Above picture Isle of Muck
SCALE AND MEASURING DISTANCE
Scale and Measuring Distances
At a scale of 1:50,000 one millimetre on the map represents a distance on the ground of 50 metres and at this scale most of the information required by the average map user can be displayed. At a scale of 1:25,0000 the distance on the ground would be 25 metres.
Straight Line Distances
Use of the Grid Lines Most maps at scales larger than 1:1000,000 carry grid lines, which are a fixed distance apart. A very quick estimation of the distance between two points is obtained by counting the grid squares between them. This method is satisfactory for distances in the north/south or east/west directions but significant errors accumalate in the diagonal direction.
- One grid square measures 1000 x 1000 metres or 1 kilometre.
- Distance horizontal or vertical across three grid squares would give you an estimated distance of 3000 metres or 3 kilometres.
- Diagonally the distance across two grid squares would be an estimated distance of 2.8 Kilometres or 2800 metres. This is because the distance diagonally across one grid square equates to 1400 metres or 1.4 kilometres.
Measuring Distances
At the bottom centre of both the 1:50,000 and 1:25,000 scale Ordnance Survey maps there are measuring scales, they are in, kilometres, statute miles and nautical miles. However you do not have a scale for measuring nautical miles on the 1:25,000 scale map.
Different Scale Maps
Look at the two maps below, both are of the same area but different scales. The top is 1:50,000 and the other one 1:25,000. You will see straight away that the 1:25,000 shows a lot more detail. You will notice the Red arrow indicating the area of The Bell and the Blue arrow indicating the location of the musuem on both maps. The area of The Bell on the 1:25,000 scale map shows you more terrain detail. The location of the musuem on the 1:50,000 scale map only shows the abbreviation for musuem, however on the 1:25,000 scale map the abbreviation is deleted and replaced by a conventional sign and indicates the exact location of the musuem.
Vertical Scale Most maps provide information relating to the the height of the terrain which they represent, and usually this is achieved by contours, ie, lines joining all points of equal height. The effectiveness of this method depends on:
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The distance between contours on a 1:50,000 is 10 metres and are normal numbered with their respective heights every 50 metres.
It is essential for the user to check the vertical interval for any particular map as this may change from one map to the next. More ever the vertical interval may change on the same map with a change of height.
Road and Path Distances
To measure a distance, which is not straight, ie, along a road or path, consider the road as a number of straight sections. Lay a piece of paper (some say a piece string) along the the first section and mark it with a tick, at the starting point and another at the end of the first section. Then pivot the paper about the tick until it lies along side the second section, mark this end of the section with another tick and repeat the process until the last point is marked.
The total distance along the road or path is then recorded as a straight line on the paper. It then can be placed along the relevant scale at the bottom centre of the map to determine the distance.
If this is done correctly it is surprising how accurate it can be.
Judging Distances
 |  Points to Remember when Judging Distances Dead Ground This is an area of ground you cannot see, it could be a dip in the ground, sloping ground in front of you, or behind a hill or wood. Basically any object or natural feature where you cannot see the ground behind it. Other things to consider are visibility and shadows especially at dawn and dusk. Pacing You need to know how many double paces it takes you to cover 100 metres. The average person takes approximately 65 double paces. By counting your paces as you walk, you can judge how far you have travelled. This is obviously useful for night navigation or bad visibility. A lot of people have different methods of recording every 100 metres they have travelled. The method I use is cheap; simple and effective, a knitting row counter attached to one of the shoulder straps on my back pack. These counters only cost about £1.50 so if you do lose it, it's not a big drama. |
Factors - Affecting your Pace
- Slope: Pace lengthens downhill, shortens uphill.
- Wind: Tailwind will lengthen your pace, headwinds will shorten the pace.
- Surface: Loose surfaces, ie, mud, gravel, snow tend to shorten the pace.
- Equipment: Heavy loads, will shorten the pace.
- Stamina: Fatigue will also shorten the pace.
- Visibility: Bad visibility, or at night will shorten the pace.
Calculating Speed Time and Distance
When planning any movement it is necessary to calculate time of arrival.
The variables are thus speed, time and distance
Speed = Distance over Time
Time = Distance over Speed
Distance = Speed x Time
Example
If the road distance on the map equals 35 miles and the average speed for this type of road in a vehicle is 30 mph. Then the time elapsed for this, as follows:
35 miles (distance) over 30 mph (speed) = 1hr 10min (time)
Naismiths Rule
When calculating how long it will take to walk to a certain point remember this simple rule:
Walking speed of 5 kilometres per hour. 5 kph + 30 min for every 300 metres of ascent.
Note: The above speed is for someone who is reasonably fit, you will have to adjust your speed to the pace of the slowest member of your team or party.
