how is the real world represented in gis?

0West Midlands Regional Observatory 2008

How is the real world represented in GIS?

Katy BregazziWest Midlands Regional Observatory


Synopsis


Linking data to a GIS

Sources of data

Layers


How is the Real World Represented in GIS?

Map Objects:

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


‘Objects’ in Maps



Point



Line



Area



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.

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).


‘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

Area

Snow Hill Stn.

Point



Some Examples:

Points Lines AreasAccident Locations; Signs; Bus stops

Roads; Motorways Emergency response 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

Aerial Photographs

Satellite Imagery

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

Scanned maps and images

Vector Data

Raster Data



NetworkA 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.

SurfaceA 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.


How is the Real World Represented in GIS?

Map Objects: Other Attributes (fields)

Attributes: descriptive information that is known about the feature

TextNumbersPictures

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


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:


Sources of Data

Paper maps -Digitising-Scanning

Indirect (secondary) Data Capture

Aerial PhotographySatellite Remote Sensing

Satellite Surveying (Global Positioning Systems GPS)

Total Station Survey

Direct (Primary) Data Capture

Digital Maps (e.g. from Ordnance survey)

Corporate Databases(e.g. in Access, Oracle etc)

Incorporation of existing digital

data

GIS


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


GIS Layers

With acknowledgements to ESRI


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




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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

how is the real world represented in gis?

Education