surveying iii (topographic and geodetic surveys) - Modern Prepper

point in the survey to provide the directional control needed in the survey network. The observation ofthe longitude and azimuth is frequently repeated in the survey to give control to the geodetic network.When combined with other geodetic measurements, astronomic observations provide a method fordetermining the deflection of the vertical. This can help in determining the figure of the earth. Places atwhich astronomic longitude and azimuth are measured are called Laplace stations.c. When making astronomic observations with an optical instrument containing a leveling device,the vertical axis of the instrument is perpendicular to the geoid. This means that the axis coincides withthe direction of the force of gravity or a plumb line at the observation site. Therefore, an astronomicobservation gives a direction with respect to the geoid.d. The normal to the ellipsoid defines geodetic latitude and longitude. Since the direction of gravityrarely coincides with the normal of the ellipsoid, astronomic coordinates do not represent positions onthe ellipsoid (Figure 1-6). This difference, the deflection of the vertical, can be used to help determinethe accuracy of the survey technique and the suitability of the ellipsoid for use in a particular surveyedarea.Figure 1-6. Deflection of the Vertical1-9 EN0593