Using Python, GDAL and NumPy for spatial analysis and modeling

 Overview of GDAL and NumPy  Where to use these instead of ArcGIS, Imagine, etc.

 Description of a real-world application  Step through two examples  Mess around with NumPy functionality  How to install the environment using Python 2.4

GDAL (Geospatial Data Abstraction Library)

 GDAL is a “translator library for raster geospatial data formats”  Open source  Used in many applications: GRASS, UMN MapServer, Google Earth, ArcGIS 9.2, etc.

 Can handle many image formats for read and slightly fewer for write: AI Grid, Imagine, GeoTiff, JPEG, PNG, NetCDF, etc.

GDAL (Geospatial Data Abstraction Library)

 Presents an “abstract data model” for processing spatial data  Can be used directly from C/C++ and can be “wrapped” for use with Python, Perl, VB, C#, R, Java …  Early developers have chosen Python as their scripting language and documentation is relatively good for this.

NumPy (Numerical Python)

 An array/matrix package for Python  Well suited for image processing – i.e. one function can operate on the entire array  Slicing by dimensions and applying functions to these slices is concise and straightforward  Nearly 400 methods defined for use with NumPy arrays (e.g. type conversions, mathematical, logical, etc.)

GDAL and NumPy

  Since GDAL 1.3(?), GDAL has implemented NG (New Generation) Python bindings which includes NumPy Process: Get raster band(s) Open GDALDataset Convert the raster band(s) to a NumPy array using ReadAsArray() Write out GDALDataset Convert the NumPy array to GDAL raster bands using WriteAsArray() Process the raster band(s) using NumPy functionality

Why use GDAL/NumPy instead of canned GIS software?

    Not advisable if what you want to do is easily handled within ArcGIS/Imagine/etc. – there is a lot of programming overhead Well suited for process model applications where the logic at a cell based is too complex Example:   Grid algebra : grid1 + grid2 (probably use GIS) Finding NN in multidimensional space (maybe use GDAL/Numpy) Also useful if your spatial data is NOT standard GIS formats (JPEG, PNG, etc.)

A real-world application

 GNNViz – Translate the Gradient Nearest Neighbor (GNN) model into a rendered 3-D environment  Game uses standard imagery such as JPEG, PNG and is easiest to ingest as unsigned 8- or 16-bit images

A real-world application

 Problem: Clip and convert spatial data (in any format) to an unsigned 8- or 16-bit image where:  No data : 0  Clipped window minimum : 1  Clipped window maximum : 255  Also needed to maintain metadata about spatial location, projection, original Z values, content

A real-world application (design)

 Class design: class Header Superclasses class Viz8BitImage class SpatialReference Subclass class VizImage class Viz16BitImage class ValueReference

Step through

 Copy the folder “geoprogramming” from T:\commons\matt.gregory to C:\temp  Open a CMD shell (Start->Run->Type “cmd”)  CD to C:\temp\geoprogramming  Run setenv.bat

 Run the GnnViz example:  Cd to gnnviz (i.e. “cd gnnviz”)  Run the example (i.e. “python extractor.py”)

NumPy examples

import numpy a = numpy.array([1,2,3]) b = numpy.array([4,5,6]) print a+b, a-b, a*b, a/b print a.min(), a.max(), b.min(), b.max() print numpy.shape(a), numpy.shape(b) c = numpy.append(a,b) d = c.reshape(2,3) print c, d Go nuts …

Resources

  GDAL:   www.gdal.org

and http://trac.osgeo.org/gdal/ Listserve: [email protected]

NumPy     Numpy homepage: http://numpy.scipy.org/ Numpy download: http://www.scipy.org/Download Numpy tutorial (not complete): http://www.scipy.org/Tentative_NumPy_Tutorial Numpy Example List: http://www.scipy.org/Numpy_Example_List_With_Doc

Building the Environment

 Somewhat painful. Incomplete docs at: http://sandbox.fsl.orst.edu/gregorym/mediawiki/index.php/Installing_GDAL-Python24-Numpy  Easier to install Numpy and GDAL binaries into your site-packages Python folder   Install Numpy for Python 2.4

Copy T:\commons\matt.gregory\install into C:\Python24\Lib\site-packages  Set environmental variables:  set PATH=%PATH%:C:\Python24\Lib\site-packages\gdal  set PYTHONPATH=%PYTHONPATH%:C:\Python24\Lib\site packages\gdal