[PROJ] Meaning of (x) numbers in proj transformations in QGIS

Jeff McKenna jmckenna at gatewaygeomatics.com
Sat Jun 29 04:32:23 PDT 2019

I agree with Nyall that users really need access to those 'remarks'.  -jeff

On 2019-06-29 3:06 AM, Nyall Dawson wrote:
> On Fri, 28 Jun 2019 at 19:03, Even Rouault <even.rouault at spatialys.com> wrote:
>> Another thing is that currently the import process of the EPSG dataset to the
>> PROJ dataset doesn't import remarks, so they are lost to the end user. I
>> didn't want to do that for now to limit the size of the database, but that
>> might be discussed.
> I'd be a big +1 to having access to these remarks through proj. It's
> really the only loss we had when porting QGIS to proj 6 and dropping
> our own bodgy db. I'd love some way to expose more information to end
> users to help inform them of the suitability of different pipelines,
> and having access to the EPSG "scope" and "remarks" seems ideal for
> this purpose.
> E.g. I'd want to expose this from the EPSG registry when giving users
> a choice between an operation involving EPSG::8447 and EPSG::8446:
> EPSG:8447
> Scope: Transformation of GDA94 coordinates when localised distortion
> needs to be taken into account, e.g. if GDA94 coordinates were derived
> survey control monuments.
> Remarks: See GDA94 to GDA2020 (1) or (3) (codes 8048 and 8446) for
> alternative conformal-only transformation without local distortion
> modelling. GDA2020 Technical Manual and fact sheet T1 give guidance on
> which to use.
> EPSG:8446:
> Scope: Conformal transformation of GDA94 coordinates that have been
> derived through GNSS CORS.
> Remarks: Gives identical results to Helmert transformation GDA94 to
> GDA2020 (1) (code 8048). See GDA94 to GDA2020 (2) (code 8447) for
> alternative with local distortion modelling included. GDA2020
> Technical Manual and fact sheet T1 give guidance on which to use.
> Both the fields are needed to fully inform users on the applicability
> of either pipeline.
> Nyall
>>> But my main question: is "which proper information an average QGIS user
>>> can use to determine what transformation to pick.." if presented with
>>> one of those dialogs (as I had....)... and not being a projection guru :-)
>> Ah ah, anyone knowning the answer would be multi-millionaire ;-)
>> But to come back to your particular transformation, the thing is that which
>> one of "Amersfoort to WGS 84 (3)" or "Amersfoort to WGS 84 (4)" should be
>> picked up depends probably more on which one was used by other data producers
>> to do similar transformations. Given the accuracy of definition of both
>> Amersfoort and WGS 84, both are OK. This is mostly an issue of using
>> consistently the same one to avoid alignment problems.
>> Even

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