Photogrammetric densification of ground control

Abstract

In many types of surveys, it is essential to have ground control points. These are points on the ground whose coordinates are known in a ground system to a certain standard of accuracy . . A situation arises when the number and location of control points is not sufficient for the intended survey. The process of increasing the control points is normally referred to as Densification of Ground Control. Traditionally, the densification has been carried out by , . ground methods through traversing, triangulation, trilateration etc. With th~ present high accuracy of some photogrammetric methods,photogrammetric densification can now be carried out for points to be used for photograinmetric or ground surveys. Control points used tor ground surveys are usually monumented and normally have coordinates of a higher standard of accuracy than those of photogrammetric control. An estimate of the accuracy of control coordinates is usually required. The require~ent for high accuracy necessitates carefully planning of the used equipment (i.e. camera, comparator and computer) and the adjustment technique to take full advantage of the high quality of the photograrnmetric measurements. To achieve a required accuracy optimally, an a-priori investigation of a densification project is necessary. This cou£d be done by applying simulation to the variable parameters to obtain the most economical and practical combination. In this project, simulation data is created for an area of 15km x 5.5. km to be covered by aerial photography with a scale of 1:10,000. The simulation data is extracted from a topograph~c m~p of a certain area. This approach ensures the closeness of the simulation data to the real environment. The parameters selected for investigation are the side lap, the number of·~easured points on each photograph, and the number and configuration of the ground control in the block. More than 100 experiments were run using the Bundle Adjustment technique. The values of the errors are presented. The results obtained in each case could be used to get an-insight into the achievable ·accuracies for a specific combination of the parameters in executing aerial triangulation for a project of the type handled in this research