The vertical angle is measured relative to the local vertical (plumb) direction. The vertical angle is usually measured as a zenith angle (0° is vertically up, 90° is horizontal, and 180° is vertically down), although you are also given the option of making 0° horizontal. The zenith angle is generally easier to work with. The telescope will be pointing downward for zenith angles greater than 90° and upward for angles less than 90°. If you make horizontal 0° then you have to work with positive and negative vertical angles, this provides an unnecessary source of error.