Datum and Grid Shift For Canadian Provinces (New Brunswick example)

Q. When I reproject to/from ATS77 do I have to use a grid shift?

A. Yes. If you're converting between a coordinate system in the ATS77 datum and another coordinate system in a different datum, you will need to do some kind of datum transformation. In all cases I'm aware of, this is a grid shift.

Q. Isnt there a fallback or just a datum math definition?

A. No.

Q. I keep getting errors such as:
|ERROR |The following Canadian grid shift file was specified, but not found: C:\Program
Files\FME2011\Reproject\GridData\Canada\ATS77\TRNB7727.dat.
To prevent inaccurate reprojection translation will be terminated
|WARN |Reprojection from 'NewBrunswick_NAD27_Foot' to 'NewBrunswick77' requires grid shift file(s) for datum transformation
|WARN |Reprojector: Could not create reprojection engine

A. This is to prevent data corruption.

FME supports the following relevant datum transformations: ATS77 <-> CSRS, NAD27 <-> ATS77, NAD27 <-> CSRS

ATS77 <-> CSRS within New Brunswisk, PEI, or Nova Scotia:

* Grid shift files are available from provincial authorities (called NB7783v2.gsb, PE7783V2.gsb, and NS778301.gsb respectively; EPSG refers to these as EPSG:1841, EPSG:1846, and EPSG:1850).
* There are no known mathematical fallbacks.
* Once grid shift file(s) are acquired, their locations need to be provide to FME. In FME Workbench, select Tools - FME Options from the menu. Choose the Coordinate Systems option, then click on the Edit button beside the ATS77_to_CSRS Grid transformation. Add the grid locations in the dialog box.

[Update - Jan 2023]
Provincial CSRS grid files can now be downloaded from this NRCAN page . You will need to create a free account to access the page.

NAD27 <-> ATS77 within New Brunswick, PEI, or Nova Scotia:

* Grid shift files are available from provincial authorities. For each province, there is one grid shift for NAD27 -> ATS77 and one grid shift for ATS77 -> NAD27. These are:
* TRNB7727.DAT (urn:ogc:def:coordinateOperation:EPSG::1947)
* TRNS7727.DAT (urn:ogc:def:coordinateOperation:EPSG::1948)
* TRPE7727.DAT (urn:ogc:def:coordinateOperation:EPSG::1949)
* TRNB2777.DAT (urn:ogc:def:coordinateOperation:EPSG::1603)
* TRNS2777.DAT (urn:ogc:def:coordinateOperation:EPSG::1604)
* TRPE2777.DAT (urn:ogc:def:coordinateOperation:EPSG::1605)

* There are no known mathematical fallbacks.

NAD27 <-> CSRS

Similar to the above, this datum transformation is configured in "NAD27 <-> CSRS" in the Coordinate Systems Option.

Q. What grid shifts relate to New Brunswick? Is this a complete list or are there others needed in order to do the transforms listed under maintain gridshifts?

A: Files required:
Nb7783v2.gsb
ATS77\TRNB7727.dat
ATS77\TRNB2777.dat
ATS77\TRNS7727.dat
ATS77\TRNS2777.dat

The files above are required to support datum shifts in New Brunswick as follows:
. for NAD27 <-> ATS77: TRNB7727.DAT, TRNB2777.DAT
. for ATS77 <-> CSRS: NB7783v2.gsb
. for NAD27 <-> CSRS directly: N/A (these exist for some other provinces)