Collimation Method / HOI Method:

In today’s era various methods are available of levelling. While surveying various equipment’s and methods used by surveyors. One of them is Collimation method/ Height of Instrument method of Dumpy level / Auto Level surveying.

While using this method we have to understand same basic rules or assumptions of this method.

1.     The first point/station of the dumpy level/auto level surveying is called as Benchmark. And the reading of that point is called as backside reading (B.S.)

2.     The last point/station of the dumpy level/auto level surveying and the reading of that point is called as foreside reading (B.S.)

3.     Points between B.S. and F.S. are known as Intermediate Points and the readings of that points is called as Intermediate Side readings. (I.S.)

The following figure will explain it in detail:

 

Example:

1.    The following staff readings were taken in succession with a level. The instrument was shifted after 3^rd ,6^th, 8^th reading. By the plane of collimation method taken R.L. and Bench Mark as 500 mts. Calculate the R.L. and All the points:

Readings are: 0.365, 1.450, 2.335, 0.855, 2.225, 2.905, 1.275, 0.725, 1.975, 2.305, 2.610, 3.335

Solution :- 

	Readings
Sr. No.	B.S.	I.S.	F.S.	P.C.	R.L.	Remark
1	0.365			500.365	500.000	Bench Mark
2		1.450			498.915
3			2.335		498.03	C.P. (1)
4	0.855			498.885	498.03	Same Point & Staff Station
5		2.225			496.66
6			2.905		495.98	C.P. (2)
7	1.275			497.255	495.98	Same Point & Staff Station
8			0.725		496.53	C.P. (3)
9	1.975			498.505	496.53	Same Point & Staff Station
10		2.305			496.2
11		2.610			495.895
12			3.335		495.17	End Point
	∑ B.S. = 4.47		∑ F.S. = 9.3

 Arithmetic Check =

SUM (B.S.) – SUM (F.S.)                            = Last R.L. – First R.L.

= 4.47 – 9.3                                                = 495.17 – 500.00

= -4.83                                             = -4.83

 both sides are equal. that means our answer is correct.