GEO 465/565 - Lecture 5
Maps as Numbers - Part 1

Maps as Numbers

The GIS places data into the computers memory in a physical data structure (i.e. files and directories).

Files can be written in binary or as ASCII text.

Binary is faster to read and smaller, ASCII can be read by humans and edited but uses more space.

The Data Model

entities ---> objects

GISs have traditionally used either field or object data models for maps and the flat-file data model for attributes.

Attribute data

i.e., matrix of numbers and values stored in rows and columns, like a spreadsheet

DBMSs use many different methods to store and manage flat files

Rasters and vectors can be flat files ... if they are simple - GRAPHIC

A field data model uses a grid data structure.

Every cell has a value, even if it is missing.

A cell can hold a number or an index value standing for an attribute

A cell has a resolution, given as the cell size in ground units.

Generic structure for a grid - GRAPHIC

The mixed pixel problem - GRAPHIC

Run Length Encoding

nearby things tend to be more similar than distant things

data entered as pairs

run length & value

40 items instead of 70

Rasters are faster...

Good for representing continuous surfaces.

Rasters are a natural for scanned or remotely sensed data

Rasters are easy to understand, easy to read and write, and easy to draw on the screen.

Spatial analytical operations are faster

Compression is easier using techniques such as Run Length Encoding and quadtrees

But...

Points and lines in raster format have to move to a cell center

Lines can become fat. Areas may need separately coded edges

Good only at very localized topology, and weak otherwise

Suffer from the mixed pixel problem

Must often include redundant or missing data

Each cell can be owned by only one feature

The Vector Model

Lines are built from sequences of points in order.

Lines have a direction according to the ordering of the points.

Polygons can be built from points or lines.

Object data model gave rise to the arc/node data structure in the 1960s

The endpoint of a line (arc) is called a node. Arc junctions are only at nodes

Vector Data Structure Featuring Arcs and Nodes - GRAPHIC

Arc-Node Combo Eliminates Redundancy of Storage - GRAPHIC

Vectors and Topology

Points, lines, and areas can each be stored in their own files, with links between them

Stored with the arc is the topology (i.e. the connecting arcs and left and right polygons)

Relationships are computed and stored

Topology

Arc/node map data structure with file - GRAPHIC

Topology, GIS, and You

Rarely are maps topologically clean when digitized or imported

A GIS has to be able to BUILD topology from unconnected arcs

The tolerances controlling snapping, elimination, and merging must be considered carefully, because they can move features

Nodes that are close together are snapped - GRAPHIC

Slivers due to double digitizing and overlay can be eliminated - GRAPHIC

The bounding rectangle - GRAPHIC

Why Topology Matters

Complete topology makes map overlay feasible

Topology allows many GIS operations to be done without accessing the point files

Topology makes spatial analysis possible

"Rasters are faster, but vectors are correcter"

Vectors are far more efficient than grids

Vectors work well with pen and light-plotting devices and tablet digitizers

Vectors are not as good with continuous coverages or plotters that fill areas

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