topic Re: Hi in Python Questions

Calculate polygon area perimeter ratio

NarayanKhatiwada1 — Tue, 14 Oct 2014 20:57:37 GMT

I want to write a python script to calculate the edge to area ratio of each county in mi_counties.shp (Shape_Leng / Shape_Area). What would be the script look like with expression to calculate the ratio? Please help. Thank you!

(Curtis Price edited title, was "Hi")

Re: Hi

ChristianWells — Tue, 14 Oct 2014 23:20:34 GMT

This can be done using the geometry column of the shapefile to return the length and area in the Calculate Field tool.

ArcGIS Help 10.1

import arcpy

shp = r"<Path>\<ShapefileName>.shp"

arcpy.CalculateField_management(in_table=shp,
                                field="EdgeRatio",
                                expression="!shape.length!/!shape.area!",
                                expression_type="PYTHON_9.3",     
                                code_block="#")

Re: Hi

Zeke — Tue, 14 Oct 2014 23:27:31 GMT

Hi Narayan. The script you want, or at least the formula you gave, is pretty simple.

def CalcRatio(len, area):
    return len/area

The differences in how you call this depend on whether you want to call this in Field Calculator or a stand alone script.

In FC, set the parser to python and check the show codeblock box. Enter the above code in the pre-logic box, and in the field = box call the function as shown below (obviously use your own field names, not my example).

You'd use the same def in a stand alone script, but the method for getting the field values for length and area are a little different. If that's what you want, post back and we can go through that.

Re: Hi

Zeke — Tue, 14 Oct 2014 23:33:57 GMT

Ok, well sure, beat me to a more elegant solution . The only caveat is if the OP isn't calculating a field, but getting the results for another process, as part of a more complex script. But your answer seems the most likely and best.

Re: Hi

NarayanKhatiwada1 — Thu, 16 Oct 2014 02:31:39 GMT

Greg and Christian, thank you so much! I will try using both of these codes and let you know later. Thank you again.Greg Keith‌ and Christian Wells