mapping projections and data conversions

Cartographic Concepts

Aim: to provide you with a basic knowledge of Cartographic Concepts

Objectives

• By the end of this session you will:

– recognise large and small scale maps

– understand level of detail that can be shown at each

scale

– understand projections and coordinate systems

– know why data in different projections need

converting before they can be used together

Map Scales

• any map is a scale representation of the Earth’s surface

• scale may be defined as:– the ratio between distance measured on the map and the

true distance on the ground

Map Scales 2

• scale may be expressed in several ways:– as a Representative Fraction

• e.g. 1:1,250; 1:250,000; 1:63,360• “One unit on the map represents 1,250 units on the

ground”

– in words• e.g. “one inch to one mile”• “One inch on the map represents one mile on the ground”

– by scale lines

100m0 50

Map Scales 3

• large scale or small scale?

• relative terms: vary by subject• no absolute definition of large and small scale

Map Scales 4

Contains Ordnance Survey data © Crown copyright and database right 2014

Map Scales 5

• … remember it’s all relative!

Map Projections

Map Projections 2

• a map projection transforms spatial relationships from a curved surface onto a flat plane using mathematical algorithms.

• projections can be divided into 3 groups:

Cylindrical Azimuthal Conic

Map Projections 3

• GB Projection– O.S. use a cylindrical projection called Transverse Mercator – Origin is 49 degrees north and 2 degrees west

• Just south of Jersey

• Height Datum– Height information expressed as a measurement above a smooth

assumed reference known as a Datum– usually Mean Sea Level at some point on coast

• GB use Newlyn, Cornwall (Ordnance Datum) • France use Marseilles• Switzerland use Lake Geneva

Referencing Systems

• to locate any point, feature or area on the Earth’s surface or a map it is necessary to have concepts and definitions of direction and distance

• three main categories of referencing system:– Geographic (applies globally)

• e.g. lines of latitude and longitude

– Rectangular (applies locally)• e.g. Ordnance Survey National Grid system

– non co-ordinate (applies locally)• e.g. Post Codes, Zip Codes

Referencing Systems 2

• Ordnance Survey National Grid– based on the Transverse Mercator Projection

– origin off-set by 100km North and 400km West to ensure that the U.K. is covered by positive co-ordinates

– this origin is as far south as the Scilly Isles and as far west as St Kilda

Referencing Systems - National Grid

Relationship between TransverseMercator graticule and O.S. National Grid System.

Red dot indicates true origin 49°N and 2°W Green dot indicates false origin of GB National Grid, offset from true origin by 100km north and 400km west

Referencing Systems - National Grid 2

The UK is covered a by a 500 x 500 km square grid. This is a remnant of a previous referencing system.


The grid is divided into 100 x 100km squares each identified by two letters.

Note that the letter I is not used.


Each 100km grid square is divided into100 x 10km squares.

These are from 0-9 from the SW corner left to right and from bottom to top.

Using this system we can identify a 10km grid squaree.g. TL63


The 10km grid square is divided into 1km squares

Each is numbered as before from 0-9

Used in conjunction with the 10km grid square reference we can identify a 1km grid squaree.g. TL6432

6

4

37


The 1km grid square is further divided into 100 metre squares.

Enables a more accurate reference to be provided. e.g. TL648321

The position of a point within square TL648321 can be identified by adding extra digits.

National Grid – Quarter Squares

For the purpose of identifying a grid square on a 1:1,250 plan, the 1km square is divided into 4 sections, NW, NE, SW and SE.

On a 1:1,250 plan the yellow square TL648321 would appear on sheet TL6432SE

Conversion• Other data can be in different projections

– Google = Web Mercator

– GPS data = WGS 84

– Different countries

– Different zones

• Different Conversions have different accuracies– Different GIS Software use different formulae

– The default isn’t always the best

Conversion 2• OSGB_1936TOWGS1984_Petroleum

– Best option “out of the box”

• National Grid Transformation OSTN02– Most accurate option– Can be added to ArcGIS via OS website

• Further Reading:– http://digimap.edina.ac.uk/webhelp/digimapgis/

index.htm#projections_and_transformations/converting_between_osgb36_and_wgs84.htm

– http://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf

mapping projections and data conversions

Education