42 | Chapter 4 Enhanced Layer Functionality
Applying Filters to Spatial Queries
A spatial query returns potentially large volumes of geographic information
pertaining to map features. By limiting the data returned by a query, a spatial
filter reduces the query result set to a particular geography. Using spatial
filters can protect against the unwanted return of much larger batches of data
than you intended.
For more information about the default Autodesk MapGuide spatial queries,
you should refer to your MapGuide user documentation, which you can
access by clicking Programs ➤ Autodesk MapGuide6.5 ➤ Documentation ➤
Autodesk MapGuide User’s Guide on the Start menu.
Turning Spatial Filtering On and Off
When you create a layer, spatial filtering is turned on by default. In this
version of Autodesk MapGuide, you can choose to turn spatial filtering on or
off.
Note You should exercise caution if you decide to turn off spatial filtering.
Removing filtering can drastically increase both the record volumes returned and
the amount of time required for data retrieval. For example, you could be observ-
ing a layer displaying San Francisco primary streets, and the grid displayed could
be extracted from a database containing the entire U.S. network of major city
streets. Turning off the spatial filter could result in an attempt to return a database
in excess of one terabyte (1,024 gigabytes!) in size. Established time limits in
retrieval functions would probably shut down the query before it completes, and
if not, you could wait a long time to retrieve the data — even if you have room
to store it!
To turn spatial filtering on and off
1 Open the map containing the layer to which you want to apply or remove
spatial filtering.
2 Double-click the name of the layer in the list to the left of the map. The
Map Layer Properties dialog box is displayed.
3 In the Map Layer Properties dialog box, click the Data Sources tab, as
shown in the following illustration.
