I am using two tranformers: Grid Inquest Reprojector and CSMapReprojector in order to reproject from OSGB36 to my local coordinate system. I am doing it in the following two-steps:
1. Transform from OSGB36 to ETRS89 using OSTN02 Grid shift. (Grid Inquest Reprojector)
2. Transform from ETRS89 to a local coordinate system using the Bursa-wolf 7 parameter transformation. (CSMapReprojector).
I performed these transformations on two different datasets: Shapefile and ESRI Ascii Grid. The Shapefile was originally derived from the ESRI Ascii Grid.
As the shapefiles are derived from the ESRI Ascii Grids, I would expect the resultant transformed files to still be relative to each other. However, the transformed ESRI Ascii GRids are now up to 13 metres displaced from the resultant transformed shapefiles.
For the interpolation settings of the ESRI Ascii Grids I used "Nearest Neighbour" and "Preserve Cells".
Does anyone know why I would be getting different resutls? I would have expected that if I use the same transformation settings on each, I would get the same results.
Thanks