A method of controlling the direction of a generally horizontal borehole drilled with a rotary drilling unit in a subterranean formation such as a coal seam. The method comprises predetermining drilling conditions for the formation being drilled and the equipment being utilized which will provide an upward buildup to the borehole trajectory, a downward change in trajectory, or a substantially level borehole trajectory. The predetermined conditions include the number of drill bit revolutions per unit length of borehole drilled. Periodic borehole surveys indicate to an operator which set of drilling conditions should be utilized for the next increment of hole drilled to maintain the hole within the boundaries of the formation being drilled.