Tips and Tricks

Tips and Tricks

Using QGIS as a frontend to GRASS

• QGIS started as a simple viewer for geodata. And that's really what it still is: a simple-to-use geodata viewer with some editing capabilities. That's what it was designed to be and that's what it can do really well. QGIS does not have built-in capabilities for geodata processing and analysis.

However, a while back, Radim Blazek, who was then a core developer of the GRASS 6 system, decided to write a plugin for QGIS that would make it possible to access GRASS functionality from within the QGIS GUI.

And that's what you get today, when you download a binary version of QGIS for your platform: QGIS + GRASS 6 plus a plugin that makes using GRASS from QGIS simple and fun.

QGIS can run as a frontend to GRASS. There is support for displaying maps, editing maps, and execution of simple GIS functions. The GDAL/OGR library is a requirement for that (but for GRASS anyway):

• QGIS homepage: http://qgis.org
• GDAL homepage: http://www.gdal.org

Here is a nice tutorial for editing vectors using QGIS as a frontend to GRASS: Editing_GRASS_vectors_with_QGIS (where did it go? unported from old wiki site? )

To use GDAL and GRASS together, the GDAL-GRASS plugin must be installed:

• Compile and install GRASS and QGIS with GDAL/OGR Plugin

Test that the GDAL-GRASS plugin is available with this command:

  gdalinfo --formats

Look for a line like "GRASS (ro): GRASS Database Rasters (5.7+)"

Enable the QGIS GRASS plugin from QGIS:

  GUI: Plugins / Plugin Manager / Check the GRASS checkbox

The GRASS toolbar should now be visible. While not a firm requirement, it is easier to start QGIS from within a GRASS session.

• QGIS GRASS Cookbook - Recipes for common tasks

Importing SRTM30plus data

See Global datasets page

GMT (The Generic Mapping Tools)

• GRASS and GMT

GMT (Generic Mapping Tools) is a Free software package for creating publication quality cartography.

GMT homepage: http://gmt.soest.hawaii.edu

Interfacing R-Statistics with GRASS

• GRASS and R

GRASS may be combined with R-Statistics to create a very powerful geostatistical analysis platform.

R-Statistics homepage http://www.r-project.org

Using GRASS with an on-line Web-GIS

see:

• GRASS and MapServer
• GRASS and PHP
• GRASS and Python

(please expand)

Starting and running GRASS from a script

See GRASS and Shell.

Running GRASS remotely on OS X

Tiger (OS 10.4) changed the default configuration of SSH from previous versions of OS X. You can no longer start an ssh session with the -X flag and display the Tcl/Tk components of the GRASS GUI remotely. If you are running grass on OS X (10.4) between hosts on a network (i.e. running it on one machine but displaying it on another), you will need to use the "trusted forwarding" mode of SSH in order for the Tcl/Tk generated graphics, such as d.m or gis.m in order for the GUI graphics to make it through your connection. This can be done using the -Y flag when you start the ssh session:

ssh -Y remotehost

or add this to ~/.ssh/config:

Host hostname
  ForwardX11 yes
  ForwardX11Trusted yes

Using the -X flag, or simply turning on X11Forwarding in the SSH configuration files, is not enough: the symptoms in this case are that a d.mon window will function fine, but none of the Tcl/Tk dialogues will work, failing with an error message complaining either about Wish not behaving as expected, or a "Bad Atom".

Quick tips

Checking categories pertainig to more than one feature

v.build op=cdump map=mymap | gawk -F"|" '{print $1}' | uniq -dc

returns, e.g.
5 35347
(5 features with cat 35347)

Compensate the lack of line ends symbology in ps.map

ps.map does not allow to specify line-end (from- and to-nodes) symbols oriented in the same direction as the last (resp. first) segment. Typically you want to add arrowheads to a line vector. Here we suggest a one-line gas (grep/awk/sed) solution. The idea is to output a bunch of selected lines ("where" argument), extract concerned couples of vertices, calculate a position and an angle for each tip, then import this set of data in a point vector. This point vector can latter be invoked in a command file.

The next example illustrates the process for line vector mylines. We want to be able to draw an arrow head at the to-node lines for which attribute code equals 3 or 5 :

for i in `v.out.ascii input=mylines format=standard where='code=3 or code=5' \
| grep -n 'L  ' | sed s/\:/' '/ | sed s/'  '/' '/ | awk '{print $1+$3-1}'`; \
do v.out.ascii input=mylines format=standard | sed -n ${i},$((i+2))p \
| awk 'NR == 2 {x=$1;y=$2} NR == 1 {xi=$1;yi=$2} NR == 3 {cat=$2} END {pi=3.14159;OFS="|";\
print cat,x,y,atan2(yi-y,xi-x)/pi*180}'; done \
| v.in.ascii -n out=myarrowheads x=2 y=3 cat=1 \
columns='cat int, x double precision, y double precision, angle double precision'

It results in a point vector named myarrowheads. Attribute field angle contains the rotation angle value fitting with to-node last segment direction.

ps.map command allowing to hardcopy arrows would look like this :

vlines mylines
	width 0.6
	color black
	end
vpoints myarrowheads
	color black
	symbol extra/n_arrow1
	rotatecolumn angle
	size 2
end