1. First order or Primary Triangulation
2. Second order or Secondary Triangulation
3. Third order or Tertiary Triangulation

**CLASSIFICATION OF TRIANGULATION SYSTEM**

The basis of the classification
of triangulation figures is the accuracy with which the length and azimuth of a
line of the triangulation are determined. Triangulation systems of different
accuracies depend on the extent and the purpose of the survey. The accepted
grades of triangulation are:

1. First
order or Primary Triangulation

2. Second
order or Secondary Triangulation

3. Third
order or Tertiary Triangulation

**1 ****FIRST
ORDER OR PRIMARY TRIANGULATION: **

The first order triangulation is of the highest order and is
employed either to determine the earth's figure
or to furnish the most precise control points to which secondary triangulation
may be connected. The primary triangulation system embraces the vast area
(usually the whole of the country). Every precaution is taken in making linear
and angular measurements and in performing the reductions. The following are
the general specifications of the primary triangulation:

1. Average
triangle closure : Less than 1 second

2. Maximum
triangle closure : Not more than 3
seconds

3. Length
of base line : 5 to 15 kilometers

4. Length
of the sides of triangles : 30 to
150 kilometers

5. Actual
error of base : 1 in 300,000

6. Probable
error of base : 1 in 1,000,000

7. Discrepancy
between two measures of a section : 10 mm kilometers

8. Probable
error or computed distance : 1 in
60,000 to 1 in 250,000

9. Probable
error in astronomic azimuth : 0.5
seconds

**2 SECONDARY ORDER OR SECONDARY TRIANGULATION**

The secondary triangulation consists of a number of points
fixed within the framework of primary triangulation. The stations are fixed at
close intervals so that the sizes of the triangles formed are smaller than the
primary triangulation. The instruments and methods used are not of the same
utmost refinement. The general specifications of the secondary triangulation
are:

1. Average
triangle closure : 3 sec

2. Maximum
triangle closure : 8 sec

3. Length
of base line : 1.5 to 5 km

4. Length
of sides of triangles : 8 to 65 km

5. Actual
error of base : 1 in 150,000

6. Probable
error of base : 1 in 500,000

7. Discrepancy
between two measures of a
section : 20 mm kilometers

8. Probable
error or computed distance : 1 in
20,000 to 1 in 50,000

9. Probable
error in astronomic azimuth : 2.0
sec

**3 THIRD ORDER OR TERTIARY TRIANGULATION:**

The third-order triangulation consists of a number of points
fixed within the framework of secondary triangulation, and forms the immediate
control for detailed engineering and other surveys. The sizes of the triangles
are small and instrument with moderate precision may be used.
The specifications for a
third-order triangulation are as follows:

1. Average triangle
closure : 6 sec

2. Maximum triangle
closure : 12 sec

3. Length of base
line : 0.5 to 3 km

4. Length of sides
of triangles : 1.5 to 10 km

5. Actual error of
base : 1 in 75, 0000

6. Probable error
of base : 1 in 250,000

7. Discrepancy
between two Measures of a section : 25 mm
kilometers

8. Probable error
or computed distance : 1 in 5,000
to 1 in 20,000

9. Probable error
in astronomic Azimuth: 5 sec.

Explain the factors to be considered while selecting base
line.

The measurement of base line
forms the most important part of the triangulation operations. The base line is
laid down with great accuracy of measurement and alignment as it forms the
basis for the computations of triangulation system. The length of the base line
depends upon the grades of the triangulation. Apart from main base line,
several other check bases are also measured at some suitable intervals. In
India, ten bases were used, the lengths of the nine bases vary from 6.4 to 7.8
miles and that of the tenth base is 1.7 miles.

Selection of Site for Base Line. Since the accuracy in the
measurement of the base line depends upon the site conditions, the following
points should be taken into consideration while selecting the site:

1. The
site should be fairly level. If, however, the ground is sloping, the slope
should be uniform and gentle. Undulating ground should, if possible be avoided.

2. The site
should be free
from obstructions throughout
the whole of the
length. The line clearing should be cheap in both labour and compensation.

3.The
extremities of the base should be intervisible at ground level.

4. The
ground should be reasonably firm and smooth. Water gaps should be few, and if
possible not wider than the length of the long wire or tape.

5. The site
should suit extension to primary triangulation. This is an important factor
since the error in extension is likely to exceed the error in measurement.

In a flat and open country, there
is ample choice in the selection of the site and the base may be so selected
that it suits the triangulation stations. In rough country, however, the choice
is limited and it may sometimes be necessary to select some of the
triangulation stations that at suitable for the base line site.

Standards of Length. The ultimate standard to which all modern
national standards are referred is the international meter established by the
Bureau International der Poids at Measures and kept at the Pavilion de
Breteuil, Sevres, with copies allotted to various national surveys. The meter
is marked on three platinum- iridium bars kept under standard conditions. One
great disadvantage of the standard of length that are made of metal are that
they are subject to very small secular change in their dimensions. Accordingly,
the meter has now been standardized in terms of wavelength of cadmium light.

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Civil Surveying - Control Surveying : Classification of Triangulation System |

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