There are two main principles of surveying:
- To work from Whole to Part
- To locate a point by measurement from two known points
Let’s discuss them one by one!
1) To work from WHOLE to PART
Figure 1: Measuring distance between point A and B
Let us take an example to understand this principle. Suppose we are asked to measure the horizontal distance between two points A and B located on a natural surface (figure 1) by using a surveying chain or tape of 20 meters.
Now, If we measure the distance along the surface of the Earth then obviously it won’t be perfectly horizontal, and hence at every 20 meters we are adding up an error “e”. These errors are different so we are denoting them as e1, e2, e3….., and en.
On reaching point B, our total measured distance will be :
D = Actual horizontal distance between point A and B
e = Error in distance measurement at every chain length
The positive and negative signs simply tell that the measured distance could be greater than or smaller than the actual horizontal distance. But most probably it is greater!
Then, How to avoid this accumulation of error ??
By working from Whole to Part !
Figure 2: Showing working from whole to part
Working from whole to part simply means that if we want to survey an area, firstly locate control points in that area. Control points are the points whose coordinates are accurately known. After locating control points, locate sub-points with respect to control points as shown in figure 2, Area 3.
This approach avoids the accumulation of errors by restricting the errors while locating the sub-points to that area itself.
Finally, errors in one area will not pass on to another area, that is why we go for working from whole to part !
2) To locate a point by measurement from two known points
Given a point P1 (4,3), Can we locate a point P2 (6,8)?
We can’t !
Because we have no idea about the reference plane. In which direction X or Y coordinate is varying?
But If we are given one more point P3 (9,5), then we can easily locate point P2. Now we know the direction in which X or Y coordinate is varying. That is why we required at least two known points to locate the third one.
There are many ways by which we can locate the third point. Some of them are:-
|If we know the distance of point C from both points A and B|
|If we know the angles towards point C from both points A and B|
|If we know one angle and one distance :> Both from one single point> Angle from one point and distance from another|
|If we know the perpendicular offset from a given point on the mainline.|
Hence you need at least two points !!2