This will ensure that future work will be supported by a reliable and consistent basic network, while

minimizing errors associated with mixed datums.

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