Digitizing and Editing Vector Data - Desktop GIS: Mapping the Planet with Open Source Tools - page 75

Keeping Your Data Safe
When editing your data with any GIS application, it's a good
idea to make sure you have a backup copy. Let's face it, disas-
ter can strike, whether it be a program crash, power outage, or
beverage incident. Keeping a current backup of your critical
data is just good practice.
All our OSGIS desktop applications that support editing allow you to
make adjustments to features by moving, deleting, and inserting ver-
tices. Once you've completed a feature, the vertices are displayed (in
QGIS they look like X's). Using the vertex tools on the editing toolbar,
you can adjust the boundary to correct any errors.
Harrison decides he also needs the streams connecting his lakes dig-
itized. Along the way, he makes a few mistakes, which we'll help him
fix. In Figure 5.5 , on the following page, you can see part of Harrison's
fist attempt at connecting the lakes using QGIS and a line shapefile for
the streams.
If you look closely at the streams (we've made them red so they stand
out), you'll notice a number of problems. For one, we have a stream
that doesn't connect with its neighbor. In another case, the line seg-
ment overshoots the intersection. These undershoots and overshoots
are called dangles. Lastly, we have a stream that runs too far into the
lake on the east. The other thing you'll notice is that Harrison was a bit
sloppy in following the stream course, especially approaching the lake
to the east.
Let's use the vertex-editing tools to clean things up a bit. All the errors
can be easily corrected just by inserting new vertices where needed and
moving existing vertices to intersect where they should. To do this, we
first need to set the snapping tolerance. This controls how QGIS snaps
to existing vertices when editing. By setting a reasonable tolerance, we
can make QGIS “jump” to the closest vertex, thereby making our job
easier as well as making sure the line segments actually touch. It's
harder than you may think to manually move a vertex and get it exactly
on the line.
To determine a proper snapping tolerance, you need to take a look at
your data and get an idea of distance between vertices in your lines.