RDNAP grid correction files for PROJ


Kadaster and Rijkswaterstaat CIV, working together under the name RDNAP, developed RDNAPTRANS™2008, the precise and official transformation between ETRS89 and the dutch national horizontal and vertical coordinate reference systems the Stelsel van de Rijksdriehoeksmeting (RD) and the Normaal Amsterdams Peil (NAP). A ‘simplified’ procedure has been developed which uses a NTv2-grid for the transformation between ETRS89 and RD as well as a VDatum-grid for the transformation between ETRS89 and NAP. This ‘simplified’ procedure has the following limitations:

 1) The rdtrans2008 NTv2-grid can only give identical results to
    RDNAPTRANS™2008 within 1 millimeter at ground level onshore and at
    mean sea 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.

Taking into account the limitations listed above, the rdtrans2008 NTv2-grid and naptrans2008 VDatum-grid can be used as an alternative to RDNAPTRANS™2008 to transform geographic ETRS89-coordinates to projected RD-coordinates with grid correction applied and NAP-heights.

Note that, although the resulting RD/NAP and ETRS89 coordinates from the transformation will be correct, geographic Bessel-1841 coordinates will differ in both procedures and should only be considered as an intermediate result.

RDNAPTRANS™2018 contains a new datum transformation based on the updated ETRS89 coordinates of realisation ETRF2000(R14). Next to this, a new and slightly more precise quasi-geoid grid model is used. This NLGEO2018 quasi-geoid model covers a larger area including a large part of the North Sea. A change with big impact is the use of a new data format of the grid files and a corresponding transformation procedure that changes the order of the steps of the transformation and uses a fixed height in the datum transformation. As a result, the transformation is now possible conform a de facto standard by including the datum transformation in the correction grid (variant 2). This allows straightforward implementation in software like GIS packages and can resolve current problems due to incorrect implementations of the transformation.

Within the administrative borders of the Netherlands, the differences in the resulting horizontal coordinates due to the changes from version 2008 to 2018 of RDNAPTRANS™ are at maximum 0.010 m at sea level (zero NAP height), and at maximum an additional 0.001 m per 50 m height above or below zero NAP height. The maximum change in the transformed height coordinates due to the slightly more precise new quasi-geoid grid model is about 0.025 m. The new correction grid has a different sampling in ellipsoidal coordinates. With this resampling, discontinuities in the correction grid are smoothed, especially outside the administrative borders of the Netherlands, to allow bilinear interpolation. As a result, changes in the resulting horizontal coordinates up to 0.05 m occur occasionally in Belgium, Germany and the Dutch EEZ. To use the same bounding box around the Netherlands including the Dutch EEZ of the North Sea of the quasi-geoid grid model for the correction grid, the correction grid has been faded out to zero correction for a seamless land-sea transition. This introduces latitude and longitude corrections up to 0.25 m in areas outside the former validity area of RDNAPTRANS™ where no corrections were defined before.

There are two variants for the implementation of RDNAPTRANS™2018. Implementation variant 1 applies the datum transformation as a separate step using a 3D similarity transformation. Implementation variant 2 includes the datum transformation in the correction grid and a different quasi-geoid grid for the height transformation is used. The advantage of implementation variant 1 is that it has no strict bounds for the area where horizontal coordinates can be transformed correctly. The disadvantage is that many software packages do not support implementation variant 1. Implementation variant 2 is supported by more software but can only be used within the bounds of the correction grid. The difference in the resulting coordinates between the two variants is well below 0.0010 m within the bounds of the RDNAPTRANS™2008 grids. Although transformation at sea and even outside the grid bounds is possible, the scale factor of the map projection increases rapidly and also the precision of transformation back and forth deteriorates. There are bounds to the recommended use of RD and NAP at sea and outside the Netherlands.

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