Geography 465 GIS Database Programming

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Geography 465 GIS Database Programming

• Getting Started with GIS Database Programming

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• See Geog 465 course web site for
• Overview and Schedule

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What is GIS Database Programming?

• Geoprocessing - application of GIS operations on
  a set of data inputs for generating information.
• Database (DB) programming the part of
  geoprocessing focusing on DB manipulation not
  quite spatial analysis but almost.
• Use scripting to automate DB programming

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Why write scripts?

• Scripts supply the added benefit of decision
  making logic and looping functionality
• Automate a work flow, for example
• Copy all incoming data into a geodatabase
• Perform a project, clip, buffer operation on
  multiple data sets (iterate)
• Easily distribute code
• A script is a self-contained, single file

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Why use Python for Scripting?

• An open-source, object-oriented, scripting
  language
• Offers IDE (integrated development environment)
  with debugging tools
• Modular, can be broken apart
• Ability to compile scripts
• Installed with ArcGIS 9 and ESRI samples provided

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Writing code in Python

• Writing Python code
• Python command line
• IDE (integrated development environment) e.g.,
  PythonWin and Komodo
• Michalis will use Komodo Edit (open source editor
  - free)
• http//www.activestate.com/Products/komodo_ide/kom
  odo_edit.mhtml
• IDE allows you to perform all jobs from one
  location
• Write, Save, Run, and Debug code

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PythonWin Interface

• Script window
• Write and save code
• Interactive window
• Test lines of code
• Report messages
• Menus and Toolbars
• standard and debugging

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Basics of Python

• Comment A non-executable line of code
• One number sign () for green and italicized
• Two number signs () for gray
• Name Michalis Avraam
• Date January 5, 2009
• Purpose To buffer a feature class
• import win32com.client
• gp win32com.client.Dispatch(esriGeoprocessing.G
  pDispatch.1)
• gp.Workspace C\\Python_Data\\SanDiego.mdb
• Gp.Buffer_analysis (Freeways, BuffFreeway,
  1000)
• Can comment and uncomment blocks of code

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Creating Scripts in Python

• Use of ArcGIS help system to find
• Usage, command syntax, and scripting examples of
  standard ArcToolbox tools,
• Usage and syntax of Geoprocessor properties and
  methods, which are only accessible through
  scripting

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Running Python Scripts Three Modes

• 1) Running scripts in IDE, e.g. PythonWin
• 2) Running scripts as script tools in ArcGIS
• 3) Running scripts as embedded model components
  in ArcGIS

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(non) Running Scripts in Python

• Problem with the schema lock
• Cannot manipulate with the same file open in
  ArcGIS and in IDE at the same time

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Running scripts in PythonWin

• Example of a simple script to buffer a feature
  class
• Name Tim Nyerges
• Date January 5, 2009
• Purpose To buffer a feature class
• import win32com.client
• gp win32com.client.Dispatch("esriGeoprocessing.G
  pDispatch.1
• gp.Workspace "C\\Python_Data\\SanDiego.mdb
• gp.Buffer_analysis ("Freeways",
  "BufferedFreeways", 1000)

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Debugging the Code

• Test code in Python Win (or other IDE)
• A check for syntax errors
• Do not test a script from ArcToolbox
• Test code in PythonWin, then add it to ArcToolbox

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Running scripts in PythonWin

• Run the script from the script window of
  PythonWin
• Check the message in the lower left corner of the
  interactive window in PythonWin
• While the script is running you will see the
  message
• running script lt.gt.py
• The indication of the successful run is the
  message
• Script lt.gt returned the exit code 0
• Display the result in ArcCatalog

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Running scripts in PythonWin

• Example of a simple script to clip a feature
  class
• Name Tim Nyerges
• Date January 3, 2007
• Purpose To clip a feature class
• import win32com.client
• gp win32com.client.Dispatch("esriGeoprocessing.Gp
  Dispatch.1")
• gp.Workspace "C\\Python_Data\\SanDiego.mdb"
• gp.Clip_analysis ("MajorAttractions",
  "SDdowntown", "SDdowntownAttractions")

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Running scripts in PythonWin

• Example of a simple script to buffer and clip
  a feature class
• Name Tim Nyerges
• Date January 5, 2009
• Purpose To buffer SD freeways first and then
  to clip the downtown section
• of freeways
• import win32com.client
• gp win32com.client.Dispatch("esriGeoprocessing.G
  pDispatch.1")
• gp.Workspace "C\\Python_Data\\SanDiego.mdb"
• if gp.Exists(gp.Workspace "\\bufferedFreeways")
  
• gp.Delete_management(gp.Workspace
  "\\bufferedFreeways")
• gp.Buffer_analysis ("Freeways",
  "bufferedFreeways", "500")
• gp.Clip_analysis ("bufferedFreeways",
  "SDdowntown", "downtownFreeways")

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Michalis will get you started in labb session
hands-on Python and Komodo edit
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Variables in Python

• Variables are dynamically typed
• No declaration required
• No type assignment required
• fc C\\ProjectData\\SanDiego.mdb\\Freeways.shp
  
• Variables are case sensitive
• fc Freeways.shp
• Fc 20000
• Variables can hold different data types
• numbers, strings, lists, files

Two different variables
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Numbers

• Variables can hold numbers and expressions
• num1 1.2
• num2 3 5

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Strings

• Variables can hold strings
• folder c/Student
• Strings are surrounded in double () or single
  () quotes
• Pathnames use two back (\\) or one forward (/)
  slash
• One backslash (\) is a reserved escape character
  and a line continuation character

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Strings

• Strings can be combined together
• gdbPath c\\SanDiego.mdb
• fc Roads
• fullPath gdbPath \\ fc
• Strings are indexed
• strings are zero-based from the left and
  one-based from the right
• fc Streets.shp
• fc0 ---gt S S is in the 0 position
• fc13 ---gt tr start at 1st, up to not
  including 3rd
• fc-4 ---gt Streets get rid of the last 4
  charaters

C\SanDiego.mdb\Roads
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Lists

• Variables can hold lists
• numList 1, 2, 3
• fcList Roads, Streets, Parcels,
  Zipcodes
• Lists are indexed
• fc1 fcList1
• fc2 fcList02 ---gt Roads, Streets
  
• fc3 fcList0-1 ---gt Roads, Streets,
  Parcels
• fc4 fcList2 ---gt Parcels,
  Zipcodes

---gt Streets
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Variable naming conventions

• Upper case versus lower case
• First word lower case, capitalize each successive
  word
• tableFieldName Street
• - Acronym at the beginning, use lower case
  letters
• gdbPath C\\SanDiego.mdb
• Acronym in the middle or at the end, use upper
  case letters
• inputFC Streets.shp
• Avoid special characters (for example / \
  !)
• Use descriptive variable names

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Line continuation

• Line continuation characters
• Parentheses ( ), brackets , and braces
• Backslash \
• Indentation is automatic
• fcList Roads, Climate, Streams,
• Zipcodes, Coastlines
• distanceValues 100, 200, 300, 400, 500, \
• 1000, 1500
• gp.Buffer_analysis(fcList2, BuffStreams1000,
  distanceValues5)