Further to the information covered within the following
sections, more-generalised least squares adjustment subjects will
be found in the Network
Adjustment - Least Squares topic.
3D Adjustments - Introduction
In addition to the 1D-Vertical and 2D-Horizontal least squares adjustment modes, there are two 3D adjustment modes as follows:
|CAUTION - In 3D Spatial/GPS - XYZ mode the adjusted points' standard deviations and error ellipses are determined and reported in the Cartesian XYZ coordinate frame and may be misleading if casually applied to the adjusted points in the normal geodetic system|
Thus adjustments to network points for GPS observations will be
made in either cartesian XYZ or local geodetic space with these two
modes, depending on the mode selected and the presence of fully- or
partially-fixed control points.
Note that if the network consists entirely of GPS observations
and they have been imported or entered from a single source where
adjustment has already been made to the network of observations by
the GPS processing software from which they were derived, there may
be little point in further attempting to adjust these observations
amongst themselves. However, transformation or localisation into
the local datum may still be required.
Observed Coordinates - In some cases, observed coordinates exported from proprietary post-processing GPS software may be unadjusted positions. Unless their standard deviations are relaxed, it is probably inappropriate to include these observations (Coord. Observations tab) in an adjustment since they may distort the adjustment and the statistical results for the network. Similarly, unless observed coordinates are adjusted values, using them to compute localisation parameters is not recommended. Refer to Leica SKI point classes for more information. To remove observations from the network, either disable individual observations, or un-tick the Active checkbox for the whole observation type, but note that a localisation cannot be computed from these points if the Observed Coords. type is set in-active.
Horizontal Distances in 3D adjustments -
Horizontal distances can be included for adjustment in 3D but are
better suited to shorter distances only (not geodetic), since the
length determined for any specific line may vary depending on which
end of the line the observed slope distance is reduced to
horizontal. It is doubtful if horizontal distances are ever
'observed' as such since they will always have been reduced from
measured slope distances.
|IMPORTANT NOTE - Horizontal distances should NOT be used if they have been reduced from slope distance observations over the same lines that will ALSO be used in the adjustment.|
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Where GPS and terrestrial observations are combined into a single network, separate 2D adjustment of the terrestrial observations and another 3D adjustment of the GPS observations should first be done to check for (and more easily eliminate) observation anomalies, errors and outliers before combining all observations into a single 3D network adjustment - this will also help to avoid errors being hidden and compensated by the adjustment. Further, to check for internal consistency, adjustment should first be made with minimum constraints to avoid placing strain on the observations - for more information refer to the Constraints and Minimally-constrained network sections in the Network Adjustment - Least Squares topic.
"Combining GPS and terrestrial data requires a common coordinate
system. When the original GPS vectors do not form a network, the 3D
network adjustment cannot be performed. In this case, in order to
integrate the GPS measurements with the terrestrial observations
and to perform a combined network adjustment, the GPS measurements
should be transformed to this common system." Derivation of some
geometric parameters of GPS measurements - Adel Alfrehat, Janka
Sabovï¿½nd Marcel Mojzes, Acta Montanistica Slovaca, Volume
10 (2005), No. 3, 310 - 316
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Three situations for 3D adjustments
Note (Cases 1 and 2) - Although the 3D adjustment is performed
in the local geodetic system, both the geodetic and geocentric
coordinates are required to compute the coefficients and
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Combining GNSS and terrestrial observations
Combining terrestrial observations with GNSS and immediately adjusting the whole network with the 3D Terrestrial + GPS option can result in a poor or un-converged adjustment if there are 'incompatible' observations present. In such a case an important point to start preparing for the adjustment is to isolate the broad observation types and compute separate 1D, 2D and 3D preliminary adjustments as follows:
If there are no conflicts, the adjustment should be satisfactory but once again may need fine-tuning and refinement of parameters and settings as in the preceding steps.
However if the adjustment now immediately diverges or there are excessive residuals, corrections, observation outliers, large error bars or ellipses, unacceptable variance factor or other statistical values, etc, then incompatibilities exist between the 2D and 3D observations which must be identified and eliminated. For example:
For related information please refer to these Help topics:
or to the
Least Squares Adjustment - Tips and Suggestions
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Localisation of point coordinates or GPS vector observations
Normally localisation will be used for post-processed GPS
baseline and point observations so that observations made on the
GPS WGS84 or ITRF datum can be localised onto the job datum prior
to least squares network adjustment. However localisation may also
be necessary in other situations, such as to derive transformation
parameters to transform points on a foreign datum (entered as
points on the Coord Observations tab with the coordinate
system defined by the Coord Obs. Type drop-down list) to the
current job datum.
For further details regarding localisations refer to Localisation (Site Calibration).
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