Data Transformations - General
|IMPORTANT - ALWAYS verify the accuracy and reliability of a transformation before applying to critical data - always test first with points whose coordinates are known in both systems.|
This topic discusses general aspects of data transformations and conversions under these headings on the current page below:
Transformation selection options
This section covers features of the various data transformation options that are common to some or all of the procedures through the Select Transformation window. The following comments relate generally to any option of the three options that performs a data transformation (i.e., Import, List Points / Export, and Transform Job options) and in the majority of cases will apply to spheroidal data, not data on a plane datum.
In many cases the direction will be determined by the To and From datums as defined and which one of these matches the current job's datum. In other cases where there is no change of datum, the user may select the direction in which to apply the transformation.
In the Source/Target coordinate system options frame, only projections which have datums matching the From or To datum for a selected pre-defined transformation will be listed in the drop-down list. This is to ensure that any grid coordinates converted from geographic coordinates will still be compatible with the transformation datum/s.
For transformations of grid-projection coordinates other than zone-to-zone, the new zone for a transformed point will be the same as for the source data.
If the spheroids are the same for both datums, the N-value can remain unchanged (assuming a small transformation shift) and the value will be just copied for each point. If the spheroids are not the same for both datums, the N-value must be reset and a new default value should be entered for application to all points. New values could be re-interpolated again later from an appropriate source of geoid data.
The type of point coordinates printed for confirmation in the transformation log file will be set according to the job type and/or the transformation.
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General Transformation Principles
The method of choice for the transformation of data between geodetic datums in regional application must be the NTv2 grid-shift transformation. Government authorities in several countries have modelled the distortion shift values between datums of large numbers of known points scattered across the country, and made available for general use in the form of files of regular grids of horizontal shift and vertical deflection values at nodes of latitude and longitude. For geodetic projects, the grid-shift method is considered to be the most accurate available because it compensates for variable distortions between the datums; however, grid-shift files may not be available for many countries, so other methods (usually 3D-Similarity or Molodensky) may be the best available. For smaller regions or more localised projects, parameters may have been derived for some of the lesser methods which will give a more reliable result. Reference should be made to the relevant government authorities for details of the transformations available and the accuracies to be expected.
Refer to the sub-heading Transformations under the topic Accuracy for comments about the relative accuracies of different transformation methods.
Geographic is best!
The better system for the representation of geodetic point data is geographic coordinates rather than grid-projection coordinates. Since the precision of point data may be degraded whenever a conversion between grid and geographic is made, exercise caution with frequent conversions between coordinate types. Best is always geographic, especially for transferring data. For example, if applying a grid transformation to geographic (or even grid) data, use (eg) Import and apply the transformation during import as this applies the transformation directly to the incoming data. But if the job was set for grid coordinates and the data was first imported without transformation, a subsequent transformation would have to convert the grid coordinates back to geographic first and the accuracy of the resulting data would be degraded. See also the next heading Loss of precision with repeated transformation.
Loss of precision with repeated transformation(Applies to: NTv2 Grid-file, Multi-parameter and Zone-to-Zone transformations)
If points are transformed to a new datum and then back again, discrepancies may result because of loss of precision due to the number of coordinate conversions required. For example, to convert grid coordinates on Datum1 to grid coordinates on Datum2 and back again, actually involves the steps:
and to reverse again:
Defining a job as geographic and defining the points in terms of geographic coords would reduce the number of conversions. The transformation is done between the geographic coordinates in each system - conversion from grid to geographic is not part of the transformation; grid coordinates are merely a derivation of geographic coordinates.
Prior Zone-to-Zone transformation not necessary (Applies to: NTv2 Grid-file, Multi-parameter and Geographic Block-shift transformations)
Although these transformations are defined for the transformation of geographic or cartesian XYZ/ECEF coordinates, grid coordinates may still be transformed from one datum to another using these methods because the transformation is actually applied to the equivalent geographic coords; if grid coordinates are present or required, they are simply converted back from the geographic coordinates in each system to the grid coordinates in the new projections. For example to convert single-zone/special-area project grid coordinates on one datum (eg PCG) to grid coordinates on another datum (eg MGA94), it is not necessary to first do a Zone-to-Zone transformation to convert the coordinates from the project grid to a Standard Transverse Mercator grid on the same datum, and then transform these coordinates to the new datum, as that would introduce a large number of repetitive conversions - simply apply the one transformation to the original project-grid coordinates and have the target set as grid coordinates on the new datum.
Grid coordinate zone checks (Applies to: Zone-to-Zone, NTv2 Grid-file and 3D-Similarity transformations)
Each of these transformations requires any points with grid coordinates to be expressed in terms of intermediate geographic coordinates during the processing. If the target system requires grid projection coordinates, checks are made on the transformed points against the zone boundaries to verify that they have fallen within the required zone; occasionally the following messages may be seen in the transformation log file:
Transformation datums must match source and target datums
For NTv2 Grid-file, Multi-parameter and Block-shift transformations, matches are made between datums, not projections, because the transformations are applied to the geographic coordinates in these cases. Whether or not the source or target system has or requires grid coordinates is irrelevant because the grid coordinates are only a derivative of geographic coordinates anyway and will be converted first. However, in the case of a Zone-to-Zone transformation, the source and target projections are defined AND must be matched. In most cases for pre-defined transformations, Geoida will list only transformations that have datums and/or projections matching the source and target systems - this is controlled by the Matching and Show All options in the Pre-defined transformations frame of the Select Transformation window; if Show All is selected, matches are not checked but the Compute button will also be disabled to prevent the application of invalid transformations.
For details relating to the definition of transformation types, refer to the Transformations option in the Definitions menu. For other details relating to specific transformation types, refer to the topics listed below.
Accuracy of transformations
While Geoida may print out transformed coordinates to several decimal places of a metre or seconds of arc, all results will be limited to and within the accuracy that may be stated or expected for the particular methods, even though additional precision may be implied by the results. This additional precision may be necessary for internal computation, but the user should be acquainted with the expected accuracy of the transformation method or the set of parameter values that may be used, and should not infer that the accuracy is better than it really is.
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For more specific topics covering the available transformation methods in detail on other pages, see other related topic pages as follows:
Other Related Topics:
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