1. Chapter 12
Basic Editing

2. Outline
What is editing
Maintaining logical consistency

3. Basic editing tasks
Digitize new features from a paper map or scanned map.
Construct new features from survey descriptions.
Map areas or objects based on air photos or satellite imagery.
Update features that have changed since they were created, or fix errors.
Update or correct attributes of features.

4. Logical consistency
Logical consistency measures how well features in the data set mimic the relationships of features in the real world.
Basic editing is concerned with two aspects of logical consistency.
Make sure that ends and corners meet at a common vertex.
Make sure that adjacent polygons share an identical boundary.

5. Maintaining logical consistency
Avoiding dangles
Automatically connects features and ensures logical consistency. Two approaches are used.
Let the vertex be placed anywhere and correct it afterwards.
Ensure that vertex is created in the right location.
Correct topology—the horizontal line intersects the vertical one, creating three lines.

6. Snapping tolerance
The snapping tolerance is the minimum distance between two vertices at which they are considered to be identical.
If the cursor gets within the tolerance of a vertex, the next mouse click will be snapped to that vertex.

7. Tolerance units Screen units Map units
User sets the tolerance as the number of pixels on the screen.
Remains consistent regardless of the zoom level.
Easy to work with at all scales
Map units
User sets the tolerance in map units (meters, feet, degrees)
Ensures consistent precision at all scales
May become difficult to work with when zoomed far in or far out from normal editing scale

8. + Types of snapping Point snapping End snapping Snapping tolerance
Edge snapping
Vertex snapping

9. Snapping for points
Make sure that sewer connections fall exactly on the house edge.
Ensure that stream gauges fall exactly on the stream.

10. Snapping for lines Make sure that road ends meet at other roads.
Ensure that stream segments connect to each other.

11. Snapping for polygons Make sure that parcels corners match.
Avoid creating spurious nooks and kinks between polygons.

12. Adjacent polygons
It is important to maintain logical consistency of adjacent polygons. It’s much easier to create than to fix later.
gap
overlap
Topological errors between two polygons
Coincident boundary between two polygons

13. Creating adjacent polygons
Two methods
Use special tool to construct adjacent polygon
Create a polygon and then cut it into pieces.

14. References Price, M. (2013). Mastering ArcGIS (6th ed.). McGraw-Hill
Price, M. (2013). Mastering ArcGIS (6th ed.). McGraw-Hill. Mastering ArcGIS, 6/e Instructor Edition Chapter 12: PowerPoint Notes and Figures