Mapping Projections and Data conversions
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This document provides an overview of cartographic concepts, including map scales, projections, and referencing systems. It discusses how scale is expressed in representative fractions or words, and how maps can be either large or small scale depending on the level of detail. It also explains the different types of map projections, highlights that the Ordnance Survey uses the Transverse Mercator projection, and discusses height and coordinate referencing systems like the National Grid used in the UK.
Mapping Projections and Data conversions
- 1. Cartographic Concepts Aim: to provide you with a basic knowledge of Cartographic Concepts
- 2. 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
- 3. 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
- 4. 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 0 50 100m
- 5. Map Scales 3 • large scale or small scale? • relative terms: vary by subject • no absolute definition of large and small scale
- 6. Map Scales 4 Contains Ordnance Survey data © Crown copyright and database right 2014
- 7. Map Scales 5 • … remember it’s all relative!
- 9. 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
- 10. 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
- 11. 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
- 12. 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
- 13. Referencing Systems - National Grid Relationship between Transverse Mercator 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
- 14. 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.
- 15. Referencing Systems - National Grid 3 The grid is divided into 100 x 100km squares each identified by two letters. Note that the letter I is not used.
- 16. Referencing Systems - National Grid 4 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 square e.g. TL63
- 17. Referencing Systems - National Grid 5 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 square e.g. TL6432 6 4 3 7
- 18. Referencing Systems - National Grid 6 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.
- 19. 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
- 20. 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
- 21. 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#pro jections_and_transformations/converting_between_osgb36_an d_wgs84.htm – http://www.ordnancesurvey.co.uk/docs/support/guide-coordinate- systems-great-britain.pdf