The combined error from curvature and refraction is -0.215 feet.
The combined error from curvature and refraction is calculated as follows:
Ehm = (d1^2 - d2^2) / 2R
where:
Ehm is the error in feet
d1 is the horizontal distance to the plus sight in feet
d2 is the horizontal distance to the minus sight in feet
R is the radius of the Earth in feet
Using the values given in the problem, we can calculate the error as follows:
Ehm = (22,235^2 - 50,165^2) / 2 * 20,903,230.55
Ehm = -0.215 feet
The combined error from curvature and refraction is -0.215 feet, or -0.215 feet to three significant figures.