Localisation (Site Calibration)


Localisation (Site Calibration)

A 3D-similarity (Helmert) transformation may be computed from GPS-derived point observations (ITRF/WGS84 datum) or other point coordinate measurements, entered into the Coord Observations tab and matching fixed control points on the local job datum and defined on the Network Points tab. Based on the computed transformation parameters, the remaining points and (optionally) GPS baseline observations can then be localised onto the job datum prior to least-squares adjustment of the network.

The computed transformation parameters are saved for re-use within the data set, and can also be saved to the Transformations library for use in any other job as a normal 7-parameter 3D Similarity transformation.

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 can also be used for other situations, most likely 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.

Figure 1 - Coord Obs. Type selection

In such a case as this, the parameters may be computed (select Compute from Current Control in the drop-down list) from:

  1. current observation data
  2. a previously-computed localisation (select Use previous localisation - Check parameters), or
  3. a 3D-Similarity transformation whose parameters are already defined in the definitions library (all transformations matching the current job datum are listed by name in the drop-down list).
Figure 2 - Localisation type selection

For NO localisation of current coordinate observations or vectors, the observations must be set 'Active' but with 'Localisation' un-ticked; if observations are NOT active or there are no coord observations, a localisation of GPS baseline observations may still be performed by selecting 'Use previous localisation - Check parameters' which will use the parameters already used previously (this option listed only when a localisation has already been computed), or an existing named transformation may be selected from the list. Click the '< Parameters' button to check the parameters of a previously-used or previously-computed localisation or any selected pre-defined transformation.

Localisation based on simple coordinate observations can also be done for standard survey jobs without any additional GPS-derived coordinate or baseline observations, or for those jobs configured for simple plane (non-ellipsoidal) coordinates. In such jobs coordinate "observations" may simply be points from another datum, however, all points on both the Network Points and Coord. Observations tabs must have valid heights; if the points have been defined with only 2D horizontal coordinates and no Height values, specify all the point heights as zero (0.000, not blank). With the Least Squares adjustment mode set to '2D Horizontal' and with 'Localisation' set to ‘Compute from Current Control’, on Compute the mode is temporarily forced into 3D-Terrestrial mode for the purpose of computing the 7 parameters for the localisation. In this case the parameters computed will effectively be 2D transformation parameters simply because there will be no vertical translation (tZ = 0.000) and no rotations about the horizontal axes (Rot-X = 0 and Rot-Y = 0). In 2D mode, set 'Coordinate Type' to 'Non-Geodetic' for Grid Projection jobs, or set to 'Job Datum' for Plane jobs. Localisation and adjustment with only point observations does not require any other observation data type to be present or active.

If it is only desired to compute the localisation or transformation parameters or to localise/transform a group of points onto the job datum, it is NOT necessary to continue with a network adjustment. When a new localisation is computed, the residuals from the adjustment of the selected matching and fixed points are displayed for verification - the user may stop further processing and return to the option at this point. See Figure 6 - Localisation parameter report.

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To localise or not?

GPS baselines measured on the WGS84 or ITRF datum should be localised or transformed to the local job datum by selecting Localisation and setting the appropriate options on the Coord Observations tab. However, even if localisation is not requested, the least squares adjustment will still adjust the observations into the required positions on the local datum (i.e. scale, rotation, translation) except that the residuals for the baseline components may appear larger than they should be and mislead the user to a wrong conclusion about their reliability.

If the local datum coincides well with the WGS84 datum in origin and rotation, then the scale and rotation differences between the two will be statistically close to zero and the need for localisation of the GPS observations onto the local datum may be optional. It may be acceptable to simply scale individual baseline observation variances/co-variances to improve the fit of the GPS vectors to the local network control points - this may be necessary simply due to residual errors or strain in the adjustment of the original control within a broader network of control stations enclosing the current area. If simple scaling is required, use the Var-Covar Scalar on the GPS Vectors tab to scale selected observations.

Figure 3 - Application of baseline variance-covariance scalar

The observation variance-covariance values can be reset to their original values by clicking the Reset button which inverts the accumulated scalars on the selected observations.

However if the two datums or ellipsoids are not closely coincident then it may be necessary to perform localisation before adjustment to transform the GPS vector observations to realign them within the local coordinate space. For example: "The GPS measurements which are the usual output of the GPS post processing software are based on the WGS84 ellipsoid and the S-JTSK local datum is based on the Bessel ellipsoid. Thus, the reduction of measurements to the S-JTSK mapping plane cannot be started from the measurements resulting from GPS post processing software because GPS and S-JTSK don't have the same ellipsoid." ['Derivation of some geometric parameters of GPS measurements' - Adel Alfrehat, Janka Sabov and Marcel Mojzes, 'Acta Montanistica Slovaca', Volume 10 (2005), No. 3, 310 - 316]

See Derivation of Transformation Parameters for Localisation for further details of localisation.

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Availability of the Localisation Function

The following table summarises the situations in which localisation can be used as defined by settings on the Coord. Observations tab of the Least Squares Network Adjustment option and according to the current job's Coordinate System and Coord. Obs Type setting.

Job Coordinate System
'Plane' 'Grid Projection' 'Geographic'
'Coord obs type' setting Comment Localisation available Localisation available Localisation available
GPS (ITRF/wgs84) The coord and/or vector obs are ITRF/WGS84 so MUST localise n/a Yes Yes
Job Datum The coord obs are on the Job Datum - localisation not required n/a No No
Non-geodetic The coord obs require localisation to the job datum (Grid job only) n/a Yes n/a
Other The coord obs require localisation to the job datum (Plane job only) Yes n/a n/a

The four observation types available on the Coord. Observations tab are as follows:

However, only two or three of these may be listed together depending on other settings . Note that Non-Geodetic is not available when set to GPS (ITRF/wgs84) mode, and GPS (ITRF/wgs84) mode is not available when set to Non-Geodetic. The reason for this is that Non-Geodetic (i.e. Plane grid) mode is not relevant when coordinate observations are GPS-derived and vice-versa. To change the setting from Non-Geodetic (Plane grid) to display points as GPS Lat/Long or XYZ coordinates, then first change the Coord. Obs Type setting to Job Datum mode, then re-display the current Grid coordinates as Geographic (Latitude/Longitude) or Cartesian XYZ when the Display (coord.type) button is enabled. Likewise, apply this procedure in a similar manner to save GPS Geographic or Cartesian XYZ coordinates as Non-Geodetic Grid. Note that the three types GPS (ITRF/wgs84), Job Datum and Non-Geodetic are only listed together when the coordinates are displayed as Grid in Job Datum mode.

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Derivation of Transformation Parameters for Localisation

If the local ellipsoid is GRS80 and/or the local datum is consistent with the GPS system, then there may be little need to transform between WGS84 and GRS80, but if the ellipsoid for the defined job datum is different from GRS80 or WGS84 then a localisation may be advisable and indeed necessary. Knowledge of the local ellipsoid is really not necessary when localising from WGS84 as the transformation parameters are simply computed on the basis of whatever control points are available to be used; thus localisation can be performed in a Plane job with simple 2D or 3D cartesian (orthogonal) coordinates.

Localisation will be required ...

At least 3 matching pairs of points are required, with the observed positions entered in Coord. Observations, either as observed GPS points or points coordinated in another system or datum, and their matching known positions in the target job datum are set fixed in Network Points. A minimum of four pairs of points must be available as both observed and fixed positions for redundancy and to provide non-zero residuals. The selected control points should be distributed evenly across the project area to derive a better and more representative set of rotation and scale parameters for the overall network.


Figure 4: Fixed (known) control points ('Network Points' tab)

Figure 5: Matching observed points ('Coord. Observations' tab)

For representative transformation values for rotation and scale, the control points selected should not be nearly co-linear and should form a well-balanced and evenly-distributed triangular network covering much of the area of interest. Excessive residuals may be a result of insufficient spatial separation between control points.

If a localisation is required and the Localisation option is set to Compute from Current Control, the parameters will be computed and applied to the observations prior to the network adjustment. Details of the new parameters will be displayed for confirmation for use in transforming the observed points and/or vectors as follows:

Figure 6 - Localisation parameter report

  1. If the computed localisation parameters and residuals are acceptable, click Yes to transform the observed coordinates and GPS vectors into the job datum and continue with the network adjustment - details will be included in the adjustment summary
  2. If the computed localisation parameters and residuals are NOT acceptable and there is a previously-computed set available, click No to revert to the previous set of parameters and continue with the adjustment - details of the existing parameters will be will be included in the adjustment summary and used to localise the coordinate and/or vector observations
  3. If there is no previous set of parameters available and the point residuals are larger than acceptable limits, click Cancel to return to the Least Squares Network Adjustment dialog to select different point-pairs to be used to determine the localisation parameters, or to select a different set of parameters.

After computing new localisation parameters and displaying a report as in Figure 6 - Localisation parameter report above, if Yes is clicked to continue the transformation as in item 1, the computed parameters are displayed again with the following prompt to save the details permanently to the Definitions library:

Figure 7 - Prompt to save localisation details as a 3D Similarity Transformation in Definitions library

If the response is Yes then the details may be saved to Transformations in the Definitions Library as a 3D-Similarity transformation for re-use in other jobs. An opportunity is provided to edit the description to be added to the transformation as saved in the library:

Figure 8 - Prompt to save localisation details as a 3D Similarity Transformation in Definitions library

To save the localisation details, the default description offered may be edited to a preferred description, or accepted as-is and modified later in the Transformations option if necessary - click OK to save and continue the computations. Click Cancel to skip saving the details and continue.

Defining datums for saved localisations: A saved localisation becomes a 3D Similarity Transformation in the Definitions library. At the point of saving, the From Datum will be 'undefined', hence the datum names OTHER and LOCAL may be used respectively depending on whether the job is defined with an existing ellipsoidal datum or is Plane, and the To Datum will be the current job datum. The new definition should be edited in Transformations and valid settings for the From and To datums selected; the user may also change the Description but it will not otherwise be changed by selecting the datums as for normal transformations, and should be kept distinctive.

Existing transformation parameters - Existing transformation or localisation parameters may be used to localise coordinate or GPS vector observations - the parameters used do not have to be derived from current data, and coordinate observations on the Coord. Observations tab do not have to be entered or enabled. However it is necessary for the Coord. Observations tab to be set Active even thought there might be no coordinate observations.

Two types of previously-defined parameter sets may be used:

  1. A previously-computed localisation from the same data set - select Use previous Localisation - Check parameters
  2. A 7-Parameter Similarity transformation defined in the Definitions library. Select the appropriate transformation - only transformations matching the current job datum will be listed, and it is up to the user to choose the correct transformation between the observed coordinate system and the job datum

Click the Parameters button to view details of the localisation/transformation selected.

Figure 9 - View localisation/transformation parameters

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 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 if the Observed Coords. type is set in-active.

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