Understanding Geographic Data: The Role of Data Models, Data Structures, and Topology, Slides of Geology

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The Nature of Geographic

Data

Based in part on Longley et al.

Ch. 3 and Ch. 4 up to 4.

(Ch. 4 up to 4.6 to be covered in Lab 8)

Library Reserve #VR 100

Bears are easily conceived as discrete

objects, maintaining their identity as

objects through time and surrounded by

empty space.

(Hal Gage/Alaskastock/Photolibrary Group Limited)

Example of representation of geographic information as a table. The locations and attributes are for each of four grizzly bears in the Kenai Peninsula of Alaska. Locations, in degrees of longitude and latitude, have been obtained from radio collars. Only one location is shown for each bear, at noon on July 31, 2000.

The discrete object view leads to a

powerful way of representing geographic

information about objects

Vectors (Arcs) and Topology

Vectors without topology are “spaghetti”

structures.

Points, lines, and areas

stored in their own files, with links between

them.

stored w/ topology (i.e. the connecting arcs

and left and right polygons).

Relationships are computed and stored

A

C B

1

2

3

4

5

0

D

6 a

b

c

d

e 7

Arc ID

Left Poly

Rt Poly

From node

To node

1 A 0 c a

2 A B b c

3 C A b a

4 0 C d a

5 C B d b

6 B D e e

7 B 0 d c

Poly

ID

No. of

arcs

List of

arcs

A 3 -1, -2, 3

B 4 2, -7, 5, -

C 3 -3, -5, 4

D 1 6

2, -7, 5, 6

Connectedness, Adjacency, Contiguity,

Geo-Relational

Why Topology Matters

Topological data structures very important in

GIS software.

Allows automated error detection and

elimination.

“Tolerances” important - features can move or

disappear

“snapping”, elimination, merging, etc.

Makes map overlay feasible.

Makes other kinds of spatial analysis possible.

Nodes that are close together are snapped.