Clipping and projecting with GDAL

QGIS, like most open-source GIS software, uses the GDAL (Geospatial Data Abstraction Library) software package to implement a lot of what ArcGIS would call "tools" or "geoprocessing". All of the GDAL functionality is also accessible as bash commands.

For historical reasons, there are GDAL commands, which operate on raster data, and OGR commands, which operate on vector data. The whole shmodz is collectively referred to as "GDAL".

Setting up GDAL access from Git Bash

(You only need to do this once.)

Start up Git Bash and run the following commands:

$ cd
$ start notepad++ .bash_profile

This will make sure you're working in your home directory (i.e. your H: drive), and then start up Notepad++ editing the file .bash_profile

If you get a popup that says

H:\.bash_profile doesn't exists. Create it?

then click Yes.

Don't forget the initial . in .bash_profile: it's part of the name.

Filenames beginning with . aren't displayed by the ls command (unless you specify the -a option.) It's a UNIX convention to use names beginning with . for files containing command-specific configuration or logging information. .bash_profile contains commands that bash runs whenever it start up.

Type or copy the following text into Notepad++:

export PATH="$PATH:/c/Program Files/QGIS Chugiak/bin"

PATH tells bash which folders to look in to find the commands that you tell it to run. We're telling bash to look in the folder containing the QGIS commands, in addtion to all the other folders ($PATH) it would usually look in.

Save .bash_profile and quit Notepad++

Quit and restart Git Bash.

Run the following command:

$ gdalinfo --version

You should see:

GDAL 1.11.0, released 2014/04/16

which means that Git Bash can now run all the commands associated with QGIS, woohoo! (c'mon, you are excited about this, admit it...)

Scripting GDAL

We're going to see how to run GDAL commands to do geoprocessing from bash. Specifically, we're going to create a shell script, which is a file containing a sequence of bash command, just like you'd type, one after another, on the bash command line. The shell script can then be run just like a command in its own right, very much like running a Python program.

If it's not still running, start up Git Bash and cd to whatever folder contains the InVEST_CV folder:

$ cd ~/esm296-4f/wk3

The ~ is a shorthand for your home folder (i.e., your H drive.)

Start up Notepad++ editing

$ start notepad++

Cutnpaste (or type, if you're a masochist) the following into Notepad++:

    # this lets you see the commands as they are executed
    # (but not the comments)
set -x

    # a list of the rasters that we want to clip and project
rasters="global_pop claybark_dem"

    # the shapefile that the rasters will be clipped to

    # arguments used by all the GDAL commands we'll run
    # -dstnotata -32768    : use -32768 as the output NO_DATA value
    # -of GTiff            : make the output raster a GeoTIFF
    # -co COMPRESS=DEFLATE : use the "DEFLATE" TIFF compression method
common_args="-dstnodata -32768 -of GTiff -co COMPRESS=DEFLATE"

    # arguments for the clip command
    # -cutline $roi    : clip the input to the shapefile $roi
    # -crop_to_cutline : clip to the shape (not just its bounding box)
clip_args="-cutline $roi -crop_to_cutline"

    # arguments for the project command
    # -t_srs EPSG:32610 : use UTM zone 10N for the output coordinate system
project_args="-t_srs EPSG:32610"

    # loop through all the files listed in $rasters
for file in $rasters
        # clip ESRI grid InVEST_CV/$file into GeoTIFF ${file}_wgs84.tif
    gdalwarp $common_args $clip_args InVEST_CV/$file ${file}_wgs84.tif

        # project GeoTIFF ${file}_wgs84.tif into GeoTIFF ${file}_utm10n.tif
    gdalwarp $common_args $project_args ${file}_wgs84.tif ${file}_utm10n.tif

Save and quit Notepad++

Change the permission on so it's executable (i.e., can be run as if it were a command):

$ chmod +x

chmod is short for "change mode". The guys that invented UNIX really didn't like to type...

OK, what did we just do?

We copied a bunch of bash commands into and made it executable as a command. If we run as a command (which we will in a minute), it will run all of the commands it contains, one after another.

Let's look at the script line by line (more or less...):

set -x

This tells bash to "echo" (type out in the bash window) each command before it's run. This is helpful so you can see what's going on, and in particular so you can tell how far a script got before it failed.

bash uses # to indicate a comment, just like Python. Everything from a # to the end of the line is ignored when the script is run.

rasters="global_pop claybark_dem"

This creates a shell variable called rasters and sets it to the words global_pop and claybark_dem, separated by a space.

Shell variables are pretty much like Python variables, except that you can't have any spaces around the =.

common_args="-dstnodata -32768 -of GTiff -co COMPRESS=DEFLATE"
clip_args="-cutline $roi -crop_to_cutline"
project_args="-t_srs EPSG:32610"

These are additional variables we'll use later. It's common in shell script to use variables for stuff that would otherwise have to be repeated (like the value of common_args), or for stuff that you might want to change later.

Note that if the value of a variable contains spaces, then you have to enclose it in quotes.

Note also that the value of clip_args uses the previously-defined variable roi. You get a variable's value by prefixing it with a $. Thus, anywhere $roi appears, bash replaces it with InVEST_CV/AOI_BarkClay.shp

for file in $rasters

This is a bash for loop, almost identical to a for loop in Python. The loop executes as many times as there are words in $rasters, with the value of file set to each word successively. In this case, the loop executes twice, once with file=global_pop and once with file=claybark_dem.

Note: Unlike Python, indentation doesn't matter in bash scripts. The indentation in this script is solely to make it easier for you to read (a highly recommended practice.)

gdalwarp is GDAL's all-purpose spatial transformer. It can do subsetting, re-projection, and a bunch of other stuff; see the gdalwarp manual page for details. First, we'll use it to clip the input ArcGIS grid file to the outline of our region-of-interest, also convert it from grid to GeoTIFF format:

gdalwarp $common_args $clip_args InVEST_CV/$file ${file}_wgs84.tif

GeoTIFF is a portable (non-proprietary) raster format that's much more suited to open-source processing than the proprietary ESRI grid format.

Note the {}s in ${file}_wgs84.tif: they tell bash that only part of the character sequence file_wgs84.tif is a variable name. We use this trick a lot to construct filenames with a common prefix but different suffixes.

Second, we'll use gdalwarp to re-project the clipped raster from WGS 84 geographic coordinates into WGS 84 UTM zone 10N projected coordinates.

gdalwarp $common_args $project_args ${file}_wgs84.tif ${file}_utm10n.tif

How do we know that the input files are in WGS 84 geographic coordinates? By asking:

$ gdalinfo -norat InVEST_CV/global_pop
Driver: AIG/Arc/Info Binary Grid
Files: InVEST_CV/global_pop
Size is 43200, 16800
Coordinate System is:
        SPHEROID["WGS 84",6378137,298.257223563,
Origin = (-180.000000000000000,83.999999999436000)
Pixel Size = (0.008333333333300,-0.008333333333300)
Corner Coordinates:
Upper Left  (-180.0000000,  84.0000000) (180d 0' 0.00"W, 84d 0' 0.00"N)
Lower Left  (-180.0000000, -56.0000000) (180d 0' 0.00"W, 56d 0' 0.00"S)
Upper Right ( 180.0000000,  84.0000000) (180d 0' 0.00"E, 84d 0' 0.00"N)
Lower Right ( 180.0000000, -56.0000000) (180d 0' 0.00"E, 56d 0' 0.00"S)
Center      (  -0.0000000,  14.0000000) (  0d 0' 0.00"W, 14d 0' 0.00"N)
Band 1 Block=256x4 Type=Int32, ColorInterp=Undefined
  Description = global_pop
  Min=0.000 Max=1346772.000 
  Minimum=0.000, Maximum=1346772.000, Mean=8.339, StdDev=138.793
  NoData Value=-2147483647

The -norat option keeps gdalinfo from printing a whole bunch more detail that we don't need; see the gdalinfo manual page for details. gdalinfo is a very handy command for finding out what's up with mystery data. (Yes, there's a corresponding ogrinfo for vector data; see the manual page)

OK, enough with the exegesis. Let's run the sucker:

+ rasters='global_pop claybark_dem'
+ roi=InVEST_CV/AOI_BarkClay.shp
+ common_args='-dstnodata -32768 -of GTiff -co COMPRESS=DEFLATE'
+ clip_args='-cutline InVEST_CV/AOI_BarkClay.shp -crop_to_cutline'
+ project_args='-t_srs EPSG:32610'
+ for file in '$rasters'
+ gdalwarp -dstnodata -32768 -of GTiff -co COMPRESS=DEFLATE -cutline InVEST_CV/AOI_BarkClay.shp -crop_to_cutline InVEST_CV/global_pop global_pop_wgs84.tif
Creating output file that is 301P x 211L.
Processing input file InVEST_CV/global_pop.
Using internal nodata values (e.g. -2.14748e+009) for image InVEST_CV/global_pop.
0...10...20...30...40...50...60...70...80...90...100 - done.
+ gdalwarp -dstnodata -32768 -of GTiff -co COMPRESS=DEFLATE -t_srs EPSG:32610 global_pop_wgs84.tif global_pop_utm10n.tif
Creating output file that is 261P x 276L.
Processing input file global_pop_wgs84.tif.
Using internal nodata values (e.g. -32768) for image global_pop_wgs84.tif.
0...10...20...30...40...50...60...70...80...90...100 - done.
+ for file in '$rasters'
+ gdalwarp -dstnodata -32768 -of GTiff -co COMPRESS=DEFLATE -cutline InVEST_CV/AOI_BarkClay.shp -crop_to_cutline InVEST_CV/claybark_dem claybark_dem_wgs84.tif
Creating output file that is 9165P x 9938L.
Processing input file InVEST_CV/claybark_dem.
Using internal nodata values (e.g. -32768) for image InVEST_CV/claybark_dem.
0...10...20...30...40...50...60...70...80...90...100 - done.
+ gdalwarp -dstnodata -32768 -of GTiff -co COMPRESS=DEFLATE -t_srs EPSG:32610 claybark_dem_wgs84.tif claybark_dem_utm10n.tif
Creating output file that is 9165P x 9938L.
Processing input file claybark_dem_wgs84.tif.
Using internal nodata values (e.g. -32768) for image claybark_dem_wgs84.tif.
0...10...20...30...40...50...60...70...80...90...100 - done.

Things to note:

  • Because you made executable, you can run it just like any other command.
  • Because of the set -x, you can see each command in the script as it executes (with a + in front of it.)
  • In the commands that you see, all the variables prefixed with $s have been replaced by their values, so you see the command that was actually executed.
  • claybark_dem has a lot more data in it than global_pop, so it takes longer to process.