Screen Digitize

Description

Screen Digitize is an ArcView extension that includes special tools for digitizing shapefiles on the screen with a mouse. With this extension, you can screen digitize point, line and polygon shapefiles that include a key field. A key field is a field reserved for storing values (usually numbers) that identify individual features in the shapefile. The Screen Digitize extension will operate based on the values stored in the key field. Typically, a shapefile only has one key field and the Screen Digitize extension requires that it be either a numeric (integer) or character field. Date, Boolean and numeric fields with decimal places are not acceptable formats for key fields. In the example below, the shapefile includes a key field named Well_Id that uniquely identifies points in the shapefile. In this shapefile, each point is a well with a unique Well_Id that matches a value in a data table with information that describes each well. The Well_Id establishes a one-to-one relationship between the points in the shapefile and the records in the separate data table. Notice there are no additional fields in the shapefile. Just the key field. Screen digitizing simply involves adding, moving, and deleting shapefile features and specifying their corresponding key field values.

Typically, the key field uniquely identifies features in the shapefile and maintains a one-to-one relationship with records in a data table. However, a shapefile with a non-unique key field is often used to classify point, line or polygon features into discrete categories when information about individual features is irrelevant. In this case, the key field is the basis of a coding scheme that describes sets of related features rather identifying individual ones. For example, if you were mapping areas where sand and gravel deposits exist, you would certainly uncover many areas of sand, gravel, sand overlaying gravel, etc. Rather than uniquely identifying each deposit, you might use the key field to store a numeric code that describes each type of deposit (for example, 1 = sand, 2 = gravel, 3 = sand overlaying gravel, and so on). These numbers match values in a data table as shown in the following example. As the example below illustrates, there is a many-to-one relationship between the polygon features in the shapefile and matching key field records in the data table. This type of table is often referred to as a lookup table. You can use lookup tables with the Screen Digitize extension to help you add new points, lines and polygons.

The Screen Digitize extension maintains either unique or non-unique key field values in a shapefile, depending on your need. For line and polygon features, there is an additional option to maintain multi-part features based on unique key field values.

Getting Started

In ArcView, add the shapefile you want to screen digitize to a view and make sure it includes a key field. Make the shapefile editable in ArcView by highlighting it in the table of contents and going to Theme|Start Editing. If you want to screen digitize a new shapefile, go to View|New Theme and make the shapefile. With the new theme editable, go to Theme|Table to open its table. Then go to Edit|Add Field to add a key field, assigning it a name, type, and field width. Be sure to assign a meaningful name to the key field. The name should be no longer than ten characters and should not include spaces. Close the table and return to the view with the editable theme. You are now ready to screen digitize.

Screen Digitize Wizard

Load the Screen Digitize extension in ArcView. Make sure the shapefile you want to screen digitize is the one that is currently editable in ArcView. If not, highlight the theme in the table of contents, and go to Theme|Start Editing. Then go to Theme|Screen Digitize Wizard to set up the edit theme for screen digitizing. The Screen Digitize wizard prompts you to specify the key field, whether the key field values are unique, how to enter key field values when screen digitizing, and so on. After using the wizard, a Digitize menu opens and it includes tools for screen digitizing the theme. As long as the theme remains in the view, the wizard remembers how you set it up for screen digitizing. This means you can re-open the wizard to modify settings for the current edit theme or re-establish settings for a previously screen digitized theme and digitize it again. This makes it easy to switch back and forth between edit themes. Note, the wizard's settings are not permanently saved with the project. To stop screen digitizing a theme, press the Stop button at the bottom of the wizard menu. This removes the settings for that theme from the wizard's memory.
Point Digitizing

Line Digitizing

Polygon Digitizing

Author Information

Howie Sternberg
State of Connecticut
Department of Environmental Protection
Environmental and Geographic Information Center
79 Elm Street
Hartford, Connecticut 06106 USA

Telephone: 860-424-3594
Fax: 860-424-4058
Email: howard.sternberg@po.state.ct.us

July, 1999