When modeling landscapes, be aware of curvature of the Earth. With perfectly flat ground surface, at 9.26 km from observer surface will be 6.7 meters below* tangent line drawn at observers location! That makes great difference from flat ground. You can easily see that on pictures below:
* along line perpendicular to tangent line
Also be aware of refraction, which at standard atmospheric conditions rises horizon by ~8% and even more at non-standard.