GEO 265
The Arc Data Model & Topology

Topology - 1

problem of common boundaries
dont want to repeat coordinates for a point shared by a number of lines, or a line shared by 2 polygons
arc-node structure is more efficient
boundary is stored only once
nodes construct arcs, arcs construct polygons
NODE - defines 2 endpoints of an arc
ARC - line segment between 2 nodes

Topology - 2

ARC - line segment between 2 nodes
is composed of its 2 nodes and a series points which define its shape
these points that define the shape of the are called VERTICES
Topology is how the computer expresses the relationship between arc and node
Expression in form of lists or tables
Data stored more efficiently
Analysis easier (e.g., identifying overlaps, tracking flow through connecting lines, etc.)

Major Topological Concepts

CONNECTIVITY - arcs connect to each other at nodes
AREA DEFINITION - arcs that connect to surround an area define a polygon
CONTIGUITY - arcs have a length and direction; arcs have a left side and a right side
ARC-NODE or POLYGON-ARC topology

Topological Commands

BUILD - builds a feature attribute table which expresses either arc-node or polygon-arc topology
CLEAN - makes sure the table is correct
assembles arcs into polygons
creates feature attribute information for each polygon
puts nodes at intersections of arcs if they arent there
uses a fuzzy tolerance (resolution or minimum distance between vertices in an arc)
uses dangle length to snip off little arc fliers

Additional Helpful Notes

Notes above from ESRI's ArcDoc Module
Last updated 1 October 1997

http://dusk.geo.orst.edu/arc/265_2.html

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