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Using Python, how do you add a new field to your shape file and calculate the area in km2?

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Use arcpy.AddField_management(). For the area calculation, check out this question: gis.stackexchange.com/questions/57448/… –  Martin Mar 24 at 15:48
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3 Answers 3

up vote 6 down vote accepted

this is an example

import arcpy
from arcpy import env 
env.workspace = "D:/"
fc = "test.shp" 
arcpy.AddField_management(fc,"area","Double")
 expression1 = "{0}".format("!SHAPE.area@SQUAREKILOMETERS!")        
arcpy.CalculateField_management(fc, "area", expression1, "PYTHON", )
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from osgeo import ogr

shapefile = "C:/StudyArea.shp"
driver = ogr.GetDriverByName("ESRI Shapefile")
dataSource = driver.Open(shapefile, 1)
layer = dataSource.GetLayer()
new_field = ogr.FieldDefn("Area", ogr.OFTInteger)
layer.CreateField(new_field)

for feature in layer:
    geom = feature.GetGeometryRef()
    area = geom.GetArea() * (0.000000092903)
    feature.SetField("Area", area)
    layer.SetFeature(feature)

dataSource = None

I assumed the coordinate system you're using is in US feet. If it's not, you'll need to change the area calculation.

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+1 It is always nice to see a pure python approach –  Aaron Mar 24 at 18:26
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You can use an Update Cursor and a shape area token SHAPE@AREA to make the calculations.

import arcpy, os

file = r'C:\temp\polygon.shp'
arcpy.AddField_management (file, "area", "FLOAT")

with arcpy.da.UpdateCursor(file, ["SHAPE@AREA", "area"]) as cursor:
    for row in cursor:
        row[1] = row[0] / 1E6 # if spatial reference in meters, convert to km
        cursor.updateRow(row)
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