Friday, 9 December 2016

How To Read A Map


Reading maps is easy, once you know what you're doing. I am writing this as a guide on how to read and understand a map.

  • What's a map and why is it useful?
A map is just basically a picture, drawing or diagram showing a location, usually as seen from above. Maps range from very simple (just a few lines or pictures) to very complex (showing many different features of the landscape).
The following are all examples of maps:
 This is a simple hand-drawn street map of a village, it shows the roads and building with labels.
 Here is a made up map from the Lord of the Rings, its still a map and shows all of the places from the story.
This one is a pirates treasure map.
And a map of the London Underground.
Finally the map type that i'll be focusing on in this post, Ordnance Survey (OS) maps. These are very detailed maps of the UK and are very useful for taking when you go on a walk somewhere.

As the examples have shown, maps can be anything that shows a location and vary in detail from just a few pictures (the pirate treasure map) to a very detailed diagram showing all the features of the land including slope and types of terrain.

Now for why maps are useful, this is an easy one. A map can help you find where to go, people use maps because they show the area and give features of the land to help with navigation.
Different maps can be useful in different cases. If you were walking around a city, you might use a map that shows all the shop names and has all of the smaller streets and footpaths marked too. If you were walking across a mountain, this type of map would be useless, you would want a map which shows a much larger area and has details such as height and steepness of the mountains. Again if you were driving, then you aren't interested in the height of nearby mountains, you just need roads and road services.
Pilots would use even larger maps showing major locations but covering the whole country (or world even) and would not need to know about all the footpaths and shops in the city they are flying over.

  • Map symbols
Ordnance survey maps use a set of symbols to show different land features which makes the map easier to understand without the need for so many labels, the map symbols are mostly pretty obvious and all maps come with a key showing all the symbols and what they mean.
Here is a copy of the Ordnance Survey map legend (the key showing what all the symbols mean)

  • Map scale
Drawing a map means shrinking things down, because it would be impossible to draw things exactly in the size that they are if you are drawing a map of anything ranging from a house to the world. To shrink this down and create an accurate picture of the landscape, a scale has to be used so that everything is the right size relative to other features (you wouldn't draw a car wider than the road it is driving on)
Different scales are used to show different amounts of information, and where one 'unit' on the map is representative of a given area in real.
The following picture is a map of the UK with a scale of 1:6000000 which allows the whole island to be shown on one page, but means that a lot of detail is lost.
The scale means that 1 unit on the map represents 6000000 (6 million) units in real. So 1cm on this map represents 60km.
Different map scales are used for different areas, the one I use a lot is the 1:25000 scale maps because this means that a significant amount of detail can be shown about an area, but you don't need to carry too many maps if you go on a walk. The whole of the lake district area is covered by four of the Ordnance Survey 1:25000 scale maps.

  • Map features
The world isn't all made up of the same thing, there are many different features of a landscape which are shown by a map.
Roads - these are shown by lines on the map, their colour depends on the type of road being shown, for example thick blue lines show motorways. All the different types of road have their own colour and style of line which is all shown in the map key.
Footpaths - These are marked as dashed lines on the map, green dashed lines show public footpaths, and other paths with symbols along the way (as shown below) show larger national trails.
This shows the Dales Way footpath
Rivers and streams - these are shown by blue lines on the map, the thickness of the blue line shows the size of the stream/river. For any river wider than 8m, or and pond/lake/tarn, the water is shown by a blue outline with light blue in between (as shown below)
This shows Red Tarn in the Lake District and the stream running down from it (towards the top right)
Woods - woods are shown as green areas on the map, and the type of wood is given by the tree shape printed over the top
The coniferous forest is shown here

  • Grid references
Ordnance Survey maps are covered in a blue grid of squares which can be used to pinpoint a position anywhere on the British Isles.
These lines have numbers shown with them and allow you to accurately pinpoint a location which can be useful when telling someone (like the mountain rescue if you are in trouble) your location.
The lines and numbers going horizontally across the map are called eastings and the vertical ones up the map are called northings.
Using these squares, which each represent a 1km square area, you can produce a 4-figure grid reference which gives your location accurate to 1km (the reference will give the location of the square which you are in but not where in the square you are) To write a 4-figure grid reference, you find your location and the numbers given for the bottom left corner are used to write the reference, starting with the horizontal position. For example, the points 1, 2, 3, and 4 are shown on the grid below, each of these points has a different grid reference. For point 1 this is 18 across and 45 up, so the 4-figure grid reference is 1845. The same can be done for all the points.
4-figure grid reference, the points 1 - 4 are as follows:
Point 1 = 1845
Point 2 = 1945
Point 3 = 1844
Point 4 = 1944
To get a more accurate location, each of these squares is split further into a 10x10 grid which is not shown on the map as it would get too crowded, but this grid allows for a much more accurate location to be given which is accurate to 100m. This is called the 6-figure grid reference as it combines the previous 4-figure reference with an extra two digits, each between 0 and 9 for the position within the 1km square. This is shown below where the red dot has the position 5 across and 3 up so it has the 6-figure grid reference of 185443
6-figure grid reference (this is inside of just one square from the 4-figure reference above)
Red dot = 185443
Blue square = 187448
While the 6-figure grid reference improves upon the accuracy of the 4-figure grid reference, it still has the problem of the larger grid going from 0 to 99, what happens when you go over 99? (because the country is more than 99km wide) When you reach 99, the grid goes back to 0 again so no matter which reference system you use (6 or 4-figure) it will give your position in multiple different p,laces in the country.
To combat this, the country is split into a series of 100km squares, each square has its own reference code which is used in the grid reference. The map below shows all of the 100km squares used over the British Isles, and the codes in the squares are used to identify the position of your reference, so the position of the red dot (185443) could be in the north of Scotland, or in Cornwall, the national grid lines and codes give this position. So if the red dot was in the north of Scotland, its reference would be NC 185443
This map shows the national grid lines and the codes for each 100km square

  • Contours and relief
Just a simple flat map is good in some cases, but if you are walking in the mountains, the map needs to give you details about the slopes, because planning a path going directly up a cliff would be a much harder walk than one going a slightly longer route but that went up a gentle slope. The relief of the land is the change in height of the land, and this is shown by contour lines on the map. the contour lines are very thin orange or brown lines on the map and each line usually shows a height difference of 10m. So if the lines are spaced far apart, then the slope is quite gentle, and if the lines are very close together, then you have a very steep slope. The contour lines are useful as they allow routes to be chosen that don't climb the hardest terrain and also allow for the extra time to be taken into account, as it always takes longer to walk if you have to climb a hill than the same distance on flat ground.
This diagram shows what the contours on a map are like in real
There are also some maps which include contours on bodies of water such as lakes where the contours are thin blue lines and are used in the same way but show depth instead of height differences.

  • Naismith's rule
Naismith's rule is a simple rule about the time taken to walk certain distances taking into account ascent. Naismith's rule states that: 'Allow 1 hour for every 3 miles forward, plus 1 hour for every 2000ft of ascent' Simplified, this basically means that we add one minute for every 10 meters climbed (for every contour line crossed)
Going downhill is slightly trickier as for gentle slopes we will be walking fast, but for steeper slopes the pace will be a lot less. Overall for the whole walk this shouldn't be too much of an effect so it is ignored.
  • Useful links
Some of this information, and a lot of the diagrams came from this pdf created by the Ordnance Survey.
It is best to have a paper map for if you are going on a walk, but to help with planning the walk, ordnance survey maps can be accessed for free on Bing maps, by selecting the layers on the right hand side and choosing OS maps.

1 comment: