raster clipping with overlapping polygons

[matseymour]

Hello,
I am quite new to using python, but...
I have a set of overlapping polygons that I would like to use to clip several raster layers

I have the following code thus far

import arcpy # set environments arcpy.env.workspace = "Path" arcpy.env.overwriteOutput = True #set parameters featureTable="Individual_catchments" baseFeature="elevation" #set cursor rows = arcpy.SearchCursor(featureClass) # loop  for row in rows:      #here I select the geometry       feat = row.Shape      arcpy.Clip_management(baseFeature, "#", "catch"+str(row.Local)+".tif",feat, "0", "ClippingGeometry")

however, when I try to run I get an error stating "ERROR 999999: Error executing function. Raster dataset already exists Failed to execute (Clip). "

thank you for your time and help

cheers

Mat

[ChristopherThompson]

A couple of things that come to mind but i haven't tested:

• try building your output raster name outside of the tool, and pass it in as a variable, putting those sorts of expressions in a tool can be confusing to read and lead to syntax errors

• If the row.Local field contains duplicate values, you could indeed be trying to overwrite an existing raster that has been created, so you might want to double check that.

• you're passing in the geometry of a polygon as the 'feat' variable - if you read the help on this tool i think it wants a feature layer or another raster layer instead of the geometry.  So create a feature layer based on the current row you are processing like this:

feat_id = row.FID #or however you want to uniquely identify a given polygon)
qry = """ "FID" = """ + feat_id
feat = arcpy.MakeFeatureLayer(featureClass,'feat_lyr',qry)
out_rast = "catch" + str(row.Local) + ".tif"
arcpy.Clip_management(baseFeature, "#", out_rast,feat, "0", "ClippingGeometry")

The MakeFeatureLayer makes an in-memory layer with a query definition based on the current row's FID and then is passed in as an argument to the Clip tool.

[MathewCoyle]

Further to what Christopher said, you want to extract your feature class data of interest using your cursor, and then access that outside the cursor. Here is an example using a dictionary.

import arcpy
# set environments
arcpy.env.workspace = "Path"
arcpy.env.overwriteOutput = True
#set parameters
featureTable="Individual_catchments"
baseFeature="elevation"
temp_featureTable = r"in_memory\temp1"
temp_baseFeature = r"in_memory\temp2"
temp_featureClass = r"in_memory\temp_fc"
temp_layer = r"in_memory\temp"

arcpy.MakeFeatureLayer_management(featureTable, temp_featureTable)
arcpy.MakeFeatureLayer_management(baseFeature, temp_baseFeature)
arcpy.MakeFeatureLayer_management(featureClass, temp_featureClass)

# Get OID field name
f_list = arcpy.ListFields(temp_featureClass)
for f in f_list:
    if f.type == "OID":
        f_oid = f.name

# Set Dictionary
feat_dict = {}

#set cursor
rows = arcpy.SearchCursor(temp_featureClass)

# loop
for row in rows:
     feat_dict[row.getValue(f_oid)] = row.getValue("Local")

for id in feat_dict:
    arcpy.MakeFeatureLayer_management(temp_featureClass, temp_layer, "%s = %s" % (f_oid, id))
    arcpy.Clip_management(temp_baseFeature, "#", "catch%s.tif" % (feat_dict[id]), temp_layer, "0", "ClippingGeometry")

[matseymour · Accepted solution]

Thanks for the input folks. After several instances of trial and error I managed to get the below script to produce the output I needed, and feel much more confident in using python to retrieve data. I am sure the code could be simplified significantly, but it is enough for my simple mind. Of course any suggestions would be welcomed.
cheers
Mat

###this first part creates individual raster files (from as single larger raster file) based on overlapping polygons import math import os import arcpy # set the workspace environment  arcpy.env.workspace = "D:/path.mdb" arcpy.env.overwriteOutput = True # set the cliping regions featureClass="Individual_catchments" # set the base feature baseFeature="elevation_swiss" #create list to store catchment elevations catchmentList = [] #create table to store catchment id and average elevation CreateTable_management ("D:/path.mdb", "Average_elevations")   # need to use a cursor argument since we are looking at a single table # this one assigns a search cursor to the Individual catchments table rows = arcpy.SearchCursor(featureClass)   # loop to create all of my individual files                            for row in rows:      #here we only want the geometry (shape)      feat = row.Shape      output="c"+str(row.LOCALITYID)          arcpy.Clip_management(baseFeature, "#", output,feat, "0", "ClippingGeometry") # delete the cursor del rows  ################################################################################################# ####### second part of script to take individual catchments and calculate some statistics ####### # i created this using a separate script from the first so the environments are retyped #################################################################################################  # make sure to put the module you are working with before the function (or it will spit back 'not defined') - one my biggest pitfalls ###here i want to update the catchemnt feature with additional information based on other layers import math import os import arcpy # set the workspace environment  arcpy.env.workspace = "D:/path.mdb"  # Overwrite pre-existing files arcpy.env.overwriteOutput = True  #parameters path = "D:/path.mdb" arcpy.Delete_management("SumTable") arcpy.CreateTable_management (path,"SumTable") arcpy.AddField_management ("SumTable", "NameID", "TEXT") arcpy.AddField_management ("SumTable", "TotalCount", "LONG") arcpy.AddField_management ("SumTable", "TotalElv", "FLOAT",'',10) arcpy.AddField_management ("SumTable", "AVG_ELV", "FLOAT",'',10) SummaryT= "D:/path.mdb/SumTable"   statsFields = [["COUNT", "SUM"],["ELEVC","SUM"]]   #create list to point to catchment elevations catchmentList = arcpy.ListRasters ("c*")   for i in catchmentList:     #make sure each raster has its own attribute table     arcpy.BuildRasterAttributeTable_management(i,"Overwrite")     #create a new field with elevation times number of occurances (COUNT * VALUE)     arcpy.AddField_management (i, "ELEVC", "LONG")     arcpy.CalculateField_management(i,"ELEVC","!Value! * !Count!","PYTHON_9.3","#")     temp_outtable = "outtable"     arcpy.Statistics_analysis (i, temp_outtable , statsFields)      #python's tedious way to get data from a table     rows=arcpy.SearchCursor(temp_outtable) #this 'points' to the first row      row=rows.next() #this 'points' to the field     print row.SUM_COUNT,"_",row.SUM_ELEVC  # Create the insert cursor and a new empty row     rowInserter = arcpy.InsertCursor(SummaryT)     newIncident = rowInserter.newRow()  # Populate attributes of new row     newIncident.NameID = str(i[0:7])     newIncident.TotalCount = row.SUM_COUNT     newIncident.TotalElv = row.SUM_ELEVC     newIncident.AVG_ELV = row.SUM_ELEVC / row.SUM_COUNT  # Insert the new row into the shapefile     rowInserter.insertRow(newIncident)     # delete catchment specific search cursor and temp sum file     del row     del rows     arcpy.Delete_management(temp_outtable)       # delete the insert cursor del rowInserter