Most digitizing methods entail human interpretation of geographic features, there are a variety of inaccuracies and digitizing errors that might arise throughout the data collection process.
A positional error happens when a feature is not recorded properly, as opposed to attribute errors, which occur when information about the feature capture is inaccurate or untrue.
These forms of positional errors are described below, with a visual representation of the various methods at the bottom of the page.
- Dangles or Dangling Nodes
Lines that are not linked but should be are referred to as dangles or dangling nodes. There are gaps in the linework where the two lines should be joined when there are dangling nodes.
Dangling nodes can also happen when a digitized polygon fails to link back to itself, leaving a gap where the two end nodes should have connected, resulting in an open polygon.
2. Switchbacks, Knots, and Loops
These errors occur when the digitizer moves the cursor or puck in such a way that the line being digitized ends up with excess vertices and/or nodes due to an unstable hand.
Extra vertices are introduced in the case of switchbacks, resulting in a bend in the line. The line folds back on itself with knots and loops, forming little polygon-like geometry known as odd polygons.
3. Overshoots and Undershoots
Overshoots and undershoots, like dangles, it is digitizing errors occur when the digitized line does not connect properly with the nearby line it should intersect with. The digitizer sets a snap tolerance during digitizing.
The snap tolerance, also known as the snap distance, is a measurement of the diameter extending from the cursor’s point of origin. Any nodes of nearby lines that come within the snap tolerance circle will have the line’s end points digitized automatically snapping to the nearest node.
When the snap distance is not set or is set too low for the scale being digitized, undershoots and overshoots occur. If the snap distance is set too high, the line terminus will snap to the incorrect node.
Slivers are holes in a digitized polygon layer that exist between adjoining polygons. Again, ensuring that the edges of adjoining polygons snap together to eliminate gaps requires setting the proper snap tolerance parameters.
The area where two adjacent polygons overlap in error is referred to as a sliver.