Principles of surveying; scales used

Principles of surveying

1. Working from whole to part. A precise control framework is made over the whole area first; the details are then surveyed inside it. Errors are thus localised and do not accumulate.
2. Fixing a point by at least two independent measurements from known points: two distances, two angles, or one angle and one distance (or an offset and a chainage). A third measurement is taken as a check.

A survey is also carried out with checks on all measurements and with accuracy suited to the purpose.

Scales used in surveying

A scale is the ratio of a length on the map to the corresponding length on the ground. It can be expressed as a representative fraction (RF), e.g. 1/50,000, a statement (1 cm = 500 m) or a graphical scale.

1. Plain scale. A line divided into main divisions and sub-divisions; it reads two units (e.g. metres and decimetres).
2. Diagonal scale. Reads three units (e.g. 100 m, 10 m and 1 m) using diagonal lines and the principle of similar triangles.
3. Vernier scale. A sliding scale that reads a fraction of the smallest main division. Least count = value of 1 main division / number of vernier divisions.
4. Comparative scale. Two scales with the same RF but different units, such as a metric and a British scale on one map.
5. Scale of chords. Used for plotting and measuring angles when no protractor is available.
6. Scale of time or speed. Used when time is to be measured from distance travelled at a constant speed.
7. Isometric scale. Used for isometric drawings.

By size, maps are large scale (1:500 to 1:5000, used for detailed plans), medium scale and small scale (1:50,000 and smaller, used for atlases).