Add example from docs

src/spatial/modules/proj/proj_module.cpp

@@ -372,6 +372,25 @@ struct ST_Transform {
 	);
 	----
 	POINT (544615.0239773799 6867874.103539125)
+
+	-- Transform a geometry from OSG36 British National Grid EPSG:27700 to EPSG:4326 WGS84
+	-- Standard transform is often fine for the first few decimal places before being wrong
+	-- which could result in an error starting at about 10m and possibly much more
+	SELECT ST_Transform(bng, 'EPSG:27700', 'EPSG:4326', xy := true) AS without_grid_file
+	FROM (SELECT ST_GeomFromText('POINT( 170370.718 11572.405 )') AS bng);
+	----
+	POINT (-5.202992651563592 49.96007490162923)
+
+	-- By using an official NTv2 grid file, we can reduce the error down around the 9th decimal place
+	-- which in theory is below a millimetre, and in practise unlikely that your coordinates are that precise
+	-- British National Grid "NTv2 format files" download available here:
+	-- https://www.ordnancesurvey.co.uk/products/os-net/for-developers
+	SELECT ST_Transform(bng
+		, '+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +units=m +no_defs +nadgrids=/full/path/to/OSTN15-NTv2/OSTN15_NTv2_OSGBtoETRS.gsb +type=crs'
+		, 'EPSG:4326', xy := true) AS with_grid_file
+	FROM (SELECT ST_GeomFromText('POINT( 170370.718 11572.405 )') AS bng) t;
+	----
+	POINT (-5.203046090608746 49.96006137018598)
 	)";
 
 	//------------------------------------------------------------------------------------------------------------------