On the Accuracy of Contours in Photogrammetry

Abstract

Most of the specifications in survey contract in Japan have some vagueness on the accuracy of contours. Usually they give the half of the contour intervals 0.5Δ as the tolerance of contouring accuracy, without further comments on the term“accuracy”. Another kind of specifications, those in USA, for example, describes that“90% of the errors of spot hight interpolated from contour lines should be less than 0.5Δ”.
After comparison of three two kinds of descriptions, the latter seems more practical and reasonable, and is equivalent to assume the standard deviation of contouring as about 0.30Δ”. Further detailed analysis considering the errors of adjacent contour lines simultaneously resulted the similar conclusion but with some spread of standard deviations, 0.25Δ-0.36Δ, for example.
Finally the contouring accuracy in photogrammetry may have different characters from that in plane table surveying, by which contours are observed on the terrain in nearly horizontal directions, while by the former procedure the observations are done from the above in the air. The numerical constants α and β in the Koppe's formula are compared to examine the difference in both procedures. The ratio β/α in photogrammetry shows smaller values than those in plane table surveying from the experimental work.
It is tried to give the ratio β/α an interpretation as the tangent of observing direction, based on the modified Koppe's formula (24) . As an example, observing directions in photogrammetry are 19°-46° from the nadir lines and 70°-85° in plane table survey.