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Scales Used In Surveying


Introduction


The consideration that is initially taken in choosing a scale of a map in surveying procedures are dependent on:
  1. The specific use for which the map is employed
  2. The extent to which the territory under consideration is to be represented

Rules in Choosing a Scale For a Map


The following are the general rules, that are to be followed in order to come up with an accurate scale:
  1. Choose a scale that is large enough. This will help to avoid reading of scale less than 0.25mm 
  2. Choose a small scale. This will ensure consistency on an accurate delineation of the smallest detail to be plotted. Table 1 shows the common scales, that are employed in a various survey.

Table.1.



Types of Scales in Surveying


The scale can be classified into following Types:
  1. Plain Scale
  2. Diagonal Scale
  3. Vernier Scale
  4. Scale of Chords
Plain Scale: A plain scale is defined as the scale which measures two dimensions only. These includes units, lengths, meters, decimeters, miles, and furlongs.

Diagonal Scale: On a diagonal scale, three dimensions can be measured. That is meters, decimeters, nd centimeters or units tens and hundreds or yard, that, feet, and inches.

Vernier Scale: The fractional part of one of the smallest division of a graduated scale can be measured with the help of a vernier scale. The principle of vernier is as that," eye can perceive without strain and with considerable precision when two graduations coincide to form one continuous straight line". This scale carries an index mark, which is the zero mark of the scale.

The types of vernier are:
  • Direct vernier
  • Retrograde vernier
The scale of Chords: To measure an angle or to set off an angle, the scale of chords are used. This is marked on a rectangular protractor or an ordinary boxwood scale.


Errors Due to the Wrong Scale


If the length is measured by means of a wrong scale, the correct length can be given by the following relation:



If the area of a map or a plan is measured by means of a wrong scale, the correct length can be given by the following relation:



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