Question

Adding Vertical Coordinate Systems to FME

  • 25 September 2019
  • 3 replies
  • 27 views
T
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I have to add 2 Vertical Coordinates systems to add to FME

These are:

WKID 6647 (CGVD 2013) Orthometric

WKID 5713 (CGND 1928) Normal-orthometric*

I have all the required grid files in .byn format and installed, that I understand to be able to work with the CSReprojector.

I'm trying to use the online Define Custom Coordinate System from FME to create a custom coordinate system for 6647 and 5713. I have the ESRI WKT info needed however I cannot submit because I need to reference a Datum.

However the datum is vertical and is:

Canadian_Geodetic_Vertical_Datum_of_2013

and

Canadian_Geodetic_Vertical_Datum_of_1928

And does not appear in the list of Datum's.

How do I do this, do I even need to or are the .byn files enough when working with the CSMapReprojector?

A second further problem is the CSMapReprojector asks for "Vertial handling" as seen here. With items like "Geocentric or ellipsoid height -> Orthometric height".

However I have: Normal-orthometric* -> Orthometric

 

*Normal Orthometric height (Hno): This terminology is not proper, but it is used to define the type of heights associated with CGVD28. These heights are neither orthometric nor normal, i.e., they are not compatible with the geoid or quasi-geoid. They are determined using normal gravity, but they are based on the formulation of orthometric heights. Normal orthometric heights were used in Canada because no actual gravity measurements were available at the time of the realization of CGVD28. The objective of the 1928 adjustment was the determination of the most accurate orthometric heights, which explains why they are commonly referred as orthometric heights. (Unit: m) -NRCAN

 

 

3 replies

daveatsafe
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Userlevel 2
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Hi @tforward,

In FME's present coordinate system handling, vertical datums are not part of the coordinate system definitions, but are handled separately in the Vertical section of the CsmapReprojector.

The transformer assumes that the supplied vertical grid is designed to convert the heights from the ellipsoid to orthometric, so if you choose 'Geocentric or ellipsoid height -> Orthometric height', the grid will be applied in the forward direction, while 'Orthometric height -> Geocentric or ellipsoid height' will apply it in reverse. I think the CGVD28 grid will work OK, even though it is not truly Orthometric, however you may wish to test on some known points first.

The .byn files can be used for a vertical transformation, but cannot be chosen directly in the CsmapReprojector. In order to allow multiple grid files to be used for a transformation, FME uses a grid catalog file (.gdc), which is an Ascii fie containing the paths of the relevant grd files. For more detailed info on creating this file, please see https://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Coordinate_Systems/CoordSys/vertical_grids.htm. This .gdc file is chosen for the Geoid Height Grid setting in the CsmapReprojector.

T
Badge

Hi @tforward,

In FME's present coordinate system handling, vertical datums are not part of the coordinate system definitions, but are handled separately in the Vertical section of the CsmapReprojector.

The transformer assumes that the supplied vertical grid is designed to convert the heights from the ellipsoid to orthometric, so if you choose 'Geocentric or ellipsoid height -> Orthometric height', the grid will be applied in the forward direction, while 'Orthometric height -> Geocentric or ellipsoid height' will apply it in reverse. I think the CGVD28 grid will work OK, even though it is not truly Orthometric, however you may wish to test on some known points first.

The .byn files can be used for a vertical transformation, but cannot be chosen directly in the CsmapReprojector. In order to allow multiple grid files to be used for a transformation, FME uses a grid catalog file (.gdc), which is an Ascii fie containing the paths of the relevant grd files. For more detailed info on creating this file, please see https://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Coordinate_Systems/CoordSys/vertical_grids.htm. This .gdc file is chosen for the Geoid Height Grid setting in the CsmapReprojector.

@daveatsafe Thank you Dave for the quick response, much appreciated.

Just to clarify, I should try using 'Orthometric height -> Geocentric or ellipsoid height' within the CSMapReprojector?

Secondly, as per the .byn files within CSMapReprojector. I do also have a HT2_2010v70_CGG2013a.byn which Conversion grids between CGVD28 and CGVD2013 off the top of your head, do you think this would be all I would need to put into the CSMapReprojector? Best, Tristan

daveatsafe
Rank
Userlevel 2
Badge +17

@daveatsafe Thank you Dave for the quick response, much appreciated.

Just to clarify, I should try using 'Orthometric height -> Geocentric or ellipsoid height' within the CSMapReprojector?

Secondly, as per the .byn files within CSMapReprojector. I do also have a HT2_2010v70_CGG2013a.byn which Conversion grids between CGVD28 and CGVD2013 off the top of your head, do you think this would be all I would need to put into the CSMapReprojector? Best, Tristan

Hi @tforward,

You will still need to create the .gdc containing the path to the .byn file, which is what you choose as the grid file in the CsmapReprojector.

For the HT2_2010v70_CGG2013a grid, please use 'Geocentric or ellipsoid height -> Orthometric height' when converting from CGVD28 to CGVD2013, and 'Orthometric height -> Geocentric or ellipsoid height' for the reverse.

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