NTv2 Grid Shift Transformation
The NTv2 Grid Shift Transformation is produced under licence from Her Majesty the Queen in Right of Canada, represented by the Minister of Natural Resources. Software is based on the National Transformation Version 2 developed by Geodetic Survey Division, Geomatics Canada.
The heart of the NTv2 Grid Shift transformation is the binary format grid shift data file which specifies grid shift values and accuracy indicators on regular grids of latitude and longitude, determined by distortion modelling of large numbers of known points scattered across the country to compensate for distortions between the datums in smaller areas. A number of grids of varying density may be contained within an NTv2 file, where larger intervals are typically used for broad expanses over less-populated areas, and smaller intervals in areas of greater human activity and interest.
To perform the transformation of a point from one datum to another, the grid square in the grid of the finest density (if more than one exists) is determined that encloses the point in question, and bilinear interpolation is used to compute the grid shift values which are simply added algebraically to the point's coordinates in the old datum to derive the transformed coordinates on the new datum.
|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.|
General Transformation Principles
General transformation principles will be found under the main topic for this section, Data Transformations - General.
NTv2 Grid Shift transformations are defined in the Transformations option under the NTv2 Grid File tab. It is necessary to first obtain a suitable NTv2 grid-shift data file from the relevant state or national authorities for the country and region of interest. For specific details relevant to particular NTv2-format grid-shift files, refer to the authority supplying the original files.
NTv2 Longitude convention
West longitudes in the NTv2 format are treated as positive and east longitudes as negative, i.e., a westerly longitude is numerically greater than an easterly longitude because the signs are reversed with this convention (applies to either West or East hemisphere). However since the opposite convention is used throughout Geoida (east longitude +ve, west longitude -ve), a reversal of the sign of longitude values is made prior to applying an NTv2 transformation.
Points do not require valid heights as the transformation is applied horizontally only, but existing heights will be retained. The vertical datum should remain the same for this transformation (eg a geoidal/orthometric datum), and a default Geoid-spheroid separation (N) value may be specified to be applied to every point (it may be desirable to interpolate new N-values later). A different vertical datum may be selected if necessary but is not recommended. If the target datum involves a different spheroid and the height datum in both systems is geoidal, new spheroidal heights can be determined in the target system if the specified default N-value or new interpolated N-values will apply to the target spheroid.
The direction of the transformation is as defined by the Definitions library (in Transformations) and may be applied in the reverse direction, but the first interpolation will not give an accurate result as the shift values are defined for application to coordinates on the defined From datum to derive transformed coordinates on the defined Target datum. So a second pass is made for each point, using the first estimate of the new target coordinates to interpolate the corrected shift values. Usually, the direction will be determined according to the selected transformation and the datum defined for the current job.
In selecting a suitable transformation, the source and target datums cannot be the same as each other and must match the datums for which the NTv2 Grid-File transformation is defined.
Grid-shift data files
NTv2 grid-shift data files are not supplied with Geoida but must be obtained from official sources. However the Geoida web site page Useful Links contains a number of links for connecting to sites where the data files may be obtained.
CAUTIONEnsure that any grid-shift file selected for an NTv2 transformation defined in Transformations corresponds to current published data and matches the geodetic datums defined for the job and for the Source or Target coordinate system. Using an inconsistent or out-of-date file, or applying to the wrong datums, may result in erroneous transformed data. 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.
Further details of the transformation options will be found in the Transform Job topic.
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