This will ensure that future work will be supported by a reliable and consistent basic network, while
minimizing errors associated with mixed datums.
5-50. Internal and External Adjustments. GPS-Ss are usually adjusted and analyzed relative to their
internal consistency and external fit with existing control. The internal consistency adjustment (such as
free or minimally constrained) is important from a mission compliance standpoint. The final (or
constrained) adjustment fits the GPS-S to the existing network. This is not always easily accomplished
since 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 adjustment
a. Internal Adjustment. An internal adjustment (also referred to as a free adjustment) is made to
determine how well the baseline observations fit or internally close. Other EDM distances or angles
may also be included in the adjustment. This adjustment provides a measure of the internal precision of
the survey.
(1) In a simplified example, a conventional EDM traverse that is looped back to the starting
point will misclose in both azimuth and position. Classical approximate-adjustment methods will
typically assess the azimuth misclosure, proportionately adjust the azimuth misclosure (usually evenly
per station), recompute the traverse with the adjusted azimuths, and obtain a position misclosure. This
position misclosure (in X and Y) is then distributed among all the points on the traverse using various
weighting methods (such as distance, latitude, or departure). Final adjusted azimuths and distances are
then computed from grid inverses between the adjusted points. The adequacy and accuracy of such a
traverse is evaluated based on the azimuth misclosure and the position misclosure after an azimuth
adjustment (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 the
points similarly to the approximate methods traditionally employed. The only difference is that a least
squares adjustment simultaneously adjusts both the observed angles (or directions) and the distance
measurements. A least squares adjustment also allows for variable weighting to be set for individual
angle or distance observations, which is a somewhat more complex process when approximate
adjustments are performed. Additionally, a least squares adjustment will yield more definitive statistical
results of the internal accuracies of each observation or point, rather than just the final closure. This
includes estimates of the accuracies of individual station coordinates, relative azimuths, and relative
distances.
(3) A series of GPS baselines forming a loop off a single point can be adjusted and assessed
similarly to a conventional EDM traverse loop described above. The baseline vector components may
be computed (accumulated) around the loop with a resultant 3D misclosure at the starting point. These
misclosures (in X, Y, and Z) may be adjusted using either the approximate or least squares methods.
The
EN0593
5-44
Previous Page
