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Open Source GIS

Geographic scripting in uDig - halfwaybetween user and developer

Geoinformation Research Group, Department of GeographyUniversity of Potsdam

March 2013

JGrasstools & GeoscriptTutor: Andrea Antonello

Raster in Geoscript - the JGrasstools project

At the time of writing of this tutorial Geoscript didn't support much of raster

data handling.

uDig can do a lot of raster processing through the Spatial Toolbox and the

JGrasstools libraries.

That means that we can access from Geoscript the exact same processing

modules and exploit their raster processing power.

The JGrasstools project is a more then a decade old project that specializes

on geomorphology and hydrology.

Since in those fields most of the processing is done on raster data, a lot of

utilities the handle rasters are available in the library.

http://www.jgrasstools.orghttp://www.jgrasstools.org

The main class that we will use is the Raster class, which wraps the data

and allows us to access the data and main properties.

The latest libraries can be downloaded from the projects download area.

The modules needed to work with geoscripts are:

jgt-jgrassgears-XXX.jar

jgt-hortonmachine-XXX.jar

jgt-modules-XXX.jar

Once donwnloaded, they need to be copied inside a folder named

spatialtoolbox in the installation folder of uDig.

http://code.google.com/p/jgrasstools/downloads/list

An example dataset

To do some tests we can download the Spearfish raster dataset.

Once unzipped and loaded in uDig and styled it should look like:

http://jgrasstools.googlecode.com/files/spearfish_elevation.zip

Reading the properties of a raster

To access raster data and use them inside scripts, the Raster class is your

friend.

path = "/home/moovida/giscourse/data_1_3/spearfish_elevation/elevation.asc"elev = Raster.read(path)

println """Spearfish properties-------------------------------- north = ${elev.north} south = ${elev.south} west = ${elev.west} east = ${elev.east} cols = ${elev.cols} rows = ${elev.rows} cellsize = ${elev.res}"""

Reading more properties of a raster

Through the Raster object we can loop over the map and calculate some

basic statistics, like average, max and min elevation:

path = "/home/moovida/giscourse/data_1_3/spearfish_elevation/elevation.asc"elev = Raster.read(path)

count = 0sum = 0min = 10000000 // initialize to something highmax = 0 // initialize to something lowfor ( col in 0..(elev.cols-1) ){ for ( row in 0..(elev.rows-1) ){ value = elev.valueAt( col, row ) if(!elev.isNoValue( value )){ count++ sum = sum + value if(value > max ) max = value if(value < min ) min = value } } }println "Max elevation: ${max}"println "Min elevation: ${min}"println "Average elevation: ${sum/count}"println "Valid cells: ${count} of ${elev.cols*elev.rows}"

Putting the raster max and min elevation in a shapefile

path = "/home/moovida/giscourse/data_1_3/spearfish_elevation/elevation.asc"dir = new Directory("/home/moovida/giscourse/data_1_3/spearfish_elevation/")// create the vector layer for the max and minlayer = dir.create('spearfish_max_min',[['geom','Point','epsg:26713'], ['type','string'],['value','double']])// get max and min from rasterelev = Raster.read(path)count = 0min = 10000000; minCol = 0; minRow = 0 // can be put on one line using semicolonmax = 0; maxCol = 0; maxRow = 0for ( col in 0..(elev.cols-1) ){ for ( row in 0..(elev.rows-1) ){ value = elev.valueAt( col, row ) if(!elev.isNoValue( value )){ if(value > max ) { max = value maxCol = col // important, keep also the maxRow = row // positions of the max and min } if(value < min ) { min = value minCol = col minRow = row } } } }// get the world position from the raster grid position minXY = elev.positionAt(minCol, minRow)maxXY = elev.positionAt(maxCol, maxRow)// add the featureslayer.add([new Point(minXY[0], minXY[1]),'min', min])layer.add([new Point(maxXY[0], maxXY[1]),'max', max])

The result should look like:

Create a new raster

A new raster can be created from an existing one by using it as a template.

To see how a new raster is created, we can extarct all those parts from the

raster map, in which the elevation lies within the elevation of 1400 and 1600

meters:

path = "/home/moovida/giscourse/data_1_3/spearfish_elevation/elevation.asc"outPath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/elev_1400_1600.asc"elev = Raster.read(path)// create a new raster using elev as templatenewElev = new Raster(elev)// loop over the elevation and pick only values between 1440 and 1600for ( col in 0..(elev.cols-1) ){ for ( row in 0..(elev.rows-1) ){ value = elev.valueAt( col, row ) if(value < 1600.0 && value > 1400.0 ) { // set the elevation in the new raster newElev.setValueAt( col, row, value ) } } }// write the raster to filenewElev.write(outPath)

The result should look like, if overlayed in greyscale style:

Neighbour operations

With the raster object it is possible to access the neighbour cell values. In

this example we will extract pits:

path = "/home/moovida/giscourse/data_1_3/spearfish_elevation/elevation.asc"dir = new Directory("/home/moovida/giscourse/data_1_3/spearfish_elevation/")// create the vector layer for the pitslayer = dir.create('spearfish_pits',[['geom','Point','epsg:26713'], ['value','double']])elev = Raster.read(path)for ( col in 0..(elev.cols-1) ){ for ( row in 0..(elev.rows-1) ){ value = elev.valueAt( col, row ) if(!elev.isNoValue( value )){ // get all neighbour values surr = elev.surrounding(col, row) isPit = true surr.each(){ neighValue -> // check if one is smaller if(neighValue

Which should look like:

Geomorphologic analyses

The JGrasstools project supplies a whole pile of modules. Two methods help

to get started.

The help method shows the help for the module in the console:

new Aspect().help()

The template method:

new Aspect().template()

on each module provides a syntax blueprint which can be used as startpoint

Aspect

Using the template as starting point, let's calculate the map of aspect from th

elevation model:

// set in and out pathspath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/elevation.asc"outPath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/aspect.asc"

// run the aspect moduleAspect aspect = new Aspect();aspect.inElev = path;aspect.outAspect = outPath;aspect.process();

Which should look like:

Contours extraction

The ContourExtractor module can be used to extract contour lines:

// set in and out pathspath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/elevation.asc"outPath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/contours.shp"

// run the module defining start, end and contours intervalContourExtractor contourextractor = new ContourExtractor();contourextractor.inCoverage = path;contourextractor.pMin = 1000;contourextractor.pMax = 2000;contourextractor.pInterval = 20;contourextractor.outGeodata = outPath;contourextractor.process();

Which should look like:

Network extraction

A network extraction can be done in 3 steps: flowdirections + tca, raster net,

vector net:

// set all in and out pathspath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/elevation.asc"flowpath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/flow.asc"tcapath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/tca.asc"netpath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/net.asc"netshppath = "/home/moovida/giscourse/data_1_3/spearfish_elevation/net.shp"

// calculate flowdirections and tcaLeastCostFlowDirections leastcostflowdirections = new LeastCostFlowDirections();leastcostflowdirections.inElev = path;leastcostflowdirections.outFlow = flowpath;leastcostflowdirections.outTca = tcapath;leastcostflowdirections.process();

// extract the network rasterExtractNetwork extractnetwork = new ExtractNetwork();extractnetwork.inTca = tcapath;extractnetwork.inFlow = flowpath;extractnetwork.pThres = 100;extractnetwork.outNet = netpath;extractnetwork.process();

// convert the network to shapefileNetworkAttributesBuilder networkattributesbuilder = new NetworkAttributesBuilder();networkattributesbuilder.inNet = netpath;networkattributesbuilder.inFlow = flowpath;networkattributesbuilder.inTca = tcapath;networkattributesbuilder.outNet = netshppath;networkattributesbuilder.process();

The first maps extracted are the ones of flowdirections and total contributing

area. The map of TCA already suggests the network paths:

In the second step the raster network was extracted:

In the third step the raster network can be transformed to shapefile:

Geographic scripting in uDig - halfway between user and developerRaster in Geoscript - the JGrasstools projectAn example datasetReading the properties of a rasterReading more properties of a rasterPutting the raster max and min elevation in a shapefileCreate a new rasterNeighbour operations

Geomorphologic analysesAspectContours extractionNetwork extraction