Data Editing & Topology Building in GIS

Explanation

Raw digitizing produces geometric inaccuracies. If an operator digitizing a road junction misses the intersecting line by two pixels, network routing algorithms fail because the roads are disconnected in the database. Topology enforces explicit mathematical rules on vector geometry. In an arc-node topological data structure, a line shared between two forest plots is stored only once in the geometry table, with topology tables explicitly defining that Polygon A lies to its left and Polygon B lies to its right, preventing sliver polygons.

Key points

• Snapping: Automated technique that binds a cursor or vertex to existing vertices or lines within a specified snapping tolerance distance.
• Topology Definition: Mathematical study of geometric properties and spatial relationships that remain invariant under continuous transformation (deformation).
• Three Core Topological Relationships: Adjacency (which polygons share boundaries), Connectivity (which arcs connect at nodes), and Containment (which features lie inside another polygon).
• Benefits of Topology: Ensures data integrity, prevents overlapping cadastral parcels, eliminates redundant shared boundary storage, and enables network routing.

Example

Code examples

/*
Arc-Node Topological Tables Representation:
Arc Table:
Arc_ID | From_Node | To_Node | Left_Poly | Right_Poly
1      | 101       | 102     | Poly_A    | Poly_B
2      | 102       | 103     | Poly_A    | External

Polygon Topology:
Poly_A is bounded by Arcs: [1, 2, 3]
Poly_B is bounded by Arcs: [1, 4, 5]
(Note: Arc 1 is stored ONCE, completely eliminating slivers!)
*/

Exam questions

• [8 Marks] What is Topology in GIS? Explain the three fundamental topological concepts (Connectivity, Directionality/Adjacency, Containment).
• [6 Marks] Identify common digitizing errors (undershoots, overshoots, slivers, dangles) and explain how snapping tolerances resolve them.