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Map, Compass and GPS.
  

Table of Contents
1. Introduction to the Ordnance Survey grid reference system.
2. How to Find an Ordnance Survey Grid Reference.
3. Check out a grid reference.
4. Getting a bearing in relation to Grid North.
5. How to travel to your destination.
6. GPS - a brief insight.
    Introduction
    Tips on use of GPS
    Thinking of getting more than a basic GPS.
    GPS in Wrist watches, smart phones and Personal Digital Assistants, (PDAs).
    Free GPS Programmes on the Internet.

  1. Introduction to the Ordnance Survey map reference system.

Generally locations on the earth are given by their latitude and longitude, in that order. The lines of latitude are a series of circles of varying radius, the largest being the equator. The circles are parallel to each other, with their centres on the earth's axis. The lines of longitude run at 90 degrees to the lines of latitude, and are furthest apart at the equator and meet at the North and South Poles so the lines of longitude are not parallel. The O.S. grid, or National Grid for England, Scotland and Wales represents a fairly small part of the earth's curved surface as a flat surface, with parallel vertical grid lines, one of which points to TRUE North. This is one of several datums in use throughout the world. Britain has just the one, which is a Transverse Mercator Projection based on an ellipsoid, representing the Earth, known as the Airy 1830 ellipsoid OSGB (1936). (The GPS uses a more accurate ellipsoid known as WGS80, which is 1km wider at the equator and flatter at the poles. The Navy has recently changed to ellipsoid WGS84.)
The grid is a rectangle 700km x 1300km and has a 'false' origin at 49° 45' 58"N 7° 33' 23"W, so as to include the Outer Hebrides and to eliminate negative numbers. (The grid for Ireland is separate.) The National Grid is split into 100km x 100km squares, each of which has drawn on it 100 horizontal and 100 vertical grid lines 1km apart. These lines are numbered 00 through to 99. Only the vertical line through 2° W passes through TRUE North. (This line stretches from Poole in the south to Berwick on Tweed, passing through Tewkesbury, Walsall and Skipton. The grid lines to the west of this line will point to the west of True North and vice versa. All of Scotland is to the west.)
The 100km x 100km squares are given letter codes. The square covering the Lake District is NY. The codings for the adjacent squares are given in the diagram below.


The vertical lines are known as Eastings and the horizontal ones are known as Northings. The Grid Reference takes on the form: letter-code Easting Northing. (Note that the vertical reference now comes before the horizontal one, the same as locations specified using longitude-latitude.) The way to remember is that Easting comes before Northing in the alphabet.

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  1. How to Find an Ordnance Survey Grid Reference.

Although far from essential, a compass with Romar scales is a convenient instrument to estimate the O.S. grid references. Both the 'Silva 54 combi' and the 'Silva expedition 54' have them.

On a 1:25,000 scale map the interval between lines representing 1 km are 4cm and on a 1:50,000 these intervals are 2 cms. The Romar scale on your compass can be used to subdivide these intervals. Splitting the interval into tenths can usually be assessed by eye. Using the Romar scale on the compass, splitting into hundredths is just possible, especially on the 1:25,000 scale map, as in the diagram below.


Romar scales on compass base plate.

Using Romar scales on compass to find map reference.


The Romar scale for the 1:25,000 scale map are the two sets of graduations at right angles on the outer edge of the baseplate of the compass. Place the junction of these graduations over the location on the map, whose grid reference is required, as shown in the diagram. Write down your estimate and check your answer here. 
If the values are rounded to the nearest digit, then the 6 digit answer will be within ±50 metres and the 8 digit answer will be within ±5 metres.
The Romar scale for the 1:50,000 scale map is placed away from the edge of the baseplate and so one should try to look directly over the graduations to avoid parallax errors.
Some compasses have baseplates with the Romar scale in one direction only. This is not so convenient because the baseplate has to be turned through 90° between measurements.

A convenient device, which can be carried in your top pocket, is a grid reference finder or Romar. This is made of transparent, rectangular plastic and has square sections calibrated in 1 : 25000 and 1 :50000 Romar scales. For more information go to Where-Wolf.

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  1. Check out a grid reference.

  • Do you have a grid reference relating to a (possibly unknown) location in the region, where the Penrith Ramblers hold their walks?
  • Do you also have no idea which map to use to find where this location is?
  • If so you can find out which map contains the grid reference by using a grid reference checking facility.
    The facility should work on most modern browsers, if Javascript is enabled.
    1. Click on the underlined link above.
    2. Enter the grid reference.
    3. Press the submit button. Silly entries will be rejected. If the grid reference is on the map, it will be plotted. Because of the small scale of tthe maps, the spot can only give a guide as to the position of the location.
    4. If the spot appears to be plotted correctly, click on the button 'Get OS Map Title' to find which OS map the grid location will appear on.
    5. If the location seems wrong, check the grid reference letter code and check if the eastings and northings are in the correct order.
    .

  1. Getting a bearing in relation to Grid North.

  • Find a flat surface, out of the wind if possible, and place your map on it, having opened the map at the correct place.
  • Estimate the approximate direction of your destination from where you are, i.e. is it west? south-east? etc. This will be used as a check later.
  • Now place your compass on the map with the 'direction-of-travel' arrow pointing roughly in the direction you wish to go.
  • Line-up the long edge of the base plate of the compass so that it runs from your present position, PP, to your destination, D, see diagram below.
  • Rotate the compass housing (coloured green), so that its arrow (coloured yellow) points to the 'North on the map' or Grid North. (On O.S. maps this is parallel with the vertical edge of the map.) The North /South lines on the base of the compass housing should now be parallel to the North/South grid lines on the map.
  • The bearing from Grid North, is shown on the index line, which in the diagram is about 113 degrees. (You could set up your own compass and check if you agree - each small division is 2 degrees on the compass illustrated.) The position of the compass needle at this stage is immaterial.
Image of compass being used to find a bearing.

Finding a bearing in relation to Grid North.

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  1. How to travel to your destination.

As you are going to use the compass needle as your reference direction, when you travel, you have to convert your bearing in relation to Grid North to a bearing in relation to Magnetic North. You will be aware that the bearing from magnetic north varies as the position of the magnetic north varies, due to the movement of the molten magnetic core of the earth.
The 3 Norths.
The three Norths.
The direction of the Magnetic North and Grid North relative to True North depends on time and place.
The diagram is that for the Lake District at the start of the 21st century.

At the moment Magnetic North is to the west of True North in Britain, (though on the other side of the earth, say in Canada, it is to the east of True North). This angle is termed the declination.
The position of Grid North in relation to True North depends on where in Britain you are. The vertical line in the centre of OS map OL4 (NW lakes) is 1° 4' west of True North. For OS maps OL5 (NE Lakes) and OL21 (South Pennines) the corresponding angles are 42' and 4' repectively. Move further east and Grid North would be to the east of True North. (The value will be zero on the line of longitude 2° W.)
Figures quoted by OS maps for the angle between Magnetic North and Grid North vary depending on the age of the map. One version of OL5 quotes a value of 5° in 1999, while an earlier version quotes 5° in 1997. Both state that the angle reduces by about ½° every 4 years. Thus a suitable value to use would appear to be 4° (or 3° 45') at the time of writing , i.e. 2007. An accurate value for the angle between Grid North and Magnetic North for any location in the British Isles can be found by going to the Geomag website. The value given for Penrith for July 2007 is 3° 7', a difference of more than ½° compared with the value derived from data given on OS maps. Fortunately it is unlikely that this small difference will prove to be of practical significance given the difficulty of walking on a bearing to this degree of accuracy using a simple compass like that above.

For this instruction, we shall use a value of 4° but you will need to amend this as time goes by. In a few years time the value will be small enough to ignore and by 2050 it should be a significant negative value and the rotation of the compass housing, mentioned below, will be clockwise - not anticlockwise.
  • Rotate the compass housing anticlockwise by 4°. This adds 4° to the reading and compensates for the difference in the angles between Magnetic North and Grid North in the British Isles.
  • Now hold the compass, so that it is flat in your hand, and turn yourself until the red end of the compass needle and the (yellow) arrow head in the compass housing line up. The 'direction-of-travel' arrow, marked on the clear base plate, now points in the direction of your destination, (D).
  • You should now try to locate a fixed distinguishing feature or 'marker' such as a tree, a bush, a rock or even a clump of grass on the line of the 'direction-of-travel' arrow. Check that this is roughly in the direction that you estimated at the start. You don't want to be going in the opposite direction to the one you want! If okay, you now head towards your marker. If you lose sight of your marker, due to undulations in the ground, check that the red end of the needle and arrow head in the housing continue to line up as you move. Always be careful not to turn the housing in relation to the baseplate.
  • When you have reached the first marker, repeat the process until (hopefully) your destination comes into sight.
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  1. GPS - a brief insight.

Introduction.

Although the basic operation of GPS receivers is similar, implementation of the user interface will differ between models and so no attempt is made here to show how to set up a GPS receiver for use, The manufacturer's manual wiil have to be consulted on how to access the facilities provided by a GPS, how to select a suitable grid system and north reference, how to enter/delete waypoints, how to load and access/edit routes, etc. Instead, the more useful facilities a GPS can provide together with a few tips on how to use a GPS are given,

Before starting it is necessary to be aware that a GPS receiver needs information from satellites orbitting the earth, which tells the receiver where the satellites are. The GPS stores this orbital information and uses it to calculate the GPS' position, so even a basic GPS has to have a sophisticated computer inside it to do deal with the complex science and mathematics. Fortunately, the walker does have to bother about this.
If left off for about 4 hours or more, the information will be out of date and the GPS has to reload the orbital information before it can calculate its position. This can take a few minutes. A page on the GPS can be selected, showing the progress of this process, the heights of vertical bars varying according to the signal strength of each satellite and changing colour when the full orbital information has been received. This is termed a 'cold' start and does not refer to the temperature of the GPS! A 'warm' start occurs when the orbital information does not need updating and the position can be calculated immediately.

GPS software has been developed to enhace the operation of a GPS which can bring an extra dimension to a walk, e.g.
  • they allow you to enter a long list of waypoints into your PC and then upload them to your GPS, quickly and accurately - much better than doing this manually.
  • they allow you to plan a route on your PC, knowing its length and cumulative ascent/descent and upload this to your GPS, which you can walk later. This is useful, for those planning to do a walk they have not done before.
  • they allow you to download a tracklog of a walk, (perhaps a recce), amend it if necessary and save it as a route for future use.
Two popular Digital mapping companies are 'Anquet' and 'Memory-Map'. These use Ordnance Survey maps under licence. You can check them out before buying by requesting a demonstration CD Rom or visit their websites. Free software is also available , which can be downloaded from the internet but this does not have maps pre-installed. You have to do this yourself. (See the section: 'Free GPS Programmes on the Internet'.)

Some ramblers disparage using a GPS receiver when rambling, saying that a map and compass are all you need but its use in some instances can sometimes be invaluable, e.g.,
  1. locating one's position when travelling over ground with few or no clear features in poor visibility.
  2. giving an accurate grid reference in the unlikely event that you or a colleague are severely injured and your position has to be passed on to Mountain Rescue.
Besides giving grid references, the basic GPS can record a log of the track one has taken on a walk, which can be useful in two ways::
  1. It enables one to retrace the walk, if considered sensible to do so, when difficulties arise, such as worsening weather conditions in snow or mist.
  2. It enables a record of the route of the walk to be saved later on a PC, if the PC is installed with suitable software.
The GPS has clear limitations, which, it seems, need to be spelt out. Its internal computer is only accurate if it gets strong satelite signals, so positions can be badly out, if satelite signals are weak or fail, as can happen in wooded areas or in gorges, etc. It does not give positions of possible hazards, such as pot holes, steep cliffs, etc. unlike a map. It relies on a battery, which has a limited charge, so it may give up completely. Therefore, to rely on it as the sole navigational aid is folly and has even been fatal. (See below.)

To Top Tips on use of GPS..
  • Fully acquaint yourself with how to access the various facilities that your GPS has to offer before using it in the field. You can get an idea of the time it takes to make a cold start. You can enter some way points in the area you live and use the GOTO function - more on waypoints below. You can check its reception under tree cover or between high buildings. Having enabled tracklog operation, you can zoom in on the track of your walk on the map screen and check on how it corresponds to the track you have taken on the ground. You can read off your position at various places and check them against the appropiate OS map. After the walk check out the distance you have walked and your average speed. You can also familiarise yourself on how to delete the tracklog memory and reset the trip metres in readiness for your next walk.

  • If waypoints have been loaded into your GPS, then these will be numbered in sequence and the GPS will display straight lines from one waypoint to the next in the sequence. (It is possible to go in reverse.) In the fells, this could mean that your path could cross dangerous ravines or deep lakes - not to be recommended. Thus extra waypoints will be needed so that you are led round such impassable places. Because it takes time and care to load waypoints by hand, some people use only a few waypoints, perhaps on prominent features. Others like to use waypoints at places where there is a choice of path or to negotiate around obstructions or points of danger. Loading a route from software on a PC is likely to produce many waypoints, their accuracy depending on how accurate the path was drawn on the PC. A disadvantage of this is that the limited number of waypoints that a GPS can hold, perhaps 500 or 1000, limits the number of routes, even though the theoretical limit may be 20 (or 50 on later models). This problem is easily overcome by storing the routes on your PC rather than your GPS.

  • The GPS needs to be set up with a suitable co-odinate system or datum. There are many in use throughout the world. The default datum is WGS84, which stands for 'World Geodetic System 1984 and if you are to use your GPS in Britain, this should be changed to the only British datum, which is OSGB (1936), Ordnance Survey Great Britain 1936, so that it lines up with the OS maps you will be using. Any software you use will need the datum setting too. Setting the GPS' datum can take many steps,, e.g. with the Garmin GPS60, you need to select the 'Main Menu' page, then 'Setup', then 'Units' and in 'Position Format' select 'British Grid' and in 'Map Datum' select 'Ord Srvy GB'. (Incidentally, having navigated this far, whilst in the 'Units' section, select the units of distance and elevation that you wish to use too.)

  • The way the GPS aligns the map can be adjusted. Choose 'Track up', if you tend to turn your map upside down when going south(!), otherwise choose 'North up'.

  • You can select whichever of the three North references the GPS uses. It is usual to select 'Grid North', if you have selected your datum as OSGB(1936) in the UK, because the position of the way points will use grid references. Use 'True North' if you are using latitude and longitude. perhaps outside the UK. Use 'Magnetic North' if you particularly want north on the GPS' electronic compass to line up with north on your magnetic compass.
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  • If you have problems loading waypoints or routes into your GPS using GPS software on your PC, then make sure you have configured both your GPS and your software so that a physical connection between your GPS and your PC has been established.

  • The basic GPS also gives distance walked, its elevation and will display a pointer, which gives the direction to your next waypoint. However
    the pointer does not work when stationary,
     so you need to carry the GPS a few steps to be able to gain the necessary information for it to calculate the correct bearing.

  • As long as you keep your GPS switched on while you walk, you can read off your speed, average speed, maximum speed and the time you have been walking. You can save power by switching on your GPS only when really needed, but this means that any readings of speed and distance walked will be more or less meaningless and the track log, if being recorded, will be very inaccurate, being a few straight lines joining the recorded positions made when the GPS was on.

  • It is important to start with fully charged batteries and ideally carry a spare set, if you are going to have the GPS on for several hours. (You won't lose any data while the batteries are swapped over.) Rechargeable batteries are cheaper and more environmentally friendly than non-rechargeable alkaline batteries. However if left unused, they will need recharging periodically. Nickel Metal Hydride, NiMH, batteries are to be preferred to Nickel Cadmium batteries as they can be recharged when only partly discharged, unlike Nickel Cadmium. Choose batteries with a high current capacity, i.e. 2500mAh rather than 1300mAh say. Eventually even your set of rechargeable batteries will need replacing. If possible select the battery setting on your GPS for either 'Alkaline' or 'NIMH' operation, whichever is appropiate.

  • The GPS needs to receive at least 3 satellite signals to get a good 2D fix and at least 4 to get elevation as well. Reception of satellite signals depends on the location of the GPS. In deep ravines or under tree cover reception can be impaired significantly. Because the satellites are positioned to the south of Britain, reception is improved if the GPS is carried with no obstructions to the south. This includes one's body. Keeping the GPS as high as possible also helps - perhaps in the top of one's rucksack, if you are not navigating with the GPS.

  • Before starting a walk, the track log memory should be cleared and the trip odometer, the time and speeds set to zero. Then enable the tracklog feature before setting out. If you do this at home, before the walk, select the GPS OFF' setting, so that it is not trying to get satellite reception. Doing this prevents the GPS having a waypoint preset before the start of the walk. Also make sure you have the GPS switched off as you travel from your home to the start of the walk, otherwise you will be logging this journey too. Don't forget to switch it off at the end of the walk either. Doing this means that the track log will be a record of just the walk, without any 'extra bits'. The log can be uploaded onto your computer and saved for future reference. If the tracklog contains more than the actual walk, it is possible to select the section you require by first splitting the track/path at the start and/or end of the walk using the Anquet or Memory-Map software and then deleting the parts you don't want.

  • When in the fells, don't set the way point of a distant location into your GPS and then use the GPS to walk straight towards the location, without first checking on a map, that there are no hazards, such as precipices, pot holes, etc., between you and it. By simply following a pointer on a GPS in thick mist, without checking for such hazards, some walkers have fallen to their deaths!

  • Some older models of GPS don't have a USB connector for connecting to a computer. This means that an (over-priced?) adapter may be needed to be able to connect such a GPS to your PC.
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Thinking of getting more than a basic GPS.

The two main improvements on the basic GPS are the incorporation of:
  1. a multi coloured display
  2. a large scale map of the area where the walk is.
If you are going to include a map with sufficient detail to be of any real use, then a multi coloured screen is essential. However, the screen on a GPS is quite small and although it is possible to load routes from a PC to a GPS, it is not possible to load a real OS map. This is because the GPS manufacturers only allow the loading of their own mapping software - 'Mapsource' from Garmin, 'MapSend' from Magellan and 'MapCreate' from Lowrance, the three main GPS manufacturers. Basemaps on a small scale will probably be already loaded. However to get larger scale maps, extra sotware, which is quite expensive, has to be purchased. You have to weigh up, if the extra cost of a more expensive GPS with the extra memory and colour display, together with the cost of the extra software, is worth the extra benefit you get.

To get round the problem of the small screen on a GPS and the unwieldiness of plastic covered OS maps in the field, some ramblers produce their own A4 sized maps. On one side is the specific area and route of each walk, (especially useful if your walks involves more than one OS map) and on the other side a smaller scale map, giving a wider view of the area. Also if the printouts are then laminated, they are durable in wet/windy weather. However, this does involve the purchase of suitable software for the PC, such as 'Anquet' or 'Memory-Map', already mentioned or scanning maps onto your computer and using 'stand alone' software.

GPS in Wrist watches, smart phones and Personal Digital Assistants, (PDAs).

Wrist watches with GPS are available, such as the Suunto X9i, which also includes an electronic digital compass, an altimeter and a barometer. Unlike many GPS models it gives cumulative ascent and descent. Waypoints, routes and tracks can be created using Memory-Map connected to a PC via a USB cable. When navigating on a pre-planned route, the Suunto X9i shows the direction and remaining distance to the next waypoint. It also indicates the current speed and distance travelled.using the GPS functions. There is a trackback facility which enables the user to retrace any recorded route and the route of a walk can be viewed after downloading it to a PC. Like many watches, it is water resistant down to 30 metres. Its main disadvantages are its cost, (about that of a top of the range GPS), its failure to give the grid reference of its position and the limited display on a watch's small dial. The Casio Pathfinder watches are examples of more limited GPS watches in only displaying a digital compass and elevation, though again they calculate and display cumulative ascent and descent..

The latest smartphones can be used to view digital maps and plan routes. Some do not possess GPS but can be linked via wireless (Bluetooth) technology to a screenless GPS. Others now possess a built in GPS, such as Mitac Mio A701, which also has pocket PC functionality. They do of course have many extras such as digital cameras and the ability to send text and voice messages. Their main disadvantage is their lack of ruggedness compared to a standard GPS.


PDAs are handheld computers. They can be bought with navigation software and are used by motorists for in-car navigation. If a PDA does not have an integral GPS, it can be connected via a lead to a standard GPS or to a screenless GPS using wireless (Bluetooth) technology, the latter allowing the GPS to be positioned for better reception away from the PDA's screen.. Their main advantage over a top of the range GPS is their larger coloured screens, their ability to be fitted with extra memory and the wide range of software available. Also they are not tied to one manufacturer as regards mapping software and can use maps produced by Anquet or Memory-Map. Unfortunately they are not designed for tough outdoor conditions by not being either weatherproof or rugged. Also the battery life of about 4 hours, is much less than that of a basic GPS..
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Free GPS Programmes on the Internet.

Besides using the software produced by digital mapping companies, such as 'Anquet', 'Memory Map' and 'Fugawi', there are a number of programmes, known as 'stand alone' software, which do not have maps included but will accept a bitmap, produced by scanning a section of a map into your computer. This bitmap has to be calibrated before the software can make use of it but once the bitmap is loaded and calibrated, the software operates in much the same way as the software of mapping companies, previously mentioned,

'Trailgauge' is a freeware programme. To calibrate a bitmap, it requires you to enter two points with their map refences. For accuracy the differences in both longtude and lattitude (Eastings and Northings), should differ as much as possible. This means that they should be as far apart as the bitmap allows on a diagonal starting and finishing in opposite corners. The first point sets the origin and the second sets the scale and rotation.

Another freeware program, 'GPS Utility' accepts between two and four calibration points. As before, the first two points are for origin, scale and rotation. The first extra point allows for any stretch or skew in the bitmap and the second extra point allows for any non-parallel grid distortion.
(Two shareware programmes of a similar nature are 'Gartrip' (once freeware before recent updates) and 'OziExplorer'. The latter has geo-referencing built in thus simplifying map calibration for the user.


Because of the need to scan a map and the need for bitmap calibration, these programms are not so easy to use as those coming with maps provided. However, getting maps from the digital mapping companies is not cheap so there may be areas where you are not covered. In such cases scanning a paper map of the area and using one of the programmes mentioned may be your best option to enter a route into your GPS.

In conclusion, the GPS should be used to complement the use of map and compass, not replace it.


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