[proj-rdnap] 05/09: Document the limitations of the grid correction files.

[Bas Couwenberg]

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sebastic-guest pushed a commit to branch master
in repository proj-rdnap.

commit aed288bd301b9e1b6a7dfa3d82a8a571b09e180b
Author: Bas Couwenberg <sebastic at xs4all.nl>
Date:   Fri Oct 17 04:27:52 2014 +0200

    Document the limitations of the grid correction files.
---
 debian/README.Debian | 35 +++++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/debian/README.Debian b/debian/README.Debian
index 286b91e..e1c0fc1 100644
--- a/debian/README.Debian
+++ b/debian/README.Debian
@@ -30,7 +30,7 @@ Direct links to the relevant pages:
 The license contained in the RDNAPTRANS2008.zip file is problematic, because
 it doesn't allow distribution. The license is documented in the file
 "Use of RDNAPTRANS2008.doc" (use `antiword "Use of RDNAPTRANS2008.doc"` to
-display it in a terminal). It says:
+display it in a terminal). Quote:
 
 "
  Use of the name RDNAPTRANSTM2008
@@ -72,7 +72,8 @@ NTv2_2008.zip file contains a different license which limits modification
 but allows distribution. The license is documented in the file 
 "Use of RDTRANS2008 and NAPTRANS2008.pdf" (use
 `pdf2txt "Use of RDTRANS2008 and NAPTRANS2008.pdf"` to display it in a
-terminal). It says:
+terminal). Quote:
+
 "
  License
 
@@ -130,3 +131,33 @@ To use these definitions with cs2s specify the rnap file:
  cs2cs +init=rdnap:rdnap +to +init=epsg:4258
  cs2cs +init=rdnap:rd +to +init=epsg:4258
 
+Beware that the transformation procedure is not identical to RDNAPTRANS™2008.
+
+The file "Use of RDTRANS2008 and NAPTRANS2008.pdf" documents a list of
+limitations of the procedure with NTv2 grid correction files. Quote:
+
+"
+    1) The rdtrans2008 NTv2-grid can only give identical results to 
+       RDNAPTRANS™2008 within 1 millimeter at ground level onshore and
+       at mean seal level offshore. The horizontal deviation is
+       approximately 1 millimeter per 50 meter height difference from
+       ground level or mean sea level.
+    2) An exception to 1) is the border of the RDNAPTRANS™2008 correction
+       grid. Transformation results within cells of the rdtrans2008
+       NTv2-grid that are intersected by the border of the RDNAPTRANS™2008
+       correction grid can result in deviations of up to 20 centimeter.
+    3) The naptrans2008 VDatum-grid cannot be used to determine deflections
+       of the vertical. For this the NLGEO2004 geoid model has to be used.
+    4) The naptrans2008 VDatum-grid is referenced to the Bessel-1841
+       ellipsoid and cannot be used stand-alone, it has to be used in
+       combination with the rdtrans2008 NTv2-grid.
+"
+
+The package build includes the mandatory tests documented in Appendix 2 of
+"Use of RDTRANS2008 and NAPTRANS2008.pdf". The tests compare the coordinate
+transformations by cs2cs to those computed with RDNAPTRANS™2008.
+
+The transformations by cs2cs produce different results, not identical to
+RDNAPTRANS™2008. Failing the test to be identical to RDNAPTRANS™2008 does
+not prevent the package build from continuing.
+

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