Spatial Data Editing Chapter 5

Introduction All digitizing involves errors In the real world, revisions are required Keeping data up-to-date is part of the job Types of errors location mechanical In order to edit, must know the structure Accuracy and metadata

Type of Digitizing Errors Location errors – by source, by human hand, or machine: not at correct location Check plot at same scale to visually note errors Standard for accuracy by Natural Resources Conservation Service says 0.01 inch line width of map. At 1:24,000 represents 20 feet on ground (6-7 meters)

Type of Digitizing Errors Human errors Scanning, tracing, follow wrong line Conversion or transformation errors (generally show in a pattern) Duplicate lines Slivers (tiny polygons) created

Topological Errors Arcs not meeting undershoot and overshoot Result is dangling arc/node Same if a polygon is not closed. Symbol is □ square. Dangling nodes can be acceptable (small streams, dead end streets) Pseudo node, symbol is ◊ diamond

Topological Errors Pseudo node when not at line end or intersection Symbol is ◊ diamond May be acceptable to show changes in attribute date, or start of isolated polygon. Direction of arc by from-node and to-node. Can be reversed by ARC/INFO

Topological Errors Polygon is linked to attribute data by label point. Symbol is + plus sign Multiple label points are not acceptable and indicate an error

Topological and Non-topological Editing Each vendor has system Topological have means for visualizing and correcting errors Some non-topological vendors provide assistance in spatial relationships Non-topological can digitize and edit, but cannot build topology

Topological Editing Steps Construct topology Note errors visually Correct errors Rebuild topology Normally an iterative process

Correcting Digitizing Errors Global or local Global uses tolerances for CLEAN dangle length fuzzy tolerance CLEAN removes overshoot and undershoots if less then the dangle length CLEAN inserts nodes and enforces arc-node relationship

Correcting Digitizing Errors CLEAN removes vertices less than fuzzy tolerance CLEAN can remove undershoots if the dangle length is too large CLEAN can snap node if too large Any action by CLEAN (or BUILD) can reduce the accuracy of the map

Correcting Digitizing Errors Local tolerances nodesnap editdistance Must be “just right” Snapping moves and distorts arc Set by value or by interactive Zoom in

Correcting Digitizing Errors Tools for local editing UNDO or OOPS can reverse, one action at a time EXTEND an arc for undershoot SPLIT an arc inserts a node Select and delete overshoot Select and delete duplicate arcs, but which? FLIP to change arc direction UNSPLIT to remove pseudo nodes Add label points or close polygon VERTEX command to reshape line

Edgematching Quad to quad transition Edgematch to join together Usually requires zoom Interactive process to “connect” components Use DISSOLVE to remove the boundry

Non-topological editing Can be “heads-up” Easier to use because can delete move cut paste Reshape lines and polygons by drag and drop of vertices

Non-topological editing Vertex edit can add new vertices Split and merge Can create variations not possible in topological data overlapping doughnut

Other types of map feature manipulation Line simplification Douglas-Peucker algorithm Line Densification why? Line smoothing mathematical SPLINE Transfer (and copy) of features GET and PUT