jeffreywolf / threshold

Threshold a raster

Introduction

Classify each pixel of an image as above or below a specified threshold value then convert the classified raster to a polygon representation.

Input

An input raster in GeoTiff format (*.tif).

Output

threshold.py

A classified raster in GeoTiff format (*.tif).

polygons.py

An polygon representation of the classified raster in shapefile format (*.shp).

Dependencies

numpy, GDAL

Examples

There are two steps: 1) threshold an image with "threshold.py" and 2) convert the thresholded raster to a vector representation with "polygons.py". The two steps can be run as a pipeline using the driver ("driver.py").

For the purpose of these examples, create a folder called output in the threshold directory.

Thresholding

The following examples classify each pixel of a canopy height model (with units in meters) as below 15.0 meters (classied as 1.0 or 1) and above 15.0 meters (classified at 0.0 or 0). The value after the -t flag represents the threshold value.

If running on UNIX-like system make sure permissions are set to executable. chmod u+x threshold.py

Then run ./threshold.py -i chm_test.tif -o output/threshold15.tif -t 15.0 -v

If using Windows, use the following:

python threshold.py -i chm_test.tif -o output/threshold15.tif -t 15.0 -v

Raster to Vector

Convert the classified raster from threshold.py into a vector representation (shapefile).

If running on UNIX-like system make sure permissions are set to executable.

chmod u+x polygons.py

Then run

./polygons.py -i output/threshold15.tif -o output/threshold15.shp -v

If using Windows, use the following:

python polygon.py -i output/threshold15.tif -o output/threshold15.shp -v

Batch Mode

The script driver.py 1) provides batch like functionality to perform classification at a series of value thresholds and 2) serves as a pipeline to both threshold and convert from raster to vector.

The example driver.cfg file will classify the chm_test.tif at all height thresholds between 2.0 m and 35.0 m using a step size of 1.0 (default). The output will be a series of files with the prefix threshold.

If running on UNIX-like system make sure permissions are set to executable.

chmod u+x driver.py

Then run

./driver.py -c driver.cfg -v

If using Windows, use the following:

python driver.py -c driver.cfg -v

Notes

The PostGIS shapefile loader shp2pgsql may have problems with loading files that contain decimal points (e.g. "threshold15.0.shp") other than the file extension separation. Therefore, it is best not to use numbers with decimal points in the file name (e.g. "threshold15.shp").