There are two basic principles upon which various methods of plane surveying are based.

Let ‘X’ and ‘Y’ be the two reference points whose position is fixed. The distance ‘XY’ can be measured accurately and the relative position of ‘X’ and ‘Y’ can be plotted on sheet using some scale.

**The two principles are**__The relative positions of the points to be surveyed should be located by taking measurement from at least two points of reference. Points of reference are points whose position is already fixed.__**1) LOCATION OF A POINT BY MEASUREMENT FROM TWO POINTS OF REFERENCE:**Let ‘X’ and ‘Y’ be the two reference points whose position is fixed. The distance ‘XY’ can be measured accurately and the relative position of ‘X’ and ‘Y’ can be plotted on sheet using some scale.

Any point ‘Z’ can be located on ground by following direct methods:

- Distance between ‘XZ’ and ‘YZ’ is measured then point ‘Z’ can be located by swinging arcs from ‘X’ and ‘Y’ of the respective distance. This principle is used in Chain Surveying.

- A perpendicular ‘ZC’ can be dropped on the reference line ‘XY’. Length of ‘XC’ and ‘ZC’ is measured. Thus point ‘Z’ is plotted using a set square. This principle is used for defining details.

- Distance between ‘XZ’ and angle between ‘YXZ’ or distance between ‘YZ’ and angle between ‘YXZ’ is measured then point ‘Z’ can be located by using protractor or applying rules of trigonometry. This principle is used in traversing.

- The angle ‘YXZ’ and angle ‘XYZ’ is measured using angle measuring instrument such as clinometer and point ‘Z’ is located using protractor or using triangle formulas. This principle is used in triangulation.

- The angle ‘ZYX’ and distance ‘XZ’ is measured. Point ‘Z’ can be located using swinging an angle arc from ‘Y’ and distance arc from ‘X’.

__It is very essential to establish first a system of control points and to fix them with higher precision. Minor control points can be established by less precise methods and the details can be located using these minor control points by running minor traverses.__

**2)SECOND PRINCIPLE IS TO WORK FROM WHOLE TO PART:**__Using this principle accumulation of error is minimized.__