Selection of chain station, large scale and small scale and Well & Ill triangle

 It should be noted that plotting triangles requires no angular measurements to be made, if the three sides are known.

Chain surveying is recommended when:

1. The ground surface is more or less level

2. A small area is to be surveyed

3. A small-scale map is to be prepared and

4. The formation of well-conditioned triangles is easy

Chain surveying is unsuitable when:

1. The area is crowded with many details

2. The area consists of too many undulations

3. The area is very large and

4. The formation of well-conditioned triangles becomes difficult due to obstacles

A. Large-Scale and Small-Scale Maps

When 1 cm of a map represents a small distance, it is said to be a large-scale map.

For example,

        

When 1 cm of the map represents a large distance, it is called a small-scale map.

For example,

A map having an RF of less than 1/500 is considered to be large-scale. A map of RF more than 1/500 is said to be small-scale.

WELL-CONDITIONED AND ILL-CONDITIONED TRIANGLES

A triangle is said to be well-conditioned when no angle in it is less than 30⁰ or greater than 120⁰ . An equilateral triangle is considered to be the best-condition or ideal triangle

Well-conditioned triangles are preferred because their apex points are very sharp and can be located by a single ‘dot’. In such a case, there is no possibility of relative displacement of the plotted point.

A triangle in which an angle is less than 30⁰ or more than 120⁰ is said to be ill-conditioned

Well-conditioned triangles are not used in chain surveying. This is because their apex points are not sharp and well defined, which is why a slight displacement of these points may cause a considerable error in plotting.

 RECONNAISSANCE SURVEY AND INDEX SKETCH

Before the commencement of any survey work, the area to be surveyed is thoroughly examined by the surveyor, who then thinks about the possible arrangement of the framework of the survey. This primary investigation of the area is termed as reconnaissance survey or reconnoitre.

During the reconnaissance survey, the surveyor should walk over the area and note the various obstacles and whether or not the selected stations are intervisible. The main stations should be so selected that they enclose the whole area. The surveyor should also take care that

The neat hand sketch of the area which is prepared during the reconnaissance survey is known as the ‘index sketch’ or ‘key plan’. The index sketch shows the skeleton of the survey work. It indicates the main survey stations, sub-stations, tie stations, base-line, arrangement for the framework of triangles and the approximate positions of different objects. This sketch is an important document for the surveyor and for the person who will plot the map. It should be attached to the starting page of the field book