surveying iii (topographic and geodetic surveys) - Modern Prepper

may (and usually does) far exceed the internal accuracy of the raw GPS observations. As with aconventional EDM traverse, the 3D misclosures may be about adjusted by proportionately distributingthem over the intermediate points. A least squares adjustment also accomplishes the same thing. Forexample, if the GPS-S is looped back to the initial point, the free-adjustment misclosure at the initialpoint may be compared with the apparent position misclosure with the other fixed point. A freeadjustmentloop misclosure is 1:100,000, whereas the misclosure relative to the two network controlpoints is only 1:5,000. Thus, the internal relative accuracy of the GPS-S is about 1:100,000 (based onthe misclosure). If the GPS-baseline observations are constrained to fit the existing control, the 0.6-meter external misclosure must be distributed among the individual baselines to force a fit between thetwo end points.If the intent of the above example was to establish 1:20,000 relative-accuracy control,connecting between these two points will not provide that accuracy given the amount of adjustment thatmust be applied to force a fit. For example, if one of the individual baseline vectors was measured at600 meters and the constrained adjustment applied a 0.09-meter correction in this sector, the relativeaccuracy of this segment would be roughly 1:6,666. This distortion is not acceptable for subsequentdesign/construction work.(3) Most GPS-S networks are more complex than a simple traverse. They may consist ofmultiple loops and may connect with any number of control points on the existing network.Additionally, conventional EDM, angles, and differential-leveling measurements may be included withthe GPS baselines, resulting in a complex network with many adjustment conditions.5-51. Partially Constrained Adjustments. In the previous example of a simple GPS traverse, holdingthe two network points rigidly caused an adverse degradation in the GPS-S because of the differencesbetween the free (loop) adjustment and the fully constrained adjustment. Another alternative is toperform a partially constrained adjustment of the net. In a partially constrained adjustment, the twonetwork points are not rigidly fixed but are only partially fixed in position. Partially constrainedadjustments are not practicable using approximate adjustment techniques.a. For example, if the relative-distance accuracy between the two fixed points is about 1:10,000, itcan be equated to a positional uncertainty between these points. Depending on the type and capabilitiesof the least squares adjustment software, the higher-accuracy GPS baseline observations can be best fitbetween the two end points. The end points of the GPS network are not rigidly constrained to theoriginal and two control points but will end up falling near them.b. Adjustment software allows relative weighting of the fixed points to provide a partiallyconstrained adjustment. Any number of fixed points can be connected, and these points may be givenpartial constraints in the adjustment.EN0593 5-46