Delivered as part of a workshop on Digimap Collections.

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!

8. Map Projections

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