Here, its worth mentioning that when the value of inclination (I) and azimuth (A) are same at both survey stations, then the denominator for some of the above equation becomes zero and hence the equation is not defined.
The error will be small and well within the accuracy needed in the field provided the distance between surveys is not too great.
The radius of curvature method is currently considered to be one of the most accurate methods available.
In directional survey we note down the inclination and azimuth at the survey point for a particular survey depth and using these data we calculate the North-South (N-S), East-West (E-W) coordinates and TVD using few mathematical calculations.
With the tangential method, the greater the build or drop rate, the greater the error.
The method assumes the wellbore course is a smooth curve between the upper and lower survey stations.