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, they need to be added to Ats77ToCsrs.gdc, configurable in the "Maintain Grid Shift Files" GUI as "ATS77 <-> CSRS". Remove any unneeded and unavailable files from this list.
NAD27 <-> ATS77 within New Brunswisk, 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
This is a special case - it can be done via a direct grid shift *or* by chaining the above two transformations to yield NAD27 <-> ATS77 <-> CSRS.
Similar to the above, this datum transformation is configured in Nad27ToCsrs.gdc ("NAD27 <-> CSRS" in the "Maintain Grid Shift Files" GUI). To provide direct NAD27 <-> CSRS grid shift files, add them normally. To select the NAD27 <-> ATS77 <-> CSRS method, specify ".\Ats77ToCsrs.gdc" instead of a grid shift file.
We ship FME with one entry ".\Ats77ToCsrs.gdc" in Nad27ToCsrs.gdc. This entry has a ",,,0.5" suffix which means "even if there are other grid shift files added to the list, ignore them".
The algorithm that chooses between multiple grid shift files in these lists, including the use of suffixes like ",,,0.5", is confusing and counterintuitive. Users generally assume that grid shift files higher in the list take precedence, and this is not the case. Therefore, we recommend that users remove all grid shift files they do not want used. We plan to revamp the "Maintain Grid Shift Files" GUI completely in FME 2012.
Q. I am seeing weird stuff under maintaining grid shifts in FME Tools
Options like:
> NAD27 <-> CSRS
> .\Ats77ToCsrs.gdc,,,0.5
> Why is this here when Ats77 has nothing to do with Nad27?
A. As above, this is a special-case way of saying, "don't use a grid file for NAD27 <-> CSRS; rather, use one grid file for NAD27 <-> ATS77 followed by another for ATS77 <-> CSRS to accomplish the transformation in two steps".
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?
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)
Q. Some of the maintain grid shift files point to .gsbs which are the actual grid shift files. Others point to gdc files such as Ats77ToCsrs.gdc, which just holds lists of gsbs. Is either of these acceptable?
A. As above, there is one special case: Nad27ToCsrs.gdc can point to Ats77ToCsrs.gdc instead of a grid file.
Q. I guess the idea is that if you specify a gdc such as Ats77ToCsrs.gdc then you need to supply the associated gsbs for it to work.
A. That's true, but again only applies in this one special case.
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