8. Staking a Spiraled Horizontal Curve

a. Establish TS and ST

Set  TS and ST points at distance T_s from PI along each tangent.

b. Stake entrance spiral

Set instrument at TS.

Backsight PI with 0°00'00" on horizontal circle.

Radially stake each spiral point through the SC.

c. Stake circular arc

Because it is offset from the tangents, the arc can't be staked from a BS using the PI as a backsight. Instead, once the entrance spiral has been staked, the circular arc must be staked from the SC.

At the flat end of the spiral, the deflection angle is (Δs)/3; at the other end it is (2Δs)/3, Figure E-15.

Figure E-15 Spiral Total Deflection Angles

To correctly orient an instrument at the SC, the surveyor would, Figure E-16:

	Set up on SC. Backsight the TS with -(2Δs)/3 on the horizontal circle.
	Rotate to 0°00'00". Instrument in now tangent to the spiral and circular arc.
	Reverse telescope about the horizontal axis. Instrument is sighting where the arc's PI would be. Stake the curve to the CS using the deflection angle information
Figure E-16 Staking the Circular Arc

b. Stake exit spiral

Set instrument at ST.

Backsight PI with 0°00'00" on horizontal circle.

Radially stake each spiral point through the CS.

The CS serves as a closure check since it was placed when the arc was staked.