Geography Reference
In-Depth Information
number]. How do you determine what that number should be? Fortu-
nately, QGIS displays the current scale in map units on the status bar.
If you look back to Figure 3.8 , on page 49 , you will see that the scale is
1:464 (look at the bottom of the QGIS frame, right under the map dis-
play). You can use this information to determine what scales you want
for the minimum and maximum settings. Use the zoom tools to get the
view you want at each level and then use the displayed scale to set up
the scale-dependent rendering.
What if we want the layer to always be visible when zoomed out (small
scales) but turn off as we zoom in? Set the scale you want it to turn
off at in the Minimum field, and set the Maximum field to a very large
number. If you want the inverse to be true, set the Minimum field to
zero and the Maximum field to the scale at which you want the layer to
turn off.
Note that the terms minimum and maximum as used in the dialog box
seem to be opposite of our definition of small vs. large scale. As long as
you are aware of this, you'll be able to set things up to meet your needs.
Spatial Indexes
The next area of interest on the General tab is the Spatial Index sec-
tion. From here you can create a spatial index, assuming the layer type
supports such an option. A spatial index speeds up drawing, selecting,
and identifying features. For example, when zooming in, QGIS uses the
spatial index to select only the features in the view window for drawing.
This is much quicker than marching through each feature in the layer
and testing it to see whether it should be drawn. The same holds true
for selecting or identifying (which really involves a select of sorts) fea-
tures. The spatial index helps quickly locate the feature(s). When you
click the button to create a spatial index for a shapefile, a new file with
a . qix extension is created. For our world_borders layer, the spatial index
file is world_borders.qix . Always make sure to create a spatial index for
layers that support it. It makes things much snappier.
Spatial Reference System
This is just a fancy name for projection. The spatial reference system
defines the projection and coordinate system of the layer. This is what
makes it possible to draw data in real-world coordinates and have dif-
fering layer's “line up.” The projection information for the layer is dis-
played in PROJ.4 format (see Chapter 9 , Projections and Coordinate
 
 
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