The reference of any point, say X, has to kept with respect to, at least, two permanent objects or well defined points, say Y and Z. Generally, this has been achieved by taking measurement of two parameters.The location of a point, say X can be done as shown in the figure below.

(a) Distances YX and ZX (Figure 1.2)

(b) Perpendicular distance OX and distance OY or OZ (Figure 1.3)

(c) Distance YX or ZX and angle YZX or ZYX (Figure 1.4(a))and (Figure 1.4(b))

(a)
(b)
   
Figure 1.4 Reference of a point using a distance and an angle

(d) Angles YZX and ZYX. (Figure 1.5)

The point of intersection of the two measured parameters defines the position of the point.

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