There are two main methods of determining distance i.e direct and indirect or computative. The direct method includes the use of chain, tape and other instrument and is very accurate. The indirect method the distances are obtained by calculations as in techometry, telemetry or triangulation.
The following are various methods used for measuring distance by direct method, but the method to be used depends largely upon the degree of accuracy required.
1) pacing:- It consists in walking over a distance and counting the number of paces. The average length of a pace is 80 cm. This method is chiefly used for Reconnaissance surveys for the preparation of military plans and for locating details in small-scale mapping.
An instrument known as passometer may be used for counting the number of paces.
2) instrumental method:-some instruments like pedometer, speedometer, odometer, perambulator may be used to find the distance approximately. The speedometer gives better results than pacing provided the route is smooth as along a highway.
3) judging distance:-this is a very rough method of determining distances. It is used in estimating distance of details in Reconnaissance survey.
4) time measurement:-the distances are also roughly determined by knowing the average time taken per km by a person at walk or horse at trot and the total time taken to cover that distance.
5) chaining:-the method of measuring distance with the help of a chain or tape is called chaining. It is the most accurate and common method of measuring distance.
The ratio of the distance on the map or drawing to the corresponding distance on the ground is called a representative fraction (R.F).
The scales maybe plain scale, diagonal scale, comparative scale, shrunk scale and vernier scale.
A plain scale is used to read only two dimensions such as metres and decimetres.
A diagonal scale is used to read three dimensions such as metres, decimetres and centimetres.
A comparative scale has a common representative fraction but read in different measures.
The vernier scale is used for measuring fractional parts of the smallest division of the main scale. There are two types of verniers i.e direct vernier and retrograde vernier. In a direct vernier the smallest division of vernier is shorter than the smallest division of its primary scale. In a retrograde vernier the smallest division of vernier is longer than the smallest division of its primary scale.
The ratio of the smallest division on the main scale to the number of divisions on the vernier is called the least count of the vernier.