Filters are a powerful means of ObjectLand which permits the user to select for display in the theme only the features which satisfy certain conditions (limitations). To check these conditions it is possible to use, firstly, spatial characteristics of features (length, area), secondly, characteristics of relative position of features (distance from a given feature to features belonging to an indicated layer or type, area of features intersection etc.) and, thirdly, data from tables and queries linked with feature types. Besides, filters permit organization of a more detailed feature differentiation joining them in groups which differ in values of some characteristics. It is possible to select a specific style of feature display for every group. It enables visualization of classification of features.
Figures 34-1 a, b, c demonstrate samples of using filters when displaying features.
a) A theme without any filter
b) A theme filtered by number of storeys
c) A theme filtered by area of buildings
Figure 34-1. Effect of filters on theme display
There is an analogy between the notion of filtering features and the notion of searching records in a table. As it was shown in chapter 26 “Searching records in a table”, it is possible to specify search condition for tables and queries which determine the records to be displayed when browsing a table or a query, basing on field values. A filter plays a similar role when browsing a theme. In the simplest case it determines which features should be displayed in a theme and which shouldn't.
A filter can be set for any type of features included into the theme. To set a filter for feature type it is necessary to specify filter condition for features of this type. This condition, unlike search condition for a table, can consist of several independent group conditions. The features for which some group condition is true make the filter group which can differ from other groups in displaying style.
There is a possibility to assign a meaningful name to each filter group and also to filter condition in general. These names are displayed in the legend in the course of theme browsing and allow the user to “read the map” easily understanding the meaning of every displaying style used.
Group condition consists of simple conditions which are combined by logical operators “AND” and “OR”. A simple condition allows the user to specify a condition (limitation) for values of one field of a table or a query linked with the present feature type, or for values of a spatial property of features. The current value of map display scale may be also included into the condition.
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