MAP READING AIDE MEMOIR

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.

Straight Line Distances

Use of the grid squares. Most maps at scales larger than 1:1000,000 carry grid lines, which are a fixed distance apart. A very quick estimate 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.

Use of grid to measure distances

Position A: One grid square measures 1000 x 1000 metres or 1 kilometre square.
Position B: Distance horizontal or vertical in this case three grid squares gives an estimated distance between two points of 3 kilometres.
Position C: Diagonally the distance across two grid squares would be an estimated distance of 2.8 kilometres. This is because the distance diagonally across one grid square equates to 1400 metres or 1.4 kilometres

VERTICAL SCALE

Most maps provide information relating to 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 1:50,000 is 10 metres and normally numbered 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. Moreover the vertical interval may change on the same map with a change of height.

This information can be found in the top left hand corner under the heading, Heights in Metres, also in the general information section on the right hand side of the Ordnance Survey 1:50,000 map.

MEASURING DISTANCES

At the bottom centre of both the 1:50,000 and 1:25,000 scale OS map sheets there is a measuring scale.

On the 1:50,000 map you have three measuring scales, they are:

Kilometres

Statute Miles

Nautical Miles.

On the 1:25.000 map you do not have a scale for measuring Nautical Miles.

ARGYLL OUTDOOR NAVIGATION