Introduction to Geographic Information Systems in Forest Resources
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Introduction to Geographic Information Systems in Forest Resources |
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In addition to changing theme legends and theme classification, a GIS should also be able to perform other activities on map displays. Depending on the vendor, there may be other theme display properties that are available. This section describes some of the other features available in ArcView.
Defining theme subsets
Setting scale thresholds
Labeling theme features
Locking theme properties
When a theme is added to a project, by default all features are displayed. However, sometimes you may wish to display only a selected set of those features. It is possible to select features to display by using a query, but this only selects those features and paints them yellow, Once the selected set is cleared, all features will once again be displayed.
Performing an underlying query on the data works like a filter, so that even when there is no active selection, only a subset of features will be displayed. Instead of performing a typical query on the data, you perform a query that is saved with the theme properties for that theme. However, the process of performing the theme subset query is the same here as it is when making a typical query.
For example, have the forest stands displayed with only stands whose age in 1999 is greater than 30. The theme subset is stored in theme properties for each feature theme. Opening the theme properties for a theme ...
and clicking the Query Builder button 
allows you to build
a query:
When this change is applied, only the older stands will be available as part of the theme. Other features are essentially hidden from the view. Even the theme table is filtered, so there will be no orphan records; only those records that meet the query criteria will be available in the theme table.
There will be more discussion on building queries later.
Certain themes may not be appropriate to show at certain scales. For example, displaying each and every road name would obscure a map of a large area printed on a letter-sized page. However, when the scale is increased, it will be possible to fit the names of stands on a map without overlap. Or perhaps you have aerial photographs for an area, but you only want the photo data to display when you are viewing an area the size of a city block. At a larger smaller scale, the photo will obscure roads and parcel lines. At a larger scale, only a series of pixels will be visible (see Spatial Data Model for an example of this). It is possible to alter the theme's scale threshold so that themes will automatically display or not display depending on the scale of the view.
Here, the scale threshold for displaying Department of Natural Resources Type 4-9 streams is set so that the streams will not display at a scale below 1:30,000.
The result: at a scale less than 1:30,000, the small streams do not appear, but the larger ones do. When zoomed into 1:25,000, the streams do appear. Note the scale representative fraction displayed on the right-hand side of the tool bar.
Adding feature text labels is often critical to the power of communication
needed for a map. There are several ways to label features in ArcView. Labels
can be added as graphical elements on the view. Theme features may be labeled
with any field in their theme attribute tables, either on a feature-by feature
basis (with the Label Features tool 
), or in
bulk by selecting Theme > Auto Label ( or by using the <CTRL-L>
keyboard combination) from the menu. Text labels added in either of these ways
are treated as graphical elements that can be resized or have font styles and
colors changed. These graphics can also be attached to themes so that they turn
on and off with the theme display. In this manner, labels can also be made scale-dependent.
Auto Labeling gives options for where the text should be placed, if overlapping labels are allowed, if duplicate labels should be dropped, and if label text should be scaled with the view. Labels can be added to features only within the current view extent.
Certain spatial data sources (ArcInfo coverages, AutoCAD drawings) may contain textual elements as georeferenced features (known as annotations). These can be added as text feature themes to the view.
You can prevent access to theme properties by locking the theme in the Theme Properties dialog. This applies password protection, so that the correct password must be supplied in order to access or change theme properties. The theme is locked only in the current project. Locking the theme will not affect permissions on either the data source on the file system or the same data source used in other views or projects.
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The University of Washington Spatial Technology, GIS, and Remote Sensing Page is provided by the College of Forest Resources and the College of Ocean and Fisheries Sciences through Unit-Specific UIF. Site administrator: Phil Hurvitz. |
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