surveying iii (topographic and geodetic surveys) - Modern Prepper

This will ensure that future work will be supported by a reliable and consistent basic network, whileminimizing errors associated with mixed datums.5-50. Internal and External Adjustments. GPS-Ss are usually adjusted and analyzed relative to theirinternal consistency and external fit with existing control. The internal consistency adjustment (such asfree or minimally constrained) is important from a mission compliance standpoint. The final (orconstrained) adjustment fits the GPS-S to the existing network. This is not always easily accomplishedsince existing networks often have lower relative accuracies than the GPS observations being fit.Evaluation of a survey's adequacy should not be based solely on the results of a constrained adjustmenta. Internal Adjustment. An internal adjustment (also referred to as a free adjustment) is made todetermine how well the baseline observations fit or internally close. Other EDM distances or anglesmay also be included in the adjustment. This adjustment provides a measure of the internal precision ofthe survey.(1) In a simplified example, a conventional EDM traverse that is looped back to the starting pointwill misclose in both azimuth and position. Classical approximate-adjustment methods will typicallyassess the azimuth misclosure, proportionately adjust the azimuth misclosure (usually evenly perstation), recompute the traverse with the adjusted azimuths, and obtain a position misclosure. Thisposition misclosure (in X and Y) is then distributed among all the points on the traverse using variousweighting methods (such as distance, latitude, or departure). Final adjusted azimuths and distances arethen computed from grid inverses between the adjusted points. The adequacy and accuracy of such atraverse is evaluated based on the azimuth misclosure and the position misclosure after an azimuthadjustment (usually expressed as a ratio to the overall length of the traverse).(2) A least squares adjustment of the same conventional loop traverse will end up adjusting thepoints similarly to the approximate methods traditionally employed. The only difference is that a leastsquares adjustment simultaneously adjusts both the observed angles (or directions) and the distancemeasurements. A least squares adjustment also allows for variable weighting to be set for individualangle or distance observations, which is a somewhat more complex process when approximateadjustments are performed. Additionally, a least squares adjustment will yield more definitive statisticalresults of the internal accuracies of each observation or point, rather than just the final closure. Thisincludes estimates of the accuracies of individual station coordinates, relative azimuths, and relativedistances.(3) A series of GPS baselines forming a loop off a single point can be adjusted and assessedsimilarly to a conventional EDM traverse loop described above. The baseline vector components maybe computed (accumulated) around the loop with a resultant 3D misclosure at the starting point. Thesemisclosures (in X, Y, and Z) may be adjusted using either the approximate or least squares methods.TheEN0593 5-44