Transcript Using Python, GDAL and NumPy for spatial analysis

Using Python, GDAL and NumPy
for spatial analysis and modeling
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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)
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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)
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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)
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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
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Since GDAL 1.3(?), GDAL has implemented
NG (New Generation) Python bindings which
includes NumPy
Process:
Get raster band(s)
Open GDALDataset
Write out GDALDataset
Convert the NumPy
array to GDAL raster
bands using
WriteAsArray()
Convert the raster
band(s) to a NumPy
array using
ReadAsArray()
Process the raster
band(s) using NumPy
functionality
Why use GDAL/NumPy instead of
canned GIS software?
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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:
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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
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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
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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
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Also needed to maintain metadata about spatial
location, projection, original Z values, content
A real-world application (design)
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Class design:
Superclasses
class Header
Subclass
class SpatialReference
class Viz8BitImage
class VizImage
class Viz16BitImage
class ValueReference
Step through
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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”)
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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
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GDAL:
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www.gdal.org and http://trac.osgeo.org/gdal/
Listserve: [email protected]
NumPy
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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
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Somewhat painful. Incomplete docs at:
http://sandbox.fsl.orst.edu/gregorym/mediawiki/index.php/Installing_GDAL-Python24-Numpy
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Easier to install Numpy and GDAL binaries into your
site-packages Python folder
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Install Numpy for Python 2.4
Copy T:\commons\matt.gregory\install into
C:\Python24\Lib\site-packages
Set environmental variables:
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set PATH=%PATH%:C:\Python24\Lib\site-packages\gdal
set PYTHONPATH=%PYTHONPATH%:C:\Python24\Lib\sitepackages\gdal