Vectors and Raster's

Spatial entity

A spatial entity is a representation of the real world

entities are the object

What are the problems with spatial entities

Scale: how the real world is modelled depends on the scale of the modelling (generalisation)

Dynamics: the world is not static, rivers flood, cities expand- things are always changing

Fuzziness problem: no distinct boundary, think trees in a forest coming to an end

Purpose: how to represent the real world depends on the purpose of the entity data

Can a curved line exist in GIS?

No

How are vector ‘lines’ created?

From joining up two coordinate points

How does rasters represent the world?

the surface of the world is divided into a grid of cells with different values

Discrete raster

raster that represent phenomena that have clear boundaries with attributes that are descriptions, classes, or categories. Generally, (but not always) integers are used for the cell values. It is assumed that the phenomena that each value represents fill the entire area of the cell. eg, land-cover

Continious raster

Raster in which cell values vary continuously to form a surface. Values exist on ascale relative to each other. It is assumed that the value assigned to each cell is what is found at the centre of the cell. eg, elevation, rainfall, temperature

gradual change, things closer together are more likely to be related

Examples of single bands (RASTER)

SINGLE BAND

▪ TIFFs, IMGs ▪ Remotely Sensed Data ▪ Spectral Data

-Digital elevation model, attribute of every cell is always the elevation

Example of multiple bands (RASTER)

MULTIPLE BAND

▪ Elevation Data ▪ Grayscale aerial imagery ▪ Scanned maps / documents

What is a node?

Location of each vertex coordinate

Vector data model

A representation of the world using points, lines, and polygons (area). Vector models are useful for storing data that has discrete boundaries, such as country borders, land parcels, and streets.

Basic types (VECTOR)

▪ Points ▪ Lines or ‘Polylines’ ▪ Area or ‘Polygons’

First and last are the same

Extended types (VECTOR)

▪ Surfaces ▪ Networks

Surface map

Types of vector data

Attribute tables, raster vs vector

in raster everything on the screen is in the attribute table (doesn’t have that flexibility) in vector it is linked to an attribute table, every layer is a point in a table

Arc-node topology

Line features can share endpoints

Region topology

Area features can overlap with other features

Polygon topology

Area feature can share boundaries

Node topology

line features can share endpoint vertices with point features

Route topology

Area features can share segments with other line features

Point events

point features can share vertices with line features

What will points connected create in vectors?

Triangles - Delaunay triangles, in a Voronoi cell

What is TIN

Triangular irregular networks

Identifies spatial relationship between spatial entities

▪ Well suited for adjacency and connectivity analyses

▪ Reduces data redundancy (all the data are useful)

▪ Allocates IDs to polygons

NOTE Digital Elevation Models (DEMs) in a TIN format have advantages over DEMs in raster format (e.g. memory allocation and speed)

harder to use

Problems with converting vectors/rasters

converting may not be accurate

Hanging nodes may arise through automation errors

Recap Raster table

Recap vector table