Extending ArcGIS with python


Extending ArcGIS with Python USGS GIS Workshop, May 10-11th, 2011 Drew Flater - Esri Does this describe you? • Comfortable using ArcGIS, geoprocessing, but want to become more efficient • Some experience with Python, or other language • Want to pick up some tips & tricks • Ask the question What else? – extend ArcGIS to do things that are not “in the box” Agenda • Python scripting essentials - Why use Python scripting? - Python 101 - What is ArcPy? - Executing geoprocessing tools - Creating workflows • Batch Processing - Listing data • Reading and Creating Data - Describe - Cursors • Spatial Analyst module - Functions - Map algebra - Raster classes • Map Automation - ArcPy Mapping module - Repair broken data sources - Create map books • Creating script tools Python Scripting Essentials Why use Python scripting? • Scripting language of ArcGIS • Free, cross-platform, easy to learn, established community • But why? Other ways to run tools • Develop, execute, and share geoprocessing workflows • Improve productivity Python 101 • Where do I write Python code? - IDE like PythonWin; Python window in ArcGIS • Which lines will run? • What are variables? - A name that stores a value; assigned using = # I am a comment, I will not execute import arcpy input = “C:/Data/roads.shp” distance = 50 both = [“C:/Data/roads.shp”, 50] # Variables act as substitutes for raw values arcpy.Buffer_analysis(input, “output.shp”, distance) Python 101 • Python has logic for testing conditions - if, else statement - Colon at end of each condition - Indentation determines what is executed - == tests equality; other operators like >, <, != var = “a” if var == “a”: # Execute indented lines print “variable is a” else: print “variable is not a” Python 101 • Techniques for iterating or looping - While loops, counted loops, list loops - Colon at end of statement - Indentation determines what is executed x = 1 while x < 5: print x x = x + 1 for num in range(1,5): print num x = [1, 2, 3, 4] for num in x: print num Python 101 • Case sensitivity - Variables, functions, etc. are case sensitive - name „X‟ is not defined, function „X‟ does not exist • For paths, use forward-slash as separator “C:/Data/Roads.shp” • Functions & Modules - Function: a defined piece of functionality that performs a specific task; requires arguments () - Module: a Python file where functions live; imported - math.sqrt(100) … 10.0 -“There‟s a module for that!” ArcPy • The access point to geoprocessing tools • A package of functions, classes and modules, all related to scripting in ArcGIS - Functions that enhance geoprocessing workflows (ListFeatureClasses, Describe, SearchCursor ) - Classes that can be used to create complex objects (SpatialReference , FieldMap objects) - Modules that provide additional functionality (Mapping ,SpatialAnalyst modules) • Builds on arcgisscripting module (pre-10.0) ArcGIS Python window • Embedded, interactive Python window within ArcGIS - Access to ArcPy, any Python functionality • Great for experimenting with Python and learning tool syntax Executing a tool in Python • ArcPy must be imported • Follow syntax: arcpy.toolname_toolboxalias() • Enter input and output parameters # Import ArcPy import arcpy # Set workspace environment arcpy.env.workspace = ”C:/Data” # Execute Geoprocessing tool arcpy.Buffer_analysis(“Roads.shp", “Roads_buffer.shp", “50 Meters”) Getting tool syntax • Results window, „Copy as Python Snippet‟ • Export Model to Python script • Drag tool into Python window • Tool documentation • arcpy.Usage(“Buffer_analysis”) Setting environments in Python • Accessed from arcpy.env • Provides finer control of tool execution; makes scripting easier • Common environments: - Workspace, coordinate system, extent arcpy.env.workspace = “C:/Data” arcpy.env.extent = “0 0 100 100” Scripting Geoprocessing Tools Exercise 1 10 min • Use the result object • Returned by geoprocessing tools • Maintains messages, parameters, and output Tips & Tricks # Use Select and Buffer tools select = arcpy.Select_analysis(veg, "select“) buffer = arcpy.Buffer_analysis(roads, “buffer”, “75 Feet”) # Erase the output of Buffer from the output of Select erase = arcpy.Erase_analysis(select, buffer, “erase") Batch Processing ArcPy functions • Perform useful scripting tasks - List data to aid batch processing (ListFeatureClasses, 12 total List functions) - Getting data properties (Describe) - Etc. • Supports automation of manual tasks Batch processing • Run a geoprocessing operation multiple times with some automation - Example: Using the Clip tool to clip every feature class in a workspace to a boundary • List functions used in Python to perform batch processing ListFeatureClasses # Set the workspace arcpy.env.workspace = “C:/Data/FileGDB.gdb/FDs" # Get a list of all feature classes fcList = arcpy.ListFeatureClasses() # Print the list of feature classes one at a time for fc in fcList: print fc Batch Processing Exercise 2 5 min • Use string formatting in Python to easily combine strings with other strings (or other types) • Placeholder using % •%s : convert item to string, then replace Tips & Tricks # Clip each feature class, name it according to the input for fc in fclist: arcpy.Clip_analysis(fc, "StudyArea", "Clipped/%s_clip"% fc) Reading and Creating Data Reading Data Properties • Describe function - Returns an object with properties • Allows script to determine properties of data - Data type (shapefile, coverage, network dataset, etc.) - Shape type (point, line, polygon) - Shape field name - Etc. # Describe a feature class desc = arcpy.Describe(“C:/Data/roads.shp") print desc.shapeType >>> “Polyline” Reading Data Values and Geometry • ArcPy cursors used to access table records - Iterate through each record - Retrieve field values of tables, feature classes, rasters - Get geometry of feature class features Cursor Explanation SearchCursor Read-only access to field values, geometry UpdateCursor Update or delete field values, geometry (write-access) InsertCursor Add new records to a table or feature class; write field values and geometry Cursors • SearchCursor • UpdateCursor scur = arcpy.SearchCursor(“C:/Data/Roads.shp”) for row in scur: print row.getValue(“Name”) ucur = arcpy.UpdateCursor(“C:/Data/Sites.shp”) for row in ucur: log = math.log(row.getValue(“Num”)) row.setValue(“Log”, log) ucur.updateRow(row) Reading Geometry • Feature classes have a geometry field - Typically (but not always) named Shape • Returns a geometry object - Has properties that describe the feature - area, length, isMultipart, partCount, pointCount, type, ... • Geometry objects can often be used in place of feature classes # Buffer each feature to a new feature class for row in arcpy.SearchCursor("C:/data/Roads.shp"): feature = row.getValue(“Shape”) name = row.getValue(“Name”) arcpy.Buffer_analysis(feature, “buffer_%s”% name, “100 Feet”) Cursors (creating new data) • InsertCursor used to add new rows, features - Insert values into attribute fields - Insert geometries into shape field icur = arcpy.InsertCursor(“C:/Data/Cities.shp”) row = icur.newRow() row.setValue(“City”,“Denver”) row.setValue(“Shape”, arcpy.Point(-104.98, 39.74)) icur.insertRow(row) ArcPy Cursors Exercise 3 10 min • Clean up cursors using a try, except, finally statement • Cursors can keep a lock on data Tips & Tricks scur = arcpy.SearchCursor(“C:/Data/Roads.shp”) try: for row in scur: print row.getValue(“Name”) except: raise finally: if scur: del scur Spatial Analyst Module Spatial Analyst Module • from arcpy.sa import * • Includes all Spatial Analyst tools • Integrates Map Algebra into Python - Defines geographic analysis as algebraic expressions - Supports mathematical, relational, other operators - Output on the left-side • Helper classes that can be used to support complex parameter demm = Raster("DEM") / 3.28 slpdeg = Slope(demm, "DEGREE") demfs = FocalStatistics(demm, NbrRectangle(3,3), "MEAN") Raster Class • Used as input to tools and map algebra expressions • Reference to raster on disk, created in two ways: - Returned output from Spatial Analyst functions - Cast using Raster() function • Creates a temporary raster dataset that must be saved to be made permanent • Has properties and method - raster.minimum, raster.format, raster.extent, etc. - raster.save() Raster Integration • NumPy is a 3rd party Python library for numeric or mathematical computing - A powerful array object - Sophisticated analysis capabilities - NumPy arrays used in other packages like SciPy • Rasters can be converted to NumPy arrays - RasterToNumPyArray(), NumPyArrayToRaster() Raster to NumPy Array • Example: Correlation Coefficient between two Rasters # Find the correlation between Temperature and Barometric Pressure # Convert Temperature and Pressure rasters to arrays tempArray = arcpy.RasterToNumPyArray(“Temperature") presArray = arcpy.RasterToNumPyArray(“Baro_Pressure") # Flatten the arrays tempArray.ravel() presArray.ravel() # Print the correlation matrix print numpy.corrcoef(tempArray, presArray) >>> [[1.0 0.98] [0.98 1.0 ]] # There is a 0.98 correlation coefficient (R) Spatial Analyst module Exercise 4 10 min ArcPy Mapping module ArcPy Mapping module • Module that contains functions, classes used to automate mapping tasks - Manage map documents, layer files, and the data within - Find and fix broken data sources - Update a layer‟s symbology across many MXDs - Export and print map documents - Map production/map series ArcPy Mapping module • MapDocument object is essential - References mxd on disk; has methods and properties • Needed to perform most mapping tasks - MapDocument as input to function OR - Functions called from MapDocument md = arcpy.mapping.MapDocument(”C:/Maps/NtlParks.mxd”) # Set map document properties md.description = “Map of National Parks” # List layers in the map maplayers = arcpy.mapping.ListLayers(md) # Fix Data Sources md.replaceWorkspaces(….) # Export the map to PDF arcpy.mapping.ExportToPDF(md, “C:/Maps/NtlParks.pdf”) ArcPy Mapping module Exercise 5 10 min ArcGIS Script Tools Script Tools • Script tools are the best way to create and share custom geoprocessing functionality • More people know how to run a tool than a Python script • Source is a script • It is a tool • Use in ModelBuilder, scripts • Has inputs, outputs; generic • Communicates with application • Layers added to map • Messages • Vehicle to serve geoprocessing tasks through ArcGIS Server Creating an ArcGIS Script Tool Exercise 6 5 min Python Scripting Resources • ArcGIS Resource Centers - resources.arcgis.com - Online documentation - Geoprocessing Resource Center: script gallery, blog, presentations • Python References - Learning Python by Mark Lutz - Core Python Programming by Wesley J. Chun • Python Organization - python.org Esri Training for Python esri.com/training • Instructor-Led Course - Introduction to Geoprocessing Scripts Using Python • Web Course - Using Python in ArcGIS Desktop 10
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