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