Topology Exercise

Oftentimes you will have to work with ESRI "coverage" and "geodatabase" data structures as you use ArcGIS (they are based on what ESRI calls the "coverage" and "geodatabase" data models, variations on the general object data model). ArcGIS uses a specific format to calculate and store spatial relationships. This exercise tests your understanding of how ArcGIS stores this spatial information, and hence establishes topology.

Lines are stored as arcs. Each arc starts and stops at a node. Because the arc starts and stops at a node, the arc has direction, defined by which node is the from node (FNODE) and which node is the to node (TONODE). Consider the following sketch of arc-node topology, and fill out the table that follows (nodes are indicated by boxed numbers, arcs by unboxed numbers).

ARC
From Node
To Node
1
5
1
2

3

4

5

6

7

8

Arcs form the boundaries of polygons. The topology of a polygon coverage contains both an ID# for each arc and the identification of the polygon bounded on the right (RPOLY) and left (LPOLY) of the arc. This topology is created when a coverage is cleaned or built (e.g., CLEAN <cover>, or BUILD <cover> in Workstation ArcInfo). Consider the following sketch of polygon-arc topology, and fill out the table that follows (polygons are indicated by boxed numbers, arcs by unboxed numbers).

Also, consider arc ID#1. How does this arc turn two corners without have nodes at these corners?? __________________

POLY
# of ARCS
ARCS
2
1
7
3

4

5

ARC
Left POLY
Right POLY
1
1
3
2

3

4

5

6

7

http://dusk.geo.orst.edu/arc/topology_ex.html