Map references, distance and time. Maps and Navigation Map references, distance and time.

Map Prefixs The whole of Great Britain is divided into a series of squares each being 100km x 100km. Each of these ‘squares’ is given a two letter prefix so that you know which map you need to be using. If you just took a ‘bare’ map reference such as ‘657 803’ it would appear once in every one if the squares you see opposite, it could be Merripit Hill on Dartmoor, Capel Bangor in mid Wales or a pine forest north of Alness in Scotland. Put the prefix NH in front of the numbers and you would know you were looking at the forest in Scotland. See if you can find the prefix letters for the maps covering Dartmoor.

Scale – Both maps below are of the same area but hopefully you can see that everything is larger in the right hand map. There is also more detail but we will come back to that, what I need you to understand is that the blue squares on BOTH maps show 1km square even though they seem smaller in the map on the left. 1:50000 1:25000

Eastings and Northings Ok, these are easy to understand!! When you look at your OS map you will see that it is covered by a series of blue squares. Each of these represent one square kilometre regardless of whether it is a 1:25000 or 1:50000 map you are using. If you look at the map section on the right you will see that the lines which run from the top to the bottom run from 58 to 63, these are called eastings because they increase to the right or east of the map. The lines which run across the map are increasing from 87 at the bottom to 94 at the top – what do you think they are called? All you need to remember is that E comes before N in the alphabet so you always give the easting number first!

Examples East Hill SX Dinger Tor Rowtor Steeperton Tor Time for you to do some work, there are four small red ‘globes’ on the map, East Hill, Dinger Tor, Rowtor and Steeperton Tor. Write down the four figure map references for each of them so that you can show which square they would be found. East Hill SX Dinger Tor Rowtor Steeperton Tor

Six figure grid references Now have a look at the new image opposite. You should be able to see four one kilometre squares but the top right one has been divided up into 100 small squares. If you have understood that each grid square represents one kilometre by one kilometre (remember a kilometre is 1000 metres)then you will see that if we divide each grid square up into 10x10 like this then each small square represents ? Metres. Now if I was trying to give the usual six figure map reference for the red square then in my head I have to divide the grid square up into ten between easting 17 and 18 (remember E before N in the alphabet) and then decide how many tenths the point is along the grid square . In this case it is five so the easting is 175. Do the same for the northing and you should get 512. So the six figure map reference for this example would be 175 512 and if it were on Dartmoor then it would be SX 175 512.  

Examples East Hill SX 59 93 Rowtor SX 58 88 Dinger Tor SX 59 91 Steeperton Tor SX 61 88 Observation Hut Right, now go back to your map and try to give an accurate six figure map reference for the same four locations. Remember that six figure map references are only accurate up to 100m so you may not all exactly agree with these. Ok, are you up for a challenge? You will rarely come across eight figure map references during training but you might so – Can you tell me how accurate they are (think if 4 figure = 1000m, 6 figure = 100m so 8 figure = ? Now try to find the army observation hut shown by the red circles on the map opposite. When you do then try to work out the 8 figure map reference for it.

Examples East Hill SX 595 938 Rowtor SX 593 916 Dinger Tor SX 586 881 Steeperton Tor SX 618 887 Observation Hut SX 6140 8846 Right, now go back to your map and try to give an accurate six figure map reference for the same four locations. Remember that six figure map references are only accurate up to 100m so you may not all exactly agree with these. Ok, are you up for a challenge? You will rarely come across eight figure map references during training but you might so – Can you tell me how accurate they are (think if 4 figure = 1000m, 6 figure = 100m so 8 figure = ? Now try to find the army observation hut shown by the red circles on the map opposite. When you do then try to work out the 8 figure map reference for it.

Measuring Distance There are several ways of measuring distance on your map, if it is a straight line you could use a six inch ruler or you may have a compass with a ‘romer’ shown on its side (see right) which you can use to calculate the distance between two points. Try this on your own map You are more likely however to get a ‘wobbly’ route such as a path or road to follow. These are more of a ‘pain’ to measure and there are several ways to do it including using a piece of string (see right) however I like the method below to measure distances. Using paper to measure distance

Examples ROUTE DISTANCE 4 5 8 Use either the edge of your compass or a ruler to measure the length of the following 3 routes Now try the ‘paper method’ to measure the length of the forest from wall junction at SX 658 818 to the wall corner at SX 653 852. ROUTE DISTANCE 4 5 8

Remember that according to the rule Working out time Just one time to workout here, how long will it you to walk route 8? Remember you are walking from the Waterfall to Sittaford Tor. Naismiths Rule William Naismith was a Scottish mountaineer who devised his ‘timings rule’ in the 1890’s. Does it work on Dartmoor? Well not really, once you are all ‘moor fit’ you will find you walk faster than the times the rule gives you (other than in the ‘bogs’ or ‘death valley’) but it is useful to use as a ‘rule of thumb’ your likely ‘slowest’ time. Remember that according to the rule You will walk 5kms in one hour – 12 minutes per kilometre. You need to add 1 minute for every 10m you go UP so in other words every contour you cross going up – easy then!!