NTv2 is a format/method to allow transform coordinates (and vector/raster layers) from one coordinate reference system to a different one (involving a change of datum) with a small error: the precision is usually around a few centimeters (or less) and is much better than the one that can be obtained by reprojecting layers using the standard coordinate reference systems definitions, as they are available in common GIS software/libraries/toolboxes.
The NTv2 method make use of binary files (.gsb) called ”grids” and is fully supported by libraries like Proj and software like GDAL/OGR. Until not long ago the availability of these grids was scarce but in recent times they have become quite easy to find and are freely available for many countries.
Unfortunately using this grid is not easy for the common GIS user: both command line utilities and Desktop GIS software need custom CRS definitions, downloading and copying files to specific locations and eventually do more configurations. See for example the following tutorials for QGIS or ArcGIS.
For the cases when grid files are freely available and redistributable we implemented an easier solution in the form of a QGIS plugin for its Processing toolbox. With this plugin the operation is as simple as just selecting the coordinate reference system of the input layer and the transformation (direct or inverse).
The plugin already comes with support for grids/transformations for a few countries and more will be added given that ntv2 grids are freely available and redistributable. If you want to have a certain grid/transformation/country added please open a request here
including all the necessary details.
Note2: The plugin was directly derived from a similar one that we helped develop for the Portuguese QGIS community/user group for the specific context of datum transformations for Portugal. At some point we felt that this plugin could have been useful also for users from other countries so we released an “international” version of it.