Line Digitizing

Below is an example showing how to use the Screen Digiting Wizard to digitize lines in a shapefile. After pressing the OK button in the wizard's last panel, a Line Digitizing Menu opens for digitizing the lines in the shapefile. Other types of Line Digitizing Menus are possible, depending on whether key field values are to be manually entered or selected from a table. This decision is made in the wizard and will determine the type of menu used to digitize the lines.

Screen Digitizing Wizard

1. Specify the Key Field in the first wizard panel. If you need to know the values already in the key field you have chosen, use the List Values check box to display them in the menu. This is a read only list for reference purposes.

   

2. In the second panel, specify how the key field identifies features in the shapefile. For line shapefiles, there are three options - Unique, Not Unique, or Unique Multi-Part. Unique only allows you to add lines with key field values that are not already in the shapefile. All lines are uniquely identified. Not Unique allows you to add more than one line with the same key field value (duplicates allowed). And Unique Mult-Part is for adding complex lines made of more than one separate segment part. Shown below is the default setup, which assigns unique key field values to lines added to the shapefile and allows them to overlap other lines in the shapefile.

   

   How does the Key Field identify features?

   • Unique - Key field uniquely identifies single part lines. One shapefile record per line.
   • Not Unique - Key field is not unique and identifies single part lines. One shapefile record per line. Duplicate key field values are allowed so more than one line may have the same value. Think road sections.
   • Unique Multi-Part - Key field is unique and identifies multi-part lines. One shapefile record per multi-part line. A multi-part line is screen digitized one part at a time. Each part can overlap another part, but they never self intersect or intersect one another and split into segments. Together, they form a single entity. Think different sized spaghetti noodles.

   How are lines structured?

   • Segment - All lines intersect. This option is only available for single part lines. Each segment is a separate shapefile record.
   • Overlap - All lines overlap (no intersections)

3. In the third panel, indicate how you want to specify key field values when digitizing. There are three options for lines. You can (1) enter the value, (2) enter a (new) value or select from values already in the shapefile, or (3) select values from records in a lookup table. The Enter Value option was chosen in the example below, which is the default setup for lines.

   

4. In the fourth panel, indicate whether you want Arcview to index the key field and shape field. The wizard reads the shapefile to determine whether these fields are indexed and displays their state in the check boxes. If they are checked, the fields are indexed. Unchecking them removes the index and checking them adds the index. Additionally, you can specify an appropriate scale range for screen digitizing if the view's map units are set in advance. This is optional. If you specify a scale range, the screen digitizing menu will be usable only when viewing within this range. This scale range does not affect the theme's display scale range. Below the screen digitize scale range is set for viewing between 1:6,000 and 1:24,000 scale.

   

Line Digitizing Menu

A line digitizing menu appears when you are finished setting up the theme and you press the OK button in the wizard's last panel. In the example below, notice the menu indicates that the key field (MyLine_Id) is unique. To digitize a new line, enter a MyLine_Id in the menu and then use the Add tool. The MyLine_Id value you enter must be unique, otherwise the Add tool won't add the line. There are other tools for moving, splitting, and deleting lines, as well as for changing their key field values while preserving uniqueness.

	Buttons
Find	Opens a menu for finding a line according to its key field value
Undo	Undo last screen digitizing edit
Undo All	Undo all screen digitizing edits since last save
	Tools
Identify	Provides information about the line you click
Add	Adds a line and assigns it the key field value entered in the menu
Delete	Opens a menu that prompts to delete the line selected with this tool. Drag a rectangle to select the line. When screen digitizing multi-part lines, you can drag a rectangle over just one part to delete just that part or all parts in the feature.
Move	Moves a line (overlapping lines only)
Split	Splits lines (unique and not unique lines only)
Change Key Field	Opens a menu that prompts you to change the key field value of a line you selected with this tool. Drag a rectangle to select the line. When screen digitizing multi-part lines, you can drag a rectangle over just one part to change the key of just that part or all parts in the feature.
Markers	Draws temporary markers for planning where to digitize

About Unique Key Fields

If the key field is unique, the Add, Split and Change Key Field tools enforce uniqueness and always prompt for values that are not already in the shapefile. This requirement is strictly enforced if the line structure is segment (as opposed to overlap) and you've added a line that intersects other lines. The new line splits the other lines into segments and consequently splits itself into segments. The Add tool prompts for unique key values for each of these segments.

Other Line Digizing Menus

Below are examples of the other types of line digitizing menus that the wizard makes possible. They offer different ways of assigning key field values to lines that are added to the shapefile as you screen digitize.

Enter or Select -

Enter a key field value or select one already in the shapefile and then digitize the line.

Lookup Table -

Select a record in a data table and then digitiz the new line.

Line digitizing Examples

The following examples illustrate what happens when you use the Add tool in the Line Digitizing Menu to add new lines under different setup conditions specified in the wizard. A single line is added in each example, but the results differ depending on whether key field values are unique or not unique, whether single or multi-part lines are desired, and whether or not lines overlap or intersect. Each example shows what the lines would look like in the view before and after adding the new one. Also, each one includes the resulting table records, which highlights some of the differences in screen digitizing overlapping, intersecting, single, and multi-part lines.

Unique key field values assigned to Overlapping lines:

Add line 1

Add line 3

Add line 89

Not Unique key field values assigned to Overlapping lines:

Add line 1

Unique key field values assigned to intersecting lines (Segments):

Add line 3 (splits line1 and 3 into 1, 3, 26, and 65)

Not Unique key field values assigned to intersecting lines (Segments):

Add line 3 (splits line 1 and line 3 into 1,3,3,1)

Unique key field values assigned to Multi-Part lines that Overlap:

Add a part to line 1