Topology Exercise Key

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 (TNODE).... 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
1
2
3
2
3
4
1
4
5
6
3
6
3
7
7
3
8
8
2
9

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.... 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?? The corners in Arc 1 are vertices that define the shape of the arc, rather than starting/ending nodes that would constitute new arcs.

POLY

# of ARCS
ARCS
2
1
7
3
4
1, -4, -5, 7
4
3
3, 4, -6
5
3
2, 5, 6

ARC

Left POLY
Right POLY
1
1
3
2
1
5
3
1
4
4
3
4
5
3
5
6
4
5
7
3
2


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