Exercise 4: Working with Fields & Selections

Goals and Objectives

The purpose of this exercise was to gain experience working with adding and calculating fields as well as using SQL statements within PyScripter. 

Methods 

This exercise carried out the exact same processes as those used in Exercise 1, only this time using Python instead of Model Builder.  While developing the script, comments were utilized to annotate each section of code.  The following steps were taken: 

• importing system modules
• importing Python modules: os, time, datetime
• creating variables
• processing data: adding fields, calculating fields, selecting features
• debugging and running script

Results

The final script is displayed below. After successfully running this script, the results were viewed in ArcMap.

#-------------------------------------------------------------------

# Name:         Ex4

# Purpose:      Add field, calculate field, and apply SQL statement via Python

#

# Author:      williaan

#

# Created:     18/07/2016

#-------------------------------------------------------------------

#import system modules

import arcpy

from arcpy import env

import os

import time

import datetime

arcpy.env.overwriteOutput = True

#create variables

print "Creating Variables" ,datetime.datetime.now().strftime("%H:%M:%S")

#input variables

ex3geodatabasePath = "Q:\StudentCoursework\CHupy\GEOG.491.801.2167\WILLIAAN\Ex_3\Ex3_results.gdb"

#filename for 'dissolve output' fc name

intersectName = "IntersectedFcs"

dissolveOutput = os.path.join(ex3geodatabasePath,intersectName) + "_Dissolved"

#output variables

selectName = "DissolveFC_Selected"

finalSelect = os.path.join(ex3geodatabasePath,selectName)

#process: add field

print "Adding a field to the dissolved fcs" ,datetime.datetime.now().strftime("%H:%M:%S")

arcpy.AddField_management(dissolveOutput,"Area_km2", "FLOAT", "", "", "", "", "NULLABLE", "NON_REQUIRED", "")

#process: calculate field

print "Calculating the area in km2" ,datetime.datetime.now().strftime("%H:%M:%S")

arcpy.CalculateField_management(dissolveOutput, "Area_km2", "[Shape_Area] / 1000000", "VB", "")

#process: select areas greater than 2km

print "Selecting only polygons with areas greater than 2km" ,datetime.datetime.now().strftime("%H:%M:%S")

arcpy.Select_analysis(dissolveOutput, finalSelect, "Area_km2 > 2")

#process: add field

print "Adding a field for calculating compactness" ,datetime.datetime.now().strftime("%H:%M:%S")

arcpy.AddField_management(finalSelect, "Compactness_Float", "FLOAT", "", "", "", "", "NULLABLE", "NON_REQUIRED", "")

#process: calculate field

print "Calculating compactness" ,datetime.datetime.now().strftime("%H:%M:%S")

arcpy.CalculateField_management(finalSelect, "Compactness_Float", "[Shape_Area] / ([Shape_Length] * [Shape_Length])", "VB", "")

print "Calculations are complete." ,datetime.datetime.now().strftime("%H:%M:%S")