A note from Sam in the Linfiniti team:
The last two weeks of FOSS GIS training courses (posted about here)
have been impressive. High-five to the trainers, Tim and Gavin, for
their hard work! Having only being at Linfiniti for a short time, I was
assisting with the courses but also learning some things about the
software myself. This is just one of the nice things I learnt from the
QGIS course.
Using the GRASS plugin in QGIS opens you to a whole host of new tools.
The plugin makes GRASS more accessible, and simplifies the complexity of
GRASS datasets, so that more users will be able to benefit from the
GRASS functionality available.
You can convert vector contour lines to a raster DEM, and then visualise
the DEM in 3D. The steps below are written for newbies (like I was!), so
skip the detail if you an experienced user ;)
1. Setting up your mapset and adding your data
When the GRASS plugin is enabled in QGIS, you will get a GRASS toolbar
added. Open your contour shapefile in QGIS as you usually would, then
click New mapset on the GRASS plugin toolbar. For those new to GRASS
mapsets, here are some guidelines:
- specify the directory (you can make a 'grass' folder in your home
directory - it is important you have full read/write access)
- create a new location - think of this as a project folder based on a
certain location (geographical extent)
- assign the projection of your dataset (your projection will be fixed
for the location and mapset)
- under the default GRASS region section, click the 'Set current QGIS
extent' button
- enter the mapset name (this will be your username - each user can
have a mapset in the location, the 'project folder')
Once you've setup, you will see a red outline around the data in your
QGIS view, this is your mapset extents.
Click the GRASS tools icon on the toolbar, and find the option to import
your vector or raster. Here's the vector import:
Once you click the function, select the layer to add (the list will show
those layers open in QGIS), type a name and click run.
You will also need to add the newly created grass layer to your QGIS
view - this will make it accessible for subsequent grass functions. Add
the layer by clicking 'view output' after running the function, or in
the browser tab in the GRASS tools dialog - click refresh, find the file
and click the add icon.
2. Creating a DEM from contours
Now for the interesting bits. Make sure your vector contour from your
grass mapset is open. Under the Module list in the GRASS tools dialog,
find and run these functions in this order:
- *v.to.rast.attr* -> converts your vector to raster based on an
attribute (use the height attribute from the contours). Open your
result once its run.
- *r.surf.contour*-> creates a surface from the rasterized
contours. Tip: edit your mapset region (last icon on the grass
toolbar) to adjust the output area to your dataset- this will save
time! Open your result once its run.
3. Visualisation in 3D using nviz
In the modules list, find *nviz*. This can be used to visualise any
DEM data, and you can add overlays. Using a nice colour map as
explained in Tim's postwill produce a catching effect. Running the
module, and adjusting the settings (z-exaggeration, lighting,
perspective etc) will give you a dynamic 3D view of your data. Nviz is
also apparently capable of 3D fly-throughs for those interested in
exploring a bit more.
Voila, and there you go! And this was all created from the comfort of
your QGIS environment, thanks to the awesome plugins that have been
added! :)
Hope this helps, and if you've got any questions, feel free to leave a
comment!
-Sam
