Extract area of polygons inside larger polygon and sum their values to a new column

Question

I have two landcover classifications (as shapefiles) created using eCognition. "Level0 (FID)" consist of larger polygons and "Level1 (FID, ClassName, Area_Pxl)" of smaller polygons within these. I would prefer a solution using arcpy but other solutions will also do (R, geopandas). Script is preferred since I need to repeat this many times.

I will describe my current workflow here emphasizing what is missing:

1. Create a new column Level0_ID to Level1
   • Done
2. Extract ID from Level0
   • I need to extract the ID of that segment in Level0 to Level1_ID column when Level0 polygon contains Level1 polygon. What tool should I use?
3. Create new field agric_perc to Level0
   • Done
4. I need to create a loop that goes through the attribute table and does the following function:
   • If Level0_ID (in Level1) == ID (in Level0) Then agric_perc (Level0) is agri_perc + Area_Pxl (in Level1)

I think this should do the trick but I am having trouble formulating the script and finding the right tools. I know there is a solution for this where the Level1 shapefile is converted to raster, but I have very large dataset (millions of segments) and original pixel size is very small so the raster solution is very slow and would rather keep the classifications as shapefiles.

---

What I was thinking looks something like this, but maybe there is better solution:

import arcpy
from arcpy import env
import os
import sys

# Set workspace
arcpy.env.workspace = "C:\HY-Data\RPIIROIN\documents\Rami_opetus\eCognitionWorkshop\DevelopingMethods\PolyPerc"

#Create datareader
Cursor1 = arcpy.UpdateCursor("Level1.dbf")

#Create column Level0_ID to Level1
arcpy.AddField_management(Level1, "Level0_ID", "SHORT", 5, "", "", "Classified", "NULLABLE", "REQUIRED")

#Extract ID from Level0
- How to do this?

#Create column agric_perc to Level0
arcpy.AddField_management(Level0, "agric_perc", "SHORT", 5, "", "", "Classified", "NULLABLE", "REQUIRED")

#Create the loop to update agric_perc column
- How do I create a loop that sums the Level0_ID values from Level1 to Level0 agric_perc column

Answer 1

Performing selections of feature layers within an update cursor is how I would accomplish this task. Create your feature layers, and then iterate through your features with an update cursor. Select each Level0 feature with an sql where clause. Perform a select by location to determine which Level1 features are within the selected Level0 feature. Perform a field calculate the Level1 feature layer with the OID. Then use a search cursor on your selected Level1 features to sum your areas. Update your row object after performing the needed math. Something like this will do, if you are using ArcGIS 10.1 or beyond...

#Create feature layers to allow selection
arcpy.MakeFeatureLayer_management (Level0, "lyr0")
arcpy.MakeFeatureLayer_management (Level1, "lyr1")

#Get layer OBJECTID field
OIDfld = arcpy.Describe ("lyr0").OIDFieldName

#Iterate through Level0 feature layer
with arcpy.da.UpdateCursor ("lyr0", ["OID@", "agric_perc", "SHAPE@AREA"]) as cursor:
    for row in cursor:

        #sql to select individual feature in Level0 feature class
        sql = '{0} = {1}'.format (arcpy.AddFieldDelimiters ("lyr0", OIDfld), row[0])

        #Select feature in Level0 feature class
        arcpy.SelectLayerByAttribute_management ("lyr0", "", sql)

        #Select intersecting Level1 features
        #Choose selection type to suit (INTERSECT, WITHIN, etc)
        arcpy.SelectLayerByLocation_management ("lyr1", "WITHIN", "lyr0")

        #Check for selection
        if arcpy.Describe ("lyr1").FIDSet:

            #Update selected features in Level1 with Level0 OID
            arcpy.CalculateField_management ("lyr1", "Level0_ID", row[0])

            #Sum areas with list comprehension and searchcursor
            AllAreas = sum([r[0] for r in arcpy.da.SearchCursor ("lyr1", "SHAPE@AREA")])
            #Do math (percent as whole number)
            AreaPercent = AllAreas / row[2] * 100

            #Update percent field
            row[1] = AreaPercent

            #Update row
            cursor.updateRow(row)

If you are using a version of ArcGIS before 10.1, look into the older version of the UpdateCursor. I hope this helps!