Observations of the vertical deflections and astronomical azimuthes at Laplace points in Japan were commenced in 1947 by the Geographical Survey Institute. On the basis of these observations, the azimuthal twist of the Japanese first-order triangulation net was studied. First, the astronomical longitude, latitude and azimuth at the geodetic datum point were compared with the values adopted as the standard datum of the Japanese geodetic coordinates. The differences in a sense of a-g were found as:dL
0=-0.75, dB
0=-0."50and dA
0=-0."50.Reducing these values for the geodetic positions of 35 triangular points where the astronomical azimuths were observed, the corrected residuals of the Laplace equation were calculated. The mean of these residuals becomes about +0."7, and it is verified that, as a whole, large or systematic azimuthal twist does not exist in our first-order triangulation net in spite of the omission of the Laplace adjustment.
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