Geog 491: Exercise 4- Working with Fields and Selections

Goal and Background

To use python to add a field, calculate a field and apply and SQL statement.  The objectives for this exercise are stated below,

• Set up the script and import the modules.
• Set up smart variables.
• Add a field and calculate a field.
• Select features based on an SQL statement.
• Add a field and calculate for selected features.

Methods

The comments in green were added along the way in order to keep the script in order.  Add and field and calculate field were used along with an SQL statement in order to produce results to be used in a future exercise.  From these fields, a select tool was used and in the end compactness was calculated.  Below is the script written for this exercise.  Exercise 3 data was used.

Results

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

# Name:        Exercise 4

# Purpose:     Find ideal locations for a ski resort by calculating #              compactness

#

# Author:      Laura Hartley, hartlelj

#

# Date:        7/18/2016

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

#Ensure the program is working

print "script initialized"

#import system modules

import arcpy

from arcpy import env

import os

import time

import datetime

#Allow the script to overwrite existing files

arcpy.env.overwriteOutput = True

#Create variables

print "Creating Variables"

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

#input variables

ex3geodatabasePath = "C:\Users\lhartley13\Documents\ArcGIS\EX3\EX3.gdb"

#This filename will be used for the subsequent 'dissolve output' fc name

intersectName = "IntersectedFcs"

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

#output variables

selectName = "DissolveFC_Selected"

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

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

#Add field

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

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

#Calculate field

arcpy.CalculateField_management(dissolveOutput, "Area_km2",  

"[Shape_Area] / 1000000", "VB", "")

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

#Select (4)

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

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

#Add field (2)

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

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

#Calculate field (2)

arcpy.CalculateField_management(finalSelect, "Compactness_Float", 

"[Shape_Area] / ([Shape_Length] )", "VB", "")

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