An experimental study on air-photo signal has been carried out for various sizes, shapes and colours in several kinds of back grounds. As the 1st stage, contrast of various colours against various vegetations, buildings, etc. has been compared on the photographs. For the 2nd stage, 57 signals of various kinds of shapes, sizes, and colours were photographed in scale 1/6, 000 and 1/10, 000, and their definitions were examined by inspection and through eyepiece of A7. Sizes of signals which were definitely recognized on the photographs of scale 1/6, 000 were as follows; signals with 3 or 4 branches of 0.4m length, squares of 0.3m side length, and black squares on white beton buildings of 0.8-1.0m side length.
