A general angle conversion strategy of the measurement on the sloping ground

Leaf area index is usually estimated by gap fraction measurement. The mean projection of the unit leaf area on a plane normal to view direction is dependent with the view zenith angle except the view zenith angle is equal to 57.5°. Researchers tried to use this particular angle to overcome the influence of the leaf inclination distribution function. However, the ring of 57.5° is relative to the normal of slope on the slope ground while the measure platform is usually designed based on the flat coordinate. A general angle conversion strategy between slope coordinate and flat coordinate is required by the measurement on the slope. The conversion method was derived in this paper and the 57.5° rings of different slope angles were drawn and compared.