GRASS Basics - Desktop GIS: Mapping the Planet with Open Source Tools - page 316

If we click the left button, we reset the corner and start over. If we click
the middle button, it establishes the second corner of our zoom window
and the display changes, zooming in as we expect. In both cases, the
third (right) button cancels the zoom function and resets all the button
labels to blank.
This takes a bit of getting used to because you are probably accustomed
to just dragging a rectangle, letting go, and watching the zoom happen.
The shortcut in this case is to use the middle button for setting both
corners of the zoom window—not that is saves you any effort, just a bit
of finger movement for those of us that are uncoordinated.
Now we know how to navigate the map, and the tools we need to get
started digitizing are on the second toolbar. The first two are for points
and lines, and the next two are the ones we want.
First we select the Digitize new boundary tool and click it. The mouse
button cues now indicate left button to create a new point and right
button to quit. Now all we have to do is click our way around the lake,
following the shoreline. When you click the first point, the mouse cues
change again. The first remains New Point, but the second changes to
Undo Last Point, and the third changes to Close Line. Now we can just
click around the lake and use undo if we need to delete a bad point.
Once we get to the origin, we use the right mouse button to close the
boundary, clicking over the first point. If we click within the tolerance,
the boundary will close; if not, we'll have to do it manually.
As soon as you right-click, a form pops up to allow you to enter the
name. At this point you are naming the boundary line. If it's not impor-
tant to you to put a name on the boundary, you can skip it; otherwise,
you can enter the name of the lake now. Once we close the boundary,
we'll see how to make it into a polygon.
If the boundary turns green when you close it, you are in luck. The start
and end points coincide. If not and it's red, we can use the Move vertex
tool to easily move the last point to close things up. To move it, select
the tool and click the last point, and then move the mouse until the
cursor is over the first point and click again. If you're lucky (or good),
the border will turn red to green, indicating closure.
Now that we have a closed boundary, we can make it a polygon by
adding a centroid. Click the Digitize new centroid tool, and then click
somewhere in the center of the lake polygon. Again the attribute form