Geography Reference
In-Depth Information
An alternative to digitizing the raster is to use the GRASS
r.to.vect
com-
mand. This will create a vector layer from the raster. The results depend
on the quality of the raster. In the case of our plat, we end up with a
huge vector layer containing 70,607 polygons. When using this method,
everything on the image gets converted, including the text. On our
example plat, the quality of the lines around the individual lots is such
that we end up with an inner and outer polygon, the outer one being as
wide as the line on the image. In addition, the process creates a polygon
for the entire image boundary. You could spend as much time cleaning
up the result as digitizing the polygons from scratch. To aid in cleaning
up the vectors, you can use
v.clean
with the
tool=rmarea
tool to remove
small areas. The other option may be to preprocess the image using
a graphics program to remove some of the noise and unwanted infor-
mation. In any case, you may find that
r.to.vect
is an effective solution
when you need to vectorize a raster.
Digitizing is an activity that you'll find insanely boring, tedious, inter-
esting, or therapeutic, depending on your outlook. It remains an impor-
tant means to create vector data from raster.
8.2
Importing Data
Another important way to get data into your GIS realm is by import-
ing it from text or other source files. Depending on the format of the
data, you may find there is a ready-made solution for importing it. A
prime example of this is delimited text that can be easily imported by
both QGIS and GRASS. QGIS supports the import of points only, while
GRASS can accommodate all feature types in the GRASS vector model.
Quite often you find yourself with some text data that contains coor-
dinates and other attribute information that's begging to go into your
GIS. Let's start with a simple example to get started. We want to create
a data layer of all the volcanoes in the world. Using a search engine, we
find a website
1
that provides a means to search for volcanoes and their
locations. By not entering any search parameters, we are presented
with a web page containing a table of all volcanoes and their locations.
Since the website doesn't provide a download of the data, the first thing
we need to do is copy and paste the results into a text editor. Doing
1.
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