Topology

GIS definitions (slightly different from graph theory in mathematics)

A single line shown in blue is part of three features: the river, the state boundary, and a county boundary.  The river is a line, and the boundaries could be either lines or polygons for perhaps states and counties.  Becasue the lines segments are shared, only the color from the last line drawn will be visible.

 

Ideally, the common line that appears in multiple features would be digitized and stored once, and all the features would have the exact same coordinates.  Then if the line was edited, the changes would propogate to all the features.  This is not possible with  Shapefiles, but is possible with much more complex data structures.  The simplicity of just storing and rapidly displaying the data outweighs the advantages of saving the topology, since those advantages really only come into play when editinig the data.

Rules for digitizing with topology:

 


last revised 12/4/2017