Polygon Digitizing

Below is an example showing how to use the Screen Digiting Wizard to digitize polygons in a shapefile. After pressing the OK button in the wizard's last panel, a Polygon Digitizing Menu opens for digitizing polygons. Other types of Polygon 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 polygon shapefiles, there are three options - Unique, Not Unique, or Unique Multi-Part. Unique only allows you to add polygons with key field values that are not already in the shapefile. All polygons are uniquely identified. Not Unique allows you to add more than one polygon with the same key field value (duplicates allowed). And Unique Mult-Part is for adding complex polygons made of more than one polygon part. Shown below is the default setup, which assigns unique key field values to polygons added to the shapefile and ensures that they are contiguous and do not overlap one another.

   

   How does the Key Field identify features?

   • Unique - Key field uniquely identifies single part polygons. One shapefile record per polygon. Think moon craters. Each one is different.
   • Not Unique - Key field is not unique and identifies single part polygons. One shapefile record per polygon. Duplicate key field values are allowed so more than one polygon may have the same value. Think beaches. Differently shaped sandy areas along the coast.
   • Unique Multi-Part - Key field is unique and identifies multi-part polygons. One shapefile record per multi-part polygon. A multi-part polygon is screen digitized one part at a time. Together, they form a single entity. Think Hawaiian Islands. They can be contiguous to or overlap other multi-part polygons.

   How are polygons structured?

   • Contiguous - Polygons abut one another like floor tiles or countries of the world.
   • Overlap - Polygons overlap like floating lily pads

3. In the third panel, indicate how you want to specify key field values when digitizing. There are three options for polygons. 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 polygons.

   

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.

   

Polygon Digitizing Menu

A polygon 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 (MyPoly_Id) is unique. To digitize a new polygon, enter a MyPoly_Id in the menu and then use the Add tool. The MyPoly_Id value you enter must be unique, otherwise the Add tool won't add the polygon. There are other tools for cutting out (removing) portions, splitting, and deleting polygons, as well as for changing their key field values.

	Buttons
Find	Opens a menu for finding a polygon 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 polygon you click
Add	Adds a polygon and assigns it the key field value entered in the menu. Use the Add tool to enlarge an existing polygon by digitizing partially inside and outside a polygon whose key field value is displayed in the menu. Also use the Add tool to digitize the parts of a multi-part polygon.
Delete	Opens a menu that prompts you to delete the polygon clicked on with this tool. When screen digitizing multi-part polygons, you can delete just the one part of the polygon you clicked on or all parts that make up the feature.
Cut Out	Removes interior portions or trims off the edges of polygons based on a polygon shape drawn with this tool.
Split	Splits polygons and prompts for a key field value for each one split
Change Key Field	Opens a menu that prompts you to change the key field value of a polygon you clicked on, When screen digitizing multi-part polygons, you can change the key of just the part you clicked on 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. If the key field is Unique Multi-Part, the Add, Split and Change Key Field tools always union polygon parts into their respective multi-part shapefile records. If a the key field value is new to the shapefile, a new record is added to store all parts having that value.

Other Polygon Digizing Menus

Below are examples of the other types of polygon 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 polygon.

Lookup Table -

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

Polygon digitizing Examples

The following examples illustrate what happens when you use the Add tool in the Polygon Digitizing Menu to add new polygons under different setup conditions specified in the wizard. A single polygon 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 polygons are desired, and whether polygons overlap or are contiguous. Each example shows what the polygons 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, contiguous, single, and multi-part polygons.

Unique key field field values assigned to Contiguous polygons:

Add polygon 1

Add to (expand) polygon 1

Add polygon 3

Add to (expand) polygon 1

Add to (expand) polygon 1 (Overlapping polygon example)

Select to Protect polygon 3 and then Add to (expand) polygon 1

Cut Out interior portion of polygon 1

Cut Out edge of polygon 1

Select to Protect polygon 3 and Cut Out polygon 1

Not Unique key field values assigned to Contiguous polygons:

Add polygon 1

Add to (expand) polygon 1

Add another polygon 1

Add another polygon 1

Add to (connect) two polygons and form a single polygon

Unique key field field values assigned to Multi-Part polygons that are Contiguous:

Add polygon 1

Add to (expand) polygon 1

Add another part to polygon 1

Add another part to polygon 1

Add to (connect) two parts in a multi-part polygon to form a single part