Manifold allows definition of custom datum grids for the NTv2 transform. Datum grids are stored in GSB "grid shift" files and are described using XML files with the same name that contain one or more datum pairs to use with the grid.
The GSB files and XML files must be placed in the folder specified in the Tools - Options file location Datum Grids value. By default, this file location specifies a Grids folder within the main Manifold System installation folder.
XML File Format for Datum Transformation Pairs
A datum pair is defined using a <transform> tag. There can be more than one <transform> tag in an XML file for the GSB file.
The definition of a datum pair is done in the following pattern:
<source>Datum Name 2</source>
<target>Datum Name 1</target>
Both <source> and <target> tags specify the names of datums, and are mandatory.
The point of providing an XML file like the above is to tell Manifold the names of the datums that are to be converted with NTv2 using this grid shift file. Since NTv2 transformations are done only for a specific region, this assures that only those datums the names of which exactly match the datum names specified will be transformed using NTv2. Other datums will be transformed using the high accuracy, general purpose routines within Manifold.
Given a pair of source and target datums, Manifold will use the custom grid file to convert coordinates both from the source datum to the target datum and from the target datum to the source datum. Switching the order of the source and target datums will cause loss of precision.
Suppose we want to use the NB7783V2.GSB grid shift file for converting from ATS77 to NAD83(CSRS) used in New Brunswick, Canada. We would provide an XML file named NB7783V2.xml that contained:
We would place both the NB7783V2.GSB file and the accompanying NB7783V2.xml file into a folder called Grids within the Manifold installation folder, or such other folder as we care to specify in the Tools - Options file location Datum Grids value.
When Manifold starts up, it will locate the .gsb file and accompanying .xml file and will load the ability to convert from ATS77 to NAD83(CSRS) using the grid file provided with NTv2.