The CSEC syllabus requires students to be able to use the scale on a map to determine actual distances between points on the ground. The scale on the map you will be presented with may be in the form of a ratio such as 1:25,000. This means that one unit on the map represents 25,000 of these same units on the ground. Therefore, on a map with this scale, one centimeter on the map represents 25,000 centimeters on the ground. One inch on this map represents 25,000 inches on the ground.
Your map should also have a linear or graphic scale. This is a black and white bar which shows distances in kilometers or miles (or both).
This linear or graphic scale shows distances on the map in both kilometers and miles
Measuring Straight Line Distance
You may be required to measure the distance in a straight line between two objects on your map. Let’s call them objects A and B. This is a relatively simple process. All you need to do is measure the distance from object A to object B on your map and use a scale to determine the actual distance.
Method 1
Let’s suppose the distance between the objects on the map when measured with a ruler is 10 cm and the scale on the map is 1: 25,000. The actual distance can be determined by multiplying the distance on the map by 25,000 as shown below.
10 cm X 25,000 = 250,000 cm = 2,500 meters = 2.5 km
Method 2
Use a piece of paper to mark the distance between objects A and B on your map. Then hold the piece of paper against the graphic scale to determine the actual distance. Ensure that you begin measuring from the point on the scale marked with a zero. The video below demonstrates this process.
Distance along a road, railway etc.
You may be asked to determine the distance along a feature on the map such as a section of road or railway which is not straight. This is only slightly more challenging than straight line distance.
All you need to do is divide the road or railway into sections which are fairly straight. Use a piece of paper to mark off your starting point and the end of your first section. Move the paper so you can mark from the end of the first section to the end of the second section. Continue in this manner until you have marked off all the sections.
If you have done this well you will now have the distance along the road or railway (or whatever the feature may be) on a straight piece of paper that you can measure against your linear scale as described in the previous section. This is demonstrated in the video below.
References
https://www.thoughtco.com/how-to-measure-distances-on-map-1435698
https://getoutside.ordnancesurvey.co.uk/guides/beginners-guide-to-measuring-distance-on-a-map/