Included Angle from WCB:

If the WCB at point of intersection of survey lines AB and BC (i.e. at station B) are not given but rather fore bearing of line AB (i.e. WCB of line AB at A) and back bearing of line BC (i.e. WCB of line BC at C) are known, then the included angle at station B between survey lines AB and BC (Figure) can be obtained as follows.

Figure

WCB of AB at B = Back bearing of line AB at B = 150^o + 180^o = 330^o. Back bearing of line BC at C = 220^o.

WCB of BC at B = Fore bearing of line BC at B = 220^o - 180^o = 40^o.    

Included angle θ₁ = 340^o - 40^o = 290^o = Exterior angle. Hence, Interior angle θ = 360^o - θ₁ = 360^o - 290^o = 70^o.