64th Birkenhead Sea Scouts

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:

Explorer (1:25 000)
Landranger (1: 50 000)
Routeplanner (1:625 000)
Road maps (1: 250 000)
Touring maps

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).

Map Symbols

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

Country park

Parking

Telephone, public/ motoring organisation/ emergency

Picnic site

Slipway

Windmill, with or without sails

Viewpoint

Wind pump; wind generator

Triangulation pillar

Public convenience

Youth hostel

Station, open to passengers

Motorway

Trunk or main road

Secondary road

Permitted footpath

Grid References

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.

Example:

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.

Example:

Point B is at grid reference 842528

Point C is at grid reference 848524

Tip:

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.

Magnetic Variation

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"

Measuring Distances

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.

Example:

Say you measured a distance of 100mm (10cm):

At 1:50000, distance = 10 ÷ 2 = 5km

At 1:25000, distance = 10 ÷ 4 = 2.5km

Tip:

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.

Journey Times

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)

Pace Counting

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).

Route Cards

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.

Leg	Grid Reference	Bearing °M	Distance in km	Height climbed in m	Total time for leg
1
2
3
etc

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.

download blank route card (pdf)

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.

REMEMBER

WHEN OUT WALKING, STICK TOGETHER AND WALK AT THE PACE OF THE SLOWEST PERSON.