By Charlie Frye, Esri Chief Cartographer
This blog entry addresses one of the new features in the ArcGIS mapmakers’ suite of tools. At version 9.2, we added the ability to store representations in your geodatabase; this has wide ranging possibilities for map makers and data publishers. You will definitely be seeing more blog entries about other uses of representations in the coming months.
A representation is data stored in a feature class and displayed as a layer’s symbology (figure 1). A representation can be a rule for symbolizing a class of features, or an override for symbolizing or modifying the symbol of a particular feature.
Figure 1. When you right click on a layer in the table of contents you can convert the symbology to represntsatons
This opens a number of doors in terms of managing how you share and publish data. The condensed version of the story for thematic maps is that you can now store, in your geodatabase, the exact symbology for a layer of thematic information to be used in a map. In fact, you can store as many such symbol specifications as you need. You can then either instruct people using your data to choose a specific representation for a given purpose, or provide them with layer files that already have that symbology assigned.
If it is important for you to avoid allowing consumers of your data to make uninformed decisions about how to present your data, then you will find that representations allow you: 1) provide the exact portrayal of data that you want people to see, 2) share only that portrayal when you share the database. Not only do representations capture your symbology, they allow you to make and store any exceptions to the normal cartographic rules that you as the map maker, or person most familiar with the data, feel are needed in order to convey the message your data carries.
How is this done? First you symbolize your data layer in ArcMap as you normally would, setting the classification, no data, definition query, etc. Then you convert the layer’s symbology to a representation which will store the symbology in your geodatabase as a representation. For the times when you need to make some exceptions, feature representations make this easier to manage. Instead of editing your data, which you may not have permission to do anyway; you edit the representation of individual features as needed.
Representations store the visual depiction of the data and that means you can remove the data field(s), to keep your raw data from being misused. This can also help if you need to protect the privacy of individuals or organizations that the data relates to, even if you are also mandated to make the information in the data public.
In many cases, thematic maps depict the results of analyses. At times, some of the analysis is done only in the ArcGIS user interface where data classification decisions are made. With representations, you are not only able to save the resulting symbology as representations, you can save alternative, contrasting, or contextual supporting views of the data as well because each of these can be stored as individual representations as well. The use and viewing instructions for such data are simple—just direct users to choose from the list of representations you’ve created to see what you want them to see. By storing multiple representations of your data, you are effectively able to store a statistical atlas in your geodatabase.
Here are some tips for setting up more than one or a set of representations for a dataset:
- Before creating any representations, make and symbolize the layers for all the different depictions of the data you will be using; this will help you to name the representations well, particularly if they are closely related to one another.
- Change the name of the representation from the default when you are creating the representation. Make it a useful name; something you would like to see as a layer name, though no special characters or spaces are allowed. See the images below to see where to change the name (top red ellipse in figure 1). Useful names in this case are clearly recognizable in the first few characters, see the last image where the names are just barely distinctive in the layer properties symbology tab.
Figure 2. The Convert Symbology to Representation dialog box
- Change the name of the representation field (bottom red ellipse in figure 1) to be a condensed form of the representation name—it may be obvious to you, as you’re creating the representations, but a month later the default values may not make sense.
- Name the classes within your data classification well, or at least format the label (figure 3). Those labels will be come the representation rule names.
Figure 3. The labels taaht will become the representations rule names