Triangulation and trilateration are two methods used to conduct land surveying by determining horizontal measurements. The basic difference between triangulation and trilateration is that, triangulation involves measurement of angles in surveying, while trilateration involves measurement of length. The process involves measurement of sides or angles of an area that is divided into triangles, polygons, quadrilateral or a combination of figures.
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TRIANGULATION 
TRILATERATION 
The triangles formed in the
area is determined by measuring their
angles. 
The triangles formed in the
area is determined by measuring their
sides. 
A base line whose value is
known is taken as a reference for proceeding the measurement. 
The azimuth of a line is the
known value and it is taken as the reference to proceed the measurement. 
To control scale error, check
base lines are measured. 
To control angular error,
check angles are measured 
Intervisibility between
stations is essential 
It is possible to measure
distances without intervisibility 
More internal checks 
Less internal checks compared
to triangulation 
Once the angles are measured,
the sides are sides of the triangles are calculated using sine rule. In the above figure, if the anyone side for example baseline ,AB and the angles A, B and C are known, then as per sine rule, BC/sin A = AC/ sin B = AB/ sin C; 
Once the sides of the
triangular figures are determined, the angles are calculated using cosine
law. 

