## What is Radius?

**Definition of Radius in Construction**

The length of a straight line, starting from the center of a circle and ending at the perimeter of the circle. The radius is used to describe the size of a circle or semi-circle such as a round top over a window or door. The radius of a circle is half the dimension across the circle or the diameter of the circle. Why the radius is very important in construction is the need to understand the concept of radius when laying out a structure, a wall or anything in construction that is built with a curve. The ability to swing the radius is an important aspect of surveying and layout in the field. If the center of the circle can be accessed, and the radius of the arc is identified on the documents, then the arc can be simply marked in the field by extending a line from the center of the circle and scribing the arc with either chalk, stakes or spray paint. In many instances, especially in the laying out of parking lots or larger structures, the physical ability to actually access the center of the circle for some very large radiuses is impossible. This is where the surveying crew must rely on their ability to use instrumentation to properly layout the arc as identified by the radius of the circle.