Displaying of map features on the screen is one of the most important functions of any GIS, which enables using the advantages of presenting geographic information visually. For that, such a system should provide the users with wide facilities to make choice what exactly is to be displayed (which feature types, in what region, what conditions are displayed features to satisfy) and in what way are the features to be displayed (what displaying style and scale should be used). The user should also have possibility to save displaying settings in order to use then later.
The GDB component intended for saving displaying settings of spatial features is referred to as a theme.
In the simplest case a theme can contain all the features of a map or of one map layer or feature type located in a certain region and displayed using styles of the corresponding map components. However the means provided by themes are much wider. These means are briefly listed below, and they will be examined in detail in subsequent chapters.
In ObjectLand it is possible to build a set of different displaying variants for one and the same map:
Every theme consists of a set of layers containing spatial features. One can select an arbitrary set of map layers and feature types for displaying, from the complete set of all layers and types to one certain type. For example, it is possible to display only buildings or only vegetation from the map “N-City” or else, for another instance, parcels plus pipelines and electric power lines. Moreover, theme layers may not necessarily belong to the same map. In general case, a theme can contain:
an arbitrary set of layers from different maps of the current GDB;
layers stored in external databases (EDB; see chapter 41 “Main EDB components”, subsection “EDB Layers”);
work layers defined only in the given theme.
Theme layers belonging to maps and to EDB's contain only the method of displaying features, and features themselves are stored in the corresponding components of the GDB. Thus, removing theme layers or removing the theme itself does not result in deleting features. On the contrary, work layers of a theme actually do store features belonging to them, i.e. these features are inaccessible beyond the theme and they will be destroyed in case of removing theme.
Layers of EDB will not be considered in this section. Their using in themes is generally the same as using map layers. Detailed description of working with EDB layers is given in the chapter 41 “Main EDB components”.
Displayed map fragment can be an arbitrary rectangle. In some cases it is advisable to display the whole map on the screen, even in a small scale, in others – enlarged display of some map fragment with the maximum number of details is required. It is possible to change the form of displayed rectangle by stretching it horizontally or vertically. Finally, rectangle sides should not be necessarily parallel to map coordinate axes: sometimes it is convenient to rotate the image to some angle. Moreover, it is possible to define coordinate transformation for theme layers. These transformations are performed when displaying the layer's features and they allow correct superposing layers of several maps which use different coordinate systems.
It is possible to select for display any set of layers and map feature types, from the complete set of all layers and types to one arbitrary feature type. For example, it is possible to display only buildings or only vegetation from the map “N-City”, or, for example, parcels plus pipeline and transmission facilities.
There is a possibility to select displaying styles of different layers, feature types and even particular features. For example, it is possible to display all pipelines by blue lines, and power networks – by red lines. Successful style selection considerably increases information density of display.
There is a possibility to choose displaying styles for different layers, feature types and even for specific features. The styles defined in a theme can differ from styles specified for the same features in the properties of the map. It gives a possibility to highlight when displaying a theme those features which are of greatest interest in that very theme. Moreover, choosing features to display and their displaying styles can depend on current displaying scale. For instance, small size features may not to be displayed at all when the scale is small.
As it will be described in detail below, table information may be linked with map features. There is a possibility to use this information when displaying a map. It is possible to display, for example, only residential buildings and paint them with different colors depending on the number of storeys. Of course, it is possible only in the case if GDB tables contain the information about the purpose of every building (residential or non residential) and about the number of storeys in them.
Variant of map displaying in ObjectLand is called a theme. Any theme is built on the basis of some map. It is possible to create any number of different themes on the basis of one map and give them names (catalog them) to be able to easily select the necessary method of map display from the list of available cataloged themes later.
There are two ways to create a new theme.
The user can create new cataloged theme directly in the navigation window. It can be either an “empty” theme initially containing no layers or a copy of one of the existing themes. The created theme becomes one of the components of the GDB. The user can determine all required characteristics of the theme via theme property sheet before the theme is opened for browsing.
The user can start browsing some map or other GDB component containing spatial features. At that, a temporary browsed theme will be created, and this theme can be saved (cataloged) later. See the next subsection for detailed considering this matter.
