Map Reading

OS Open Data under Creative Commons BY-SA 4.0 license

Many people become totally immersed in maps, others find it helpful to use mapping Apps on smartphones or navigation systems in cars. Between paper and electronic maps, many of us are now using maps on an almost daily basis. With all the information provided electronically, it is possible to forget the mapping skills and knowledge that you need to fully understand what is being displayed. That is not to mention the benefits in being able to read a paper map when your phone has run out of battery!

In the UK the Ordinance Survey (OS) are probably best known for paper maps. Over the last 20 years, OS have also worked on geographical big data in a digital format. But the basis for the work of the organisation was paper maps – so understanding how these work can help with electronic data.

Contour map - copyright free, Pilsdon Pen - Jim Champion under Creative Commons BY-SA 4.0 license A black and white line drawing of a contour map and a trig point at the summit of Pilsdon Pen.

Maps are usually two dimensional representations of a three dimensional world. To get around this problem and ensure that the information is there, we use contour lines. These are lines that join together areas of the same height above sea level. If you go out for a walk following a contour line (they aren’t marked on the ground!) you will stay at the same level – neither going up or down. The closer together the contour lines the steeper the slope. If a map shows a steep coastal cliff face you will see the contour lines virtually touching each other. Contour lines allow a two dimensional map to let the viewer see topographical features (such as steep hills).

To use maps there are some key elements to support understanding and allow maps to be used efficiently outside your front door.

On contour lines you will also see numbers. These are the height above sea level that is represented by that contour line. In addition, you may see black dots with numbers above them. These are spot heights above sea level. Although today these heights can be calculated by GPS (Global Positioning System) technology they were traditional calculated with the use of triangulation pillars, commonly known as trigpoints.

OS maps are drawn to scale meaning that they provide an accurate representation of the real world. On an OS map the distance between two of the grid lines is always one kilometre. With a 1:25,000 map 1cm on the map represents 25,000cm (or 250 metres) on the ground. So on a 1:25,000 map the gridlines are 4cm apart.

Copyright: GFDL - Milhaus Example of a graphical scale

Maps are criss-crossed with vertical and horizontal lines to form a grid. The lines are numbered and using these lines a grid reference is given. On an OS map a four figure grid reference identifies an area of 1km². The lines running top to bottom are called eastings and the number increases the further to the east (right hand side) you move along. Those running across the map are northings the value increases the closer to the top of the map you are.

To work out the point on a grid square on a map – such as point ‘A’ - using a four digit reference figure you first find the easting value (17) and then the northing value (41). So for square ‘A’ the four figure reference number is 1741. Have a go and see if you can work out point ‘B’. Remember start with the easting value and then the northing value.

Most maps also let you know where north is on the map. This is usually straight up – but not always!

To get more information from a map there are extra layers of data that are valuable for different users. These can show a range of different things – depending on how the map is intended to be used. As a driver, the roads are important, as a walker the footpaths, as a canoeist it may be the rivers. This, and much more data, is found on OS maps.

Maps employed for other uses have different types of data represented. When used to think about human factors, these can relate to population distribution, structure or movement. It could also cover vegetation, climate or geology when used in relation to consider data for more environmental factors.