# Principles of surveying in civil engineering

Surveying is the technique of determining the relative position of the object and the nature of the land. Without a survey, it is difficult or almost impossible to design any structure on the land.

The survey is important for the construction of roads on land, construction of buildings, airports,s, etc.

## Principles of surveying in civil engineering

Basically, there are two principles of surveying in civil engineering.

1. To locate the third point with the help of two points.
2. To working from whole to part

Let us describe both the principle in detail below with the help of the figure.

### 1) To locate the third point with the help of two different points

If we have two know points in position and elevation, then we can easily locate another third or fourth point.

For example, our one point is A(x1, y1) and another point is B(x2,y2), then a third point can be located with the help of the direction and position of the given two points as a reference.

We can say that the third point C(x3,y3) is located at 50 meters from point (A) in the x direction and 30 meters from point (B) in the negative y direction.

Similarly, we can locate the third point C(x3,y3), which is located perpendicular to the line joining point (A) and (B) and passes through the midpoint at a distance of 20 meters.

Let’s see in the figure,

The third point in the survey can be located by the following clue.

1. The third point is located at some distance from the two points.
2. The third point is located at some angle from the line joining two-point and at some distance from any two points.
3. The third point is located at some angle from both two points. etc.

### 2) To work from whole to part

Let we have to measure the distance of a 1000-meter road with the help of a 30 m chain. Then, what we do is, we will follow the road line with the help of a 30 m chain each time, and finally we will reach the end of the line.

So, there may be some errors in each time. let all the errors be like e1, e2, e3, e4 ad so on. and finally, we will get the total length is equal to 1000± Σerror.

Here you can see, that as you move toward your destination the error is also increasing. So, this approach to measuring the land is uncomfortable and wrong.

To remove this type of error, we work from whole to part. For this, we will create two control points as shown in the figure below. Controle points are those whole bother elevation and position are known. Let our control points be CP1, CP2, and CP3.

With the help of these control points, we can survey their surrounding area. In this case, we can see the area can be divided into three parts. So, the error in CP1 will not affect the CP2 or CP3 and we will be able to localize the error in each area.

Hence, we follow the principle of working from whole to part.