[GIS] Iterating Intersect_analysis over polygon shapefile using ArcPy

arcgis-10.3arcpyintersection

I am using ArcGIS 10.3.1 for Desktop and I have two shapefiles:

  1. A set of stream (vector line) data
  2. A fishnet of polygons

I want to know, for each polygon in that fishnet, what the total length of streams is within that polygon. So far, I've tried iterating the intersect_analysis tool over the fishnet table, summing the lengths of the output and writing that to the relevant field, but each iteration performs the intersect on the entire grid, not the selected polygon in that grid.

Code:

import arcpy

#set some environmental variables
arcpy.env.workspace = "C:\Users\Alex\Documents\SoilMoisture"
arcpy.env.overwriteOutput = 1
#define variables for all necessary feature classes and fields


streams = "pathto/str_split_lamb.shp"  ## shapefile of stream network split at vertices, in Lambert projection, with Length value field pre-calculated
rainGrid = "pathto/net_test.shp" ## polygons for rain cells, with empty "DRN_LENGTH" field

cursor = arcpy.UpdateCursor(rainGrid) ## will iterate across every row of the rainfall grid feature

for row in cursor:

    intStreams = arcpy.Intersect_analysis([streams,rainGrid],"output.shp","ALL","","LINE") ## intStreams becomes all stream segments inside polygon

    strCursor = arcpy.SearchCursor(intStreams) ##iterate over streams in intersection

    totalLen = 0
    for rowmore in strCursor:
        totalLen += rowmore.length ## sum all length values into one variable

    print totalLen # this is just here to check what's going on

    row.DRN_LENGTH = totalLen ## add value to field

    cursor.updateRow(row)


print "Done!"

Any ideas?

Best Answer

You should not iterate any SQL selections at all, since that is very slow and totally unnecessary. Instead intersect all lines by all polygons, then use a search cursor on the intersect results to build a dictionary with the polygon FID as the dictionary key with values that contain the sums of the line lengths. Finally use an update cursor to write to each polygon after looking up the polygon FID dictionary key. Once the Intersect is complete, the actual process of summarizing the intersect data for the full grid and writing the lengths to the target dataset will finish in seconds for 10,000 records.

See my Blog on Turbo Charging Data Manipulation with Python Cursors and Dictionaries. In particular look at the section on "Using a Python Dictionary Built using a da SearchCursor to Replace a Summary Statistics Output Table" at the end of the Blog.

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