The geosoftware NTv2Creator provides a powerful development tool for the modeling and creation of accurate NTv2 files from identical points in two different Reference Systems The software analyses two files with identical points and suggests the boundaries and mesh sizes which can then be changed manually. Finally, a highly accurate NTv2 file is generated using small-scale three-dimensional 7-parameter Helmert transformations around each grid point and subsequent Natural Neighbour Interpolation for residual gap distribution. NTv2Creator provides three different mathematical models for the creation of NTv2 files.
Modeling of NTv2 files
NTv2 means "National Transformation Version 2" and describes a method for the transfer of georeferenced data from one Reference System to another. Originally developed in Canada, it has grown to a worldwide standard. Many countries use NTv2 files to homogeneously transform country-specific geotopographic data into new, continent-wide Reference Systems.
The advantage of the NTv2 method is that it is available as a standard method in many geodetic transformation programs and GIS. Furthermore, there are some RunTime-Libraries, which support an implementation in own software. Examples are the program TRANSDATpro for worldwide coordinate transformations and the Geodetic Development Kit GeoDLL by KilletSoft.
The NTv2 concept is based on a regular grid of geographic coordinates for which shift values are stored as the difference between two Reference Systems. For a coordinate transformation between the Reference Systems, the grid mesh first is determined, in which a point to be transformed lies. The point lying inside the grid mesh is interpolated from the shift values of the four grid points in the corners of this mesh. The interpolated shift values of the longitude and latitude are then added to the coordinate of the Source Reference System to obtain an equivalent coordinate in the Target Reference System. The use of an NTv2 grid file with suitable software offers the advantage that the coordinate transformation can be done quickly and, depending on the mesh size, very accurately.
Modeling of accurate NTv2 files from Identical Points
So much about using already existing NTv2 files. But how is an NTv2 file created from a cloud of irregularly distributed geodetic points in two base point files? First of all, all points of the base point field must be available as so-called identical points, which have been precisely measured and calculated in two different reference systems. Suitable for this are Cartesian Coordinates and also Geographical Coordinates, Gauss Kruger coordinates and UTM Coordinates with or without ellipsoidal height using the earth ellipsoid dimensions. From the identical points, the shift values of the grid points for a defined working area are calculated in a one-time process and stored in an NTv2 file. Since modeling an NTv2 file is a time-consuming process, high computational time expenditure is to be expected depending on the size of the working area, the number of identical points in the base point files and the fineness of the grid meshes.
As results the program NTv2Creator creates two variants of NTv2 files. One variant is a file in readable ASCII format, which has the file name suffix "gsa". The other variant is a binary NTv2 file with the file name suffix "gsb", which can be processed very quickly by coordinate transformation software and a GIS.
In order to create an NTv2 file, first of all some characteristics concerning the extension of the grid, the mesh size, the ellipsoids of the source and target reference systems and some designations for the reference systems and for the grid have to be defined. The program NTv2Creator writes this data in a file header and in a grid header of the NTv2 file. In the ASCII variant, you can see this clearly in plain text. The shift values of the grid points and quality characteristics are entered behind the headers. There are four decimal numbers for each grid point. These are the shift values for the latitude and longitude of the grid point and the quality characteristics for the latitude and longitude of all grid points.
At the beginning of the processing, the coordinates of the identical points are tested for outliers and deviations using statistical methods. Then all grid points are calculated. To calculate a grid point, the program NTv2Creator collects all identical points from the point files of the source reference system and the target reference system, which lie in a certain radius around the grid point. With these points a small spatial three-dimensional 7 parameter Helmert transformation is performed. During the subsequent coordinate transformation of the grid point of the source reference system, the Helmert parameters determine the coordinate of the same grid point in the target reference system. To increase the accuracy the Natural Neighbour interpolation is used for the distribution of the residual gaps. The difference between the coordinates of the grid point in both reference systems is entered as a shift value in the NTv2 file. The calculated shift values include the compensation of regional and local tensions and they consider the different dimensions and bearings of the ellipsoids used in both reference systems.
The quality feature of the grid point results from the weighted distance of the identical points that have been collected to determine the 7 parameters of the Helmert transformation. The quality feature varies from 1.0 for the highest quality and goes close to zero for lower qualities.
The program NTv2Creator provides three different mathematical models for the creation of NTv2 files:
- Model "Grid-wide transformation"
- Model "Polygonal Validity Scopes"
- Model "Forced grid point values"
An NTv2 file always contains a "rectangular" grid with defined boundaries. The three mathematical models differ only in the handling of so-called missing points. Missing points are defined as grid point that lie inside the rectangle but outside the actual working area, which is usually represented in a polygonal structure. Furthermore, missing points occur when the identical points found around a grid point are too inhomogeneous to realize a small-spatial Helmert transformation.
The model "Grid-wide transformation" replaces missing points with coordinates that have been transformed using previously determined grid-wide Helmert parameters.
The model "Polygonal Validity Scopes" marks missing points with so-called "exopolygonal values" in the quality feature as invalid. Polygonal Validity Scopes are supported by all geodetic programs and development tools of KilletSoft. So e.g. the program TRANSDATpro issues a warning, if it is tried to transform a coordinate outside of the Polygonal Validity Scoope.
The model "Forced grid point values" forces the calculation of missing points even from far away identical points. If calculations fail, the program replaces the missing points with valid neighboring grid points.
The newly created NTv2 binary file is verified by cross-calculation and the result is stored in a cluster field. Bad matching identical points are determined and suggested for rectification. Such so-called outliers can also be excluded from the calculation by a program setting.
The program NTv2Creator is equipped with an up-to-date graphic user surface which handling can be understood intuitively. For each window detailed help texts can be shown. The country dependent layout of the user surface can be chosed from different languages.
