As it was mentioned above, layers and feature types of a theme are selected from the set of layers and feature types of a map or of an external database, i.e. the presence of a certain component in a theme means only that the corresponding map or EDB component is to be displayed in this theme.
There is, however, an important exception. The user can create one or more work layers in the structure of a theme, and these layers are not components of any map. For that, it is needed to select radio button Create work layer in the dialog box “Adding layers” (Figure 14-2) and enter name of the layer being created.
Figure 14-2. Adding a work layer
A work layer of a theme has much in common with a layer of a map. In the structure of such a layer the user can create feature types of any spatial categories, as in the structure of a map layer, however feature types of a work layer (and the features themselves) will not be components of any GDB map.
Thus, whereas usual theme layers only specify displaying of map features in the given theme, work layers actually contain features.
Features of a work layer of a theme will be displayed only when browsing that unique theme that contains this layer. These features are not displayed when browsing maps and when browsing any other theme.
Work layers are aimed to store features which are either temporary or highly specialized in their nature, and therefore including these features in the structure of a map is unsuitable.
As an example of using work layer, one can consider the theme “Pollution of the Environment” which is built on the base of the map “N-City” in the demo GDB. This theme comprises, besides of the map's layers, one work layer. This layer contains a point feature type “Emission Location” which contains features indicating locations of polluting matters emission. Displaying such locations over map layers allows locating pollutions relative to the urban objects. The section “Filters” contains description of the tools which give possibility to highlight map features by different displaying styles depending of their distances from the pollution locations.
One can consider (or, at least, one can hope) that polluting emissions are not stable map features, they will be eliminated in a short time. In future, emissions can happen in quite other locations. The theme “Pollution of the Environment” will most likely be used when necessary for on-line analysis of arising pollutions. In that case a work layer of the theme seems to be more convenient place to hold pollution information then the map “N-City” itself.
Another possible application of work layers is storing of decorative elements such as frames, photos, vignettes, additional inscriptions etc.
In the case of shared access to GDB work layers can be used as convenient stores for the features having private status. As it will be described in the section “Access control”, there is a possibility to grant access to a specific theme (and, accordingly, to the work layers of this theme) to one user or to a restricted group of users.
When displaying a theme, the features of work layers do not practically differ from features contained in map layers. The user can specify displaying styles for a work layer and for feature types of such a layer. If these styles are not specified, the system style is inherited.
Operations of adding, deleting and editing features (see the section “Map editing”) are performed in the same way for the map features and for the features of the work layers.
Some distinctions of the work layer properties will be marked further in corresponding places of the manual. One substantial difference is to be highlighted now. It lies in the fact that deleting a work layer or one of feature types of such a layer results in an irreplaceable loss of all features belonging to this layer or type. This situation cardinally differs from the deleting of other theme layers which are retained in the structure of a map, that is, no feature loss take place in that case.
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