AbstractBecause lengths measured by electromagnetic instruments are supplementing or replacing angular measurements, it is essential that all required geometric corrections be applied in their reduction to the ellipsoid in accordance with Helmert's projection method. A correction to sea level distance for an approximate geoidal height is in general insufficient. It is necessary to determine thedifferencesbetween heights above the ellipsoid of the ends of steep lines, for which astro-geodetic deflections are needed.