Quick wxGUI tutorial

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_\|/_ GRASS 6.4.0 _\|/_


GRASS (Geographic Resources Analysis Support System) is a raster/vector GIS, image processing system, and graphics production system. GRASS contains over 400 programs and tools to render maps and images on monitor and paper; manipulate raster, vector, and sites data; process multi-spectral image data; and create, manage, and store spatial data. GRASS uses both an intuitive graphical interface as well as command line syntax for ease of operations. GRASS can interface with commercial printers, plotters, digitizers, and databases to develop new data as well as manage existing data.


To run GRASS on the Live DVD, click on the GRASS link on the desktop. From the "Welcome to GRASS" window select either the Spearfish or North Carolina (nc_spm_08) dataset for the location, and "user1" for the mapset, then click on [Start Grass].

This will launch GRASS with our brand new GUI written in wxPython. At the time of writing we have gotten just about all of the kinks out of it and are nearly ready to call it (and GRASS 6.4.0) complete. The old Tcl/Tk GUI is still available if you prefer to use that; you can start it by typing g.gui --ui on the command line.

Displaying maps

Once inside add a raster map layer such as "elevation" from the PERMANENT mapset. To do this click on the checkerboard toolbar button with a "+" on it and then select the map name from the pull-down list which appears, and click [Ok].

In a similar fashion add the "roads" vector layer from the PERMANENT mapset by clicking on the toolbar button with a "+" and a bent poly-line which looks a bit like a "V".

Over in the Map Display window toolbar click on the eyeball button to render the view.
You should now see the maps displayed.

Plot an elevation profile

Back in the GIS Layer Manager window click on the elevation raster map name to select it. Then in the Map Display window, to the right of the zooming buttons on the Map Display toolbar is an icon with a line graph and checkerboard on it. Click on that and select Profile Surface Map. If it isn't automatically listed again pick an elevation map as the raster layer and press [Ok]. The second button in from the left allows you to set out the profile line, click it then mark out a few points on the Map Display canvas. When done go back to the Profile window and click on the eyeball button to create the plot. Click on the I/O button of the far right to close the profile window.

Create a random surface

Now let's create a new map. Select Raster → Generate

 surfaces → Fractal surface from the menu (near the bottom);
 give your new map a name; adjust any options you like in the Options
 tab (the defaults are fine); and click [Run].

Adjust colors

Now add your new raster layer to the layer list as you did before with the elevation raster map, except this time it will be in your "user1" working mapset. You might un-tick the elevation layer check-box now so that the two don't draw over the top of each other. Click on the eyeball to view your new map. <p>The colors might not be as you'd like so let's change them. In the Raster menu select Manage colors → Color Tables. In the "Colors" tab click on the pull-down list for the "Type of color table" option, and pick one from the list. "srtm" is a nice choice. Once done click the [Run] button and close the r.colors dialog window. <p>Because you have altered the map's metadata, this time to re-render it you will have to fully flush the display cache. So click on the little refresh button next to the eyeball button to re-render all layers and you should see your map with its new colors.

Create a shaded relief map

 <p>Next we'll create a shaded relief map of the elevation layer we saw
 earlier. Start by selecting the elevation @PERMANENT map from before
 by clicking on its name. In the Raster menu select Terrain
 analysis → Shaded relief (Terrain analysis is about half way
 down), and the module control dialog will appear. With the elevation
 map name selected as the input map click [Run]. Now add the new
 elevation.shade @user1 map into your layer list and un-tick the other
 raster layers, then click the eyeball to re-render. (If you get sick of
 clicking the eyeball all the time you can tick the "Render" box in the
 bottom right of the Map Display window to have that happen automatically)

Watersheds and steams

 Once again select the elevation @PERMANENT map and in the Raster menu
 choose Hydrologic modeling → Watershed analysis. Set
 the elevation layer as your input map, in the 'Input Options' tab
 set the sub-basin threshold to 10000 cells, then in the 'Output Options'
 tab enter "elev.basins" for the watershed basin option and "elev.streams"
 for the stream segments option just below it. Then click [Run].
 <p>Back in the Layer Manager window add those two new raster maps to the
 layer list and make sure that they are the only two which are ticked
 for display in the box to the left of the layer name. Right click on
 the elev.basins raster map layer name and select "Change opacity level".
 Set it to about 50% then re-render the Map Display.
 <p>In the GIS Layer Manager window click on the third button in from the
 right to add a grid layer. For size of grid put 0:03 for 0 degrees and
 3 minutes (format is D:M:S), then in the "Optional" tab tick Draw
 geographic grid and press [Run] and re-render.
 <p>To add a scalebar go to the Map Display window and press the "Add
 Overlay" button to the right of where you selected the Profile tool
 earlier and select "Add scalebar and north arrow". Read the instructions
 then click [Ok]. A scalebar will appear in the top right. Drag it down
 to the bottom left. From the same toolbar menu select "Add legend" and
 in the instructions window click the Set Options button to set the
 raster map name to create the legend for. After picking one click [Ok]
 and [Ok] again.
 <p>Now you may be thinking to yourself that these fonts are a bit bare.
 That's easily fixed in the GIS Layer Manager menus open Config →
 Preferences and in the Display tab click the [Set font] button and
 then [Apply] in the Preferences window. You will have to do a full
 re-render to see the change so click on the re-render button next to the
 eyeball. The fonts will now be much prettier.

Vector modules

 <p>The above tasks have only covered a few raster modules. Don't let this
 give you the idea that GRASS is just for raster maps -- the vector engine
 and modules are every bit as full-featured as the raster ones. GRASS
 maintains a fully topological vector system which allows all sorts of
 very powerful analyses.

3D visualization

 <p>Start the 3D visualization suite from the File → NVIZ
 menu item and have fun exploring. You will have to select those raster
 and vector maps again from the Raster and Vector tabs.
 <p>Select Visualize→Raster Surfaces from the menu, and
 set the fine resolution to "1", then move the positioning puck and height
 slider around to get different views.

Shutdown and the command line

 <P>When finished, exit the GRASS GUI with File→Exit.
 Before you close the GRASS terminal session as well, try a GRASS
 module by typing "g.manual --help" which will give you a list
 of module options. The GRASS command line is where the true power of
 the GIS comes into its own. GRASS is designed to allow all commands
 to be tied together in scripts for large bulk processing jobs. Popular
 scripting languages are Bourne Shell and Python and some neat tricks
 are included for both to make this scripting easier.
 "g.manual -i" will then launch a web browser
 with the module help pages. When done close the browser and type "exit"
 at the GRASS terminal prompt to leave the GIS environment.

Further reading

_\|/_ Happy mapping!