- Introduction to OS Maps
- Map Symbols
- Grid References
- Parts of the Compass
- Magnetic Variation
- Orientating the Map
- Taking a Bearing
- Measuring Distances
- Journey Times
- Pace Counting
- Route Cards
- Using a Watch as a Compass
Introduction to OS Maps
Ordnance Survey (OS) maps are a whole collection of different maps, each covering a section of the UK. Like any map, they are a 2 dimensional representation of the real world.
When you first look at one of these maps you may be inundated by symbols, lines and different colours; but all the meanings of these and other useful information is contained on the front of the map. Also, when starting out, you are unlikely to need to make use of all these things at once.
Ordnance Survey maps actually come in several different types:
The important thing to notice here is the scale....
The smaller the scale, the less area the map covers and the more detailed it is.
The two most commonly used for walking are Explorer (1: 25000) and Landranger (1:50000).
All the relevant symbols are displayed in the information pane of the map itself but some of the common ones are shown below:
|Camp site/ caravan site|
|Church with steeple|
|Church with tower|
|Information centre, all year/ seasonal|
|Radio or TV mast|
|Telephone, public/ motoring organisation/ emergency|
|Windmill, with or without sails|
|Wind pump; wind generator|
|Station, open to passengers|
|Trunk or main road|
An OS map is split up in to a number of grid squares by a series of horizontal and vertical lines known as Eastings and Northings. These lines are numbered with the eastings scale increasing from west to east (left to right) and the northings increasing when read from south to north (bottom to top).
Each square can be identified by reading off the eastings scale followed by the northings scale. Or if you prefer; "along the coridor and up the stairs".
Reading off the scale in this way will give you a 4 figure grid reference.
Point A is at grid reference 8553
This gives us a problem since at grid reference 8452 there are two points; point B and point C.
To distingusish between several features within a single grid square, we need a system which has greater accuracy. We therefore use 6 figure grid references.
With a 6 figure grid reference, the grid square is further broken down in to an imaginary 10 by 10 grid.
Point B is at grid reference 842528
Point C is at grid reference 848524
Six figure grid references can be easily obtained by using the appropriate (i.e. correct scale) guide in the top right corner of most good compasses.
Whereas a 4 figure grid reference only gives an accuracy to within 1 square km, 6 figure grid references have an improved accuracy of 100 square m.
Parts of the Compass
|Base Plate||Bottom of the compass; the part which you hold. Includes various scales for working out distances and 6 fig. grid references.|
|Compass Housing||Centre part of the compass which is sealed. Contains the magnetic needle, suspended in a liquid.|
|Direction of Travel Arrow||Points to the direction you need to travel after setting a bearing.|
|Magnetic Needle||Red and white needle, the red end of which always points to magnetic north.|
|Degrees Scale||Indicates the angle between a measured point and north, 000° to 360°, in 2° increments.|
|Orienting Lines||Parallel lines inside the compass housing.|
|Orienting Arrow||Stationary arrow inside the compass housing.|
It is important to be aware that there are three types of north:
- Magnetic North
- True North
- Grid North
A compass will always point to magnetic north
OS maps are referenced to grid north
The difference between grid north and true north is very small and can almost always be ignored.
The difference between true north and magnetic north is known as Variation or Declination.
The level of variation changes with location as well as changing annually. This information is given on the map. It is therefore important to make sure that maps stay up to date.
The current level of variation is approximately 3° W
[It decreases by a rate of approximately 1° in every six years]
Orientating the Map
Orientating or Setting the map is a method of aligning the vertical grid lines (northings) on the map to north as indicated by the compass. This way, the landmarks which appear on the map will match what you are seeing around you. This makes things less confusing and helps to minimise silly mistakes.
- Rotate the compass dial so that the orienting arrow is aligned with the direction of travel arrow.
- Place the compass on the map and line up one edge with a vertical grid line.
- Holding the compass on the map, rotate both map and compass as a whole so that the north end of the magnetic needle is aligned with the orienting arrow.
- The map is now facing north (ignoring variation) and is said to be orientated.
Taking a Bearing
Bearings are used to accurately indicate a direction, obtained by measuring the angle between 2 points. Bearings are useful when navigating in poor visability or over featureless landscapes. All bearings are read clockwise from north.
Taking a bearing between 2 points:
- Orientate the map if preferred.
- Place the compass on the map with its edge lined up against the place you are at and the place you wish to go to. Ensure the direction of travel arrow points towards your destination.
- Turn the compass dial until the orienting lines line up with the vertical grid lines on the map.
- Read off the bearing from the degrees scale
- Adjust for variation
- Holding the compass in front of you, turn until the north end of the magnetic needle is aligned with the orienting arrow.
- The direction of travel arrow now points in the direction you need to walk.
Remember that although OS maps are referenced to grid north, the difference between this and true north is small so can be ignored.
Bearings obtained from maps are therefore referred to as being in degrees true.
In the above example:
The bearing is 54 degrees true; written as 054° T
This is not the end of the story though. Remember we have to take in to account magnetic variation. Since the variation is West, we Add the variation.
In our example:
The bearing is 54 + 3 = 57 degrees magnetic, written as 057° M
If you are taking bearings from the real world and translating them to the map in order to fix your position, you have to do the opposite (i.e. subtract the variation).
To help, remember the rhyme:
"Grid to mag add, Mag to grid, get rid"
As mentioned previously, maps come in different scales; most commonly 1:25000 and 1:50000.
Regardless of the scale, each grid square is always 1km wide in real life. The only difference is the size of the square as it appears on the map.
at 1:25000, 1km = 4cm
at 1:50000, 1km = 2cm
To work out the real word distance, all you need to do is measure the route on the map in cm and divide by 4 or 2 depending on which scale map you are using.
Say you measured a distance of 100mm (10cm):
At 1:50000, distance = 10 ÷ 2 = 5km
At 1:25000, distance = 10 ÷ 4 = 2.5km
If you need to measure a curved route, use a piece of string; say the cord attached to the compass itself: and measure it against the ruler on the side of the compass or the scale at the bottom of the map.
You can also buy special map measurers which do the same job but perform the conversion automatically.
Being able to estimate the time it takes to complete various legs of a journey or indeed the total time is an essential part of route planning but also very useful in terms of safety. An example might be; when leaving an ETA with a home contact or deciding on the next course of action in the event of failing light.
The underlying principle is that of Naismith's Rule. The rule was devised in 1892 by a Scottish mountaineer called William Naismith and is a formula for working out average walking time. It can be summerised as:
20 minutes per km + 1 minute for every contour crossed (10m height gain)
This is a rough estimate as a number of factors affect speed. These include:
- Different terrain (slower on boggy ground)
- Fitness level (slower at the end of the day)
- Weight carried (slower when backpacking)
This is a method of fairly accurately measuring distance on the ground while on the move and is especially useful when there are no obvious landmarks at which to make the next course change or in poor visability.
All you need to do is work out before hand how many paces it takes for you to cover 100m. It is best to do this several times and work out an average. You are looking to cover the distance somewhere between 50 and 80 paces.
Now when you walk, count every 100m and every kilometre (i.e. 10 lots of 100m). Record both these distance measurements in a way that works best for you (e.g. knotted rope, beads etc).
A route card is used as an aid to navigation and route planning.
The planned route is broken down into sections called legs. Wherever possible, each leg should start and finish at a obvious landmark such as a lake, saddle, stream junction etc.
For formal expeditions such as DofE, the following information should be included:
- The members of the team
- Actual start time and date
- Estimated time and date of arrival
- Mobile telephone number and/or radio frequencies in use
- Notes regarding special equipment carried, tents, emergency shelters, food etc.
A copy of the route card is then left with a responsible person so as to assist the emergency services in the event that the group should suffer a mishap.
|Distance in km||Height climbed in m||Total time for leg|
The table above shows the sort of information required to be filled in for each leg. The methods required to obtain this information have already been covered in previous sections.
Using a Watch as a Compass
In the northern hemishphere, the sun rises in the east and sets in the west; at midday it will be due south.
An analogue watch can be used to roughly find north by pointing the hour hand at the sun; the south will be half way between the hour hand and the number 12. This method works between the hours of 0600 and 1800, provided it is not too cloudy.
WHEN OUT WALKING, STICK TOGETHER AND WALK AT THE PACE OF THE SLOWEST PERSON.