Scale and Measuring Distances

 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. Now visualise something that is 25 metres  long for example, a swimming pool!

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. 

Compare with map below

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:

• The vertical interval between contours.
• The nature of the terrain.
• The scale of the map.  

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.

 Pacing

Everyone should know how many double paces they take to cover 100 metres, knowing this is essential when it comes to navigation in bad visibility or at night and also micro navigation.

The average person will normally be in the range of 65 to 70 double paces, ie, every time the left foot hit the ground.

There are several methods of keeping count of how many 100 metres you have covered to your objective things like tying a knot in a piece of string, even dropping one pebble from one pocket to another, or purchase to one of those fancy pace counters.

Me personally I use a knitting row counter which has been in use in the forces for years, cheap so it's no big loss if you loose it.

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. 

Judging Distances

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.