# How to set out curve by perpendicular offsets from tangent lines

In this construction video tutorial, the renowned SL Khan, has briefly explained how to lay out road or railway line curve with perpendicular offsets from tangent lines method. The process is very useful for land surveying.

In survey work, it is frequently required to specify right angles or perpendicular lines on the field. In the sections that pursue, a few practical methods point out how this is performed. These methods contain the following:

- the 3-4-5 method: applied to lay out a right angle out of a specific point on the base line;

- the rope method: applied to lay out a line perpendicular to the base line that originates from a point not situated on the base line;
- the single prismatic square and the double prismatic square: Applied to lay out right angles and perpendicular lines simultaneously.

The concept of this method is explained briefly as follow :-

The first step is to find out the curve points by defining the tangent length of the curve. Afterwards, divide the tangent line with some intermissions and choose perpendicular offsets from tangent line. Then, join the ends of all offsets which produce a good looking and smooth curve.

To learn the detail process, watch the following video.

Video Source: SL Khan