Vertical Curvature of the Earth
Selected in MICRODEM:
In screen profiles the computer can include in its calculations curvature of the earth's surface along the profile. This can be seen on very long profiles through areas of low relief, where the ticks on both sides of the profile will be visibly at different levels (e.g. a 20 km profile through West Point will have a 50 m drop due to curvature of the earth with a 500 m vertical scale). The program will not allow curvature in very long profiles, such as those from SRTM30 where the curvature can be much greater than the actual relief.
The algorithm choices are:

Vertical Earth Curvature (m)  
Distance (m)  TM5441  Yoeli  Radio, K=4/3 
100  0.0  0.0  0.0 
500  0.0  0.0  0.0 
1000  0.1  0.1  0.1 
2500  0.4  0.4  0.4 
5000  1.7  1.7  1.5 
10000  6.8  6.8  5.9 
25000  42.3  42.7  36.9 
50000  169.0  170.7  147.4 
100000  676.0  682.9  589.7 
The differences between Yoeli and TM5441 are probably due to the low resolution of the constants in the equations, and should not be considered significant.
Earth curvature (over 3d shading from the DEM). Create on Required antenna height of Calculate, Intervisibility menu. Provides an estimate of how good a flat earth approximation will be. 
Graphs of the vertical earth curvature.
Last revision 7/29/2011