Hello,
I'm trying to gather preliminary census data within a 15 minute walkshed and a 15 min bikeshed boundary that we have drawn around a development site. As expected, neither of the "shed" layers fit perfectly into the Census Block Groups (CBGs) we have. Now I know this is far from scientific, but we were thinking for the CBGs that only partially overlap the shed layers, calculate the percentage of the overlap, place that percentage in a field, and multiply statistics by that percentage. I've been working on this script for over a week now and I simply cannot get it to work right. Basically what I want it to do is:
-Create an Update Cursor in the CBGs layer
-Select each CBG polygon as it scrolls through
-Clip the "shed" layer to the selected polygon
-Measure the shape area for the clipped layer
-Divide the area of the clipped layer by the CBG
-Set the result as the value in the percentage field in CBG
-Delete the clipped polygon
-Repeat
In its current form the script is completing without error, but with whole numbers that clearly are not the desired percentage. Every time I try to create an AddMessage so that I can see what values are being divided I then get an error and am still unable to see where the values are coming. This is the first time I've ever built a python script without step-by-step instructions so I'm very lost. If anyone sees the errors in my code, any pointers would be greatly appreciated. I have the script written below and have attached a zipped folder with the .py, the bikeshed, a few selected CBGs, and the toolbox I'm executing it with.
import arcpy
censuslayer = arcpy.GetParameter(0)#census unit layer
percentField = arcpy.GetParameter(1)#field where percentage will be input
studylayer = arcpy.GetParameter(2)#walk or bikeshed layer
#currently unused#arcpy.AddField_management (censuslayer, "perc_cov", "float", "3", "3") #adds blank percentage field to census units
censussize = arcpy.AddField_management (censuslayer, "shapearea", "FLOAT")#adds shape area to census
arcpy.CalculateField_management (censuslayer, "shapearea", "!shape.area@squarefeet!", "PYTHON") #calculates the geometry
#for polygon in studylayer:
arcpy.AddField_management (studylayer, "shapearea", "FLOAT") #adds a shape area to walk or bikeshed
#search = arcpy.SearchCursor(censuslayer)
update = arcpy.UpdateCursor(censuslayer)#creates update cursor to move through census layers
for row in update:
ceblocksize=float(row.getValue("shapearea"))#places shape area value in variable
currentCen=int(row.getValue("OBJECTID"))#places object ID field name in variable
holder=("in_memory\holder")#output
whereClause=str("'OBJECTID' = " + "'"+ str(currentCen)+"'")#formula to indicate record to select
selectBlock=arcpy.Select_analysis(censuslayer, holder, whereClause)#select record where cursor should be
temppolygon = arcpy.Clip_analysis (studylayer, selectBlock, "templayer") #clip shape from shed layer to selected feature
studysize = arcpy.CalculateField_management (temppolygon, "shapearea", "!shape.area@squarefeet!", "PYTHON") #calculate geometry of clipped feature
studysearch = arcpy.SearchCursor(temppolygon)# new cursor in clipped feature
for only in studysearch:
shedsize = only.getValue("shapearea")#store the shape area in a variable
#arcpy.AddMessage(shedsize"/"ceblocksize)
percent = float(shedsize)/ceblocksize #get the value to place in new field
row.setValue(percentField, percent)#sets the value in field
update.updateRow(row)
arcpy.Delete_management(temppolygon)#deletes the clipped polygon
row = update.next
Edit: Formatted Code Block
