REPRESENTATION OF SCALE IN SURVEYING (StudyCivilEngg.com)
REPRESENTATION OF SCALE IN SURVEYING
SUBJECT : SURVEYING
The following three methods are used to represent the scale of a map.
- Engineer’s scale
- Representative fraction
- Graphical scale
The first two methods represent the scale numerically, whereas the third method, as the name indicates, is graphical.
Engineer’s scale
It is indicated by a statement. For example, 1 cm = 50 m. According to this scale, a specified distance on the map represents the corresponding distance on the ground. In this example, 1 cm on the map represents 50 m on the ground.
Representative Fraction (R.F.)
It is indicated by a ratio. The representative fraction is the ratio of the distance on the map to the corresponding distance on the ground. For example, for a scale of 1 cm = 50 m, 1 cm on the map represents 50 m (5000 cm) on the ground. Therefore, the representative fraction (R.F.) is 1/5000 or 1: 5000. It may be noted that the larger the denominator of RF., the smaller is the scale of map.
Graphical scale
A graphical scale is a line drawn on the map so that its map distance corresponds to a convenient units of length on the ground. A graphical scale is constructed on the map by drawing a line and marking the ground distances directly on it.
Below figure (a) shows a graphical scale corresponding to a scale of 1 cm = 5 m. In this case a line 12 cm long is drawn and divided into 6 equal parts of 2 cm each. Thus each part represents 10 m on the ground. The first part is further divided into 10 divisions, each division representing 1m. The units of measurement must always be written on the graphical scale.
The zero of the scale is generally placed between the units and subdivisions so that the measured distances from the map can be determined conveniently. Figure (a) shows a distance of 44 m marked on the scale. In Figure (b) the zero is placed at the end. It is not convenient to measure the distance directly. Figures (c) and (d) show two other types of graphical scales commonly used.
SUGGESTED SCALES FOR DIFFERENT TYPES OF SURVEY
The graphical scale has the advantage over the numerical scales that the distances on the maps can be determined by actual scaling even when the map has shrunk or has been reproduced to some other scale. In such cases, the graphical scale also changes with the map, and, therefore, the ratio is unaffected.
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