How is the real world represented in GIS?

How is the real world represented in GIS?

Katy Bregazzi
West Midlands Regional Observatory

0
West Midlands Regional Observatory 2008

Synopsis


Linking data to a GIS

Sources of data

Layers

1

How is the Real World Represented in GIS?

Map Objects:

- Point
- Line
- Area
- Other vector geometry (network, surface)
- Grid (raster/image)

2

‘Objects’ in Maps

3


Point

4


Line

5


Area

6


Co-ordinate Geometry:
- Shape and location can be defined in terms of co-ordinate pairs from a given grid
system (e.g. British National Grid).

Point
- An object that occurs in one physical location in space.
- Defined by a single pair of co-ordinate values.

Line
- An object which spans between two or more points.
- Defined by a sequence of co-ordinate pairs defining each point through which the
line is drawn.

Area (or Polygon)
- An object which has area.
- Defined by a sequence of co-ordinate pairs, with the first and last points joined to
make a complete enclosure.

7

‘Objects’ in Maps – effects of scale

Depending on the scale of the map and its intended purpose, ‘real life’
things can be represented by different object types

- Railway station

Point Area

Snow Hill
Stn.

8


Some Examples:
Vector Data
Points Lines Areas
Accident Locations; Roads; Motorways Emergency response
Signs; Bus stops districts; Parking lots
Port locations Rivers; Streams; Canals Lakes; Harbours;
Wetlands
Rail crossings Rail tracks Rail corridors

Utility poles; Pylons Utility lines: pipes/cables Utility service areas

Picnic sites; toilets Footpaths; cycle routes Parks; Forests; Fields

Raster Data
Aerial Photographs

Satellite Imagery

Ordnance Survey background maps e.g. 1:50K

Scanned maps and images

9


Network
A feature defined by a series of line segments connected to form a
continuous branching system of links.

Networks enable the calculation of optimal routes through road networks,
or the simulation of flow through rivers or pipes.

Surface
A feature which requires three dimensions to define it. A series of
coordinate pairs define the surface, each pair with a vertical ‘z’ value.

The ‘z’ value may represent: height, rainfall, population density etc.

10

How is the Real World Represented in GIS?

Map Objects: Other Attributes (fields)

Attributes: descriptive information that is known about the feature

Text
Numbers
Pictures

River; Name, Length, Chemical Quality, Biological Quality

Road; Name, Length, Classification (e.g. A or B road), Speed limit

This extra information within a GIS enables some very powerful
interrogation/analysis of the data

11

Linking Existing Data to a GIS

Can incorporate simple spreadsheet files or more complex relational
databases

GIS links with the data through the geography/location field/column
e.g. Postcode, Region, SOA

Example of joining two tables:

12

13

14

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Sources of Data

Direct (Primary)
Satellite Surveying
Data Capture (Global Positioning Systems GPS)

Total Station Survey

Satellite Remote Sensing
Aerial Photography

Indirect (secondary)
GIS
Data Capture

Incorporation of
existing digital
data Paper maps
-Digitising
Corporate Databases
-Scanning
(e.g. in Access, Oracle etc)
Digital Maps
(e.g. from Ordnance survey)

16

Sources of Data for Use in GIS

Getting hold of digital geographic data can be difficult:
- Create it Yourself (field survey, digitise, scan image etc)
- Buy it from other organisations
- Web services (e.g. ONS)
- GIS Software package - may include some basic free data
- Obtain specialist GIS data products (e.g. OS digital products;
mastermap, landline etc)

Consider: availability, existing agreements, time, costs, quality,
purpose, data conversion, file sizes & performance, copyright

Much existing data held by organisations contains location and is
therefore able to be mapped/used in a GIS

17

GIS Layers

With acknowledgements to ESRI

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GIS Layers

Map Objects (Records) Stored in GIS Layers (Files)

- Open any combination in any order (including raster and vector)

- Control style, labels, visibility etc

- Temporary drawing/cosmetic layer

- Combine layers for display, query and analysis

19

Further Information

‘Useful Websites’ Handout
- General information on GIS

‘Examples of some existing internet GIS tools’ handout
- Links to examples of national/regional/local scale web-GIS tools

25

How is the real world represented in GIS?

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