It’s a good idea to spend a little time preparing your map for editing. You’ll be a lot more productive and save clicks if you set up your map and data before you really get in and make edits. This post is part of a series on the subject.
Choosing where to store your data
ArcGIS allows you to create and edit several kinds of data. You can edit feature data stored in geodatabases and shapefiles, as well as various tabular formats. When gathering your data in preparation for a geographic information system (GIS) project, make sure all the data you want to edit is stored in the same workspace, which is a single geodatabase or a folder of shapefiles, since you can only edit one workspace at a time. If you are still using shapefiles, consider migrating to a geodatabase, such as a file geodatabase, which provides more functionality and storage capacity with as much speed and simplicity as shapefiles. You can use the geoprocessing tools for importing and exporting data, as well as often simply copying and pasting feature classes in the Catalog tree, to get data into a geodatabase.
With the Catalog window now embedded in ArcMap, you can perform data management tasks and access your data without having to open the separate ArcCatalog application. This is useful in itself, but a few additional settings can make your use of the Catalog window even more productive. For example, the directory where you save a map document is tagged as the Home location and is always promoted to the top of the Catalog window when that map is open. Therefore, if you put the geodatabase in the same folder as the map document you are working on, you can quickly find your data in the Home location without having to navigate through the whole folder tree. Doing this also keeps your GIS project better organized since all the data, maps, and other supporting materials are in the same place. In addition, you can set your geodatabase as the map’s default geodatabase (right-click it in the Catalog window and click Make Default Geodatabase) so any outputs will be saved in that location automatically.
Choosing the projection for your data
As you compile your data, you need to consider the projections. First, you should make sure the feature classes that you will be editing all have the same coordinate system. In addition, if you have data in a geographic coordinate system, you may want to change to an appropriate local projection. This will improve accuracy when editing and make it easier to enter lengths and other measurements since values are specified in the map units of the coordinate system by default. For example, if your map uses the geographic coordinate system of WGS 1984, when you are editing, ArcMap interprets any entered values as decimal degrees because those are the map units for that coordinate system. So when you type 100 for the length of a segment, as shown below, ArcMap interprets that as 100 degrees and will likely present you with a series of error messages. On the other hand, with a projected coordinate system, the map units will be in a more useful unit, such as meters or feet. Also, a projected coordinate system is flexible because it allows you to specify distances in units other than the map units by including an abbreviation with the value; you can only enter values in the coordinate system’s map units (typically, decimal degrees, as just discussed) when working with a geographic coordinate system.
The coordinate systems of the layers also need to match the coordinate system of the data frame. If the coordinate systems of the data frame and layers are different, the layers will be projected on the fly to the coordinate system of the data frame. Projecting on the fly can be problematic because it may cause unexpected alignment issues when making edits. For example, when editing, you may digitize some lines that look like they connect to other lines. While the lines appear to be snapped to edges when projecting on the fly, the lines may be dangling when you display them in their native projection. In addition, you cannot perform shared editing of coincident features through a map topology for layers that are being projected on the fly.
To avoid all these issues, make sure you are not projecting on the fly while editing. When you have an empty data frame, it automatically takes on the coordinate system of the first layer added to it. To change the data frame’s coordinate system, right-click the data frame name in the table of contents, click Properties, then click the Coordinate System tab. In the Select a coordinate system box, you can quickly set the coordinate system of the data frame to match that of a layer in it by clicking the Layers folder and navigating to the coordinate system listed underneath one of the layer names. Since the coordinate system of the data frame and the layers will now match, the layers will not be projected on the fly.
Starting an edit session
When you are finally ready to edit your data, turn on the Editor toolbar (if it’s not already displayed), click the Editor menu, then click Start Editing. This begins an edit session, which you will end when you are done. When you start an edit session on a geodatabase workspace, you have the ability to edit all the feature classes and tables in that geodatabase at the same time. With an edit session on a shapefile folder workspace, you can edit all the shapefiles that are stored in that directory.
If you start editing in a map that contains data from more than one workspace, you are prompted to choose the workspace you want to edit. On the dialog box, click a layer at the top to select its workspace source at the bottom of the window (notice that the database symbols change color), or click a workspace at the bottom of the window to view the layers in it at the top. Once you have picked the workspace, click OK to start the edit session. Later, if you need to edit data in the other workspace, stop editing, then start a new edit session and choose that workspace. Keep in mind that you can also right-click a layer in the table of contents, point to Edit Features, then click Start Editing, which automatically starts an edit session on the entire workspace containing that layer.
Once you choose the workspace to edit, sometimes you may see another dialog box appear about problems that ArcMap encountered when you started editing. This dialog box will list the layers that are being projected on the fly, as well as any other issues such as missing licensing, layers that cannot be edited because they are read-only or inside a basemap layer, and so on. You can double-click each message to open a help topic with more information.
For more on the Catalog window, projection considerations, and edit sessions in ArcGIS 10, see the following:
What’s new for accessing your data in ArcGIS 10
About editing data in a different projection (projecting on the fly)
About edit sessions