitely a raster... Does anyone know how to make it work?

  • 1
    What GIS software are you using? What have you tried so far?
    – PolyGeo
    Jun 23 '16 at 11:23
  • I am using ArcGIS 10.1. I've tried the raster calculator, but that doesn't work because it only reads raster. And joining tables and raster doesn't work either. Jun 23 '16 at 11:29
  • If you need to retrieve the table value automatically, you will need arcpy. I would read the value in the csv table with a Search Cursor, then feed it in the Times tool along with the raster.
    – GISGe
    Jun 23 '16 at 11:55

What I understand is you want to use the raster values to do some calculations with the values in the csv file. This is one way to do it.

Step1: Convert the raster into a point file. Use the raster values as column info of the point file.

Step2: Use that point file info to make calculations with the csv.

Step3: Once calculations are done. Either convert back the point file into the raster.

  • thanks! But now I have a problem with step 2... I can't find how to write to sql expression for the fieldCalculator in arcpy. How can I express hat I would like to have all the values from that field minus a certain value? Apparently arcpy.CalculateField_management("gwf","test","test"-2) doesn't work... Jun 28 '16 at 10:18
  • @NathalieLea I think you can make the equation with simple SQL expression in field calculator itself. If you can share the details of equation, it would make it more clear exactly what u aim to do.
    – Chetan
    Jun 28 '16 at 12:48
  • My problem actually lies in this simple SQL expression. I can't find anywhere how to express in arcpy.CalculateField that each row in one field (f.ex. "test") is subtracted by a certain value (here "Gwt", that I defined in the script above with value = [row[0] for row in arcpy.da.SearchCursor(path,gwf)]). So I tried many different possibilities but nothing worked. For example: arcpy.CalculateField_management("gwf","test","test"-"Gwt"). The thing is also, that I need it to work automatically in a script that I will insert in a modell. Jun 29 '16 at 8:56

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