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

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

Richard Duivenvoorde
Hi List,

coming from this QGIS Dev thread:

https://lists.osgeo.org/pipermail/qgis-developer/2019-June/057806.html

I was wondering which proper information an average QGIS user can use to
determine what transformation to pick...

And what information (numbers and text) in these dialogs come from proj
or from QGIS.

In the above thread I point to the images in this post:

http://www.qgis.nl/2019/06/27/english-proj-select-datum-transformations-for-epsg28992/?lang=en

First screenie being a 'choose transformation' dialog in QGIS 3.8 (with
proj5), the second one from QGIS master with proj6.

According to Nyall, he uses the info from proj to label the 'best fit'
green, but I also see 'higher order' numbers in the descriptions of the
options (numbers between brackets). My main reason to come here:
apparently (3) is by proj/Nyall seen as current, but then I wonder why
there is a (4) option...

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 :-)

Regards,

Richard Duivenvoorde
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Re: Meaning of (x) numbers in proj transformations in QGIS

Even Rouault-2
Hi Richard,

> According to Nyall, he uses the info from proj to label the 'best fit'
> green, but I also see 'higher order' numbers in the descriptions of the
> options (numbers between brackets). My main reason to come here:
> apparently (3) is by proj/Nyall seen as current, but then I wonder why
> there is a (4) option...

Tough topic. The numbers in brackets (3), (4) come directly from the name of
the transformation from the EPSG dataset. I don't think they have a particular
meaning, except to distinguish transformations which go from the same source
to the same target. The latest number is probably the most recent, but PROJ
doesn't make any such hypothesis.
So when PROJ collects all possible transformations and tries to filter and
sort them from the most relevant to the less, it takes into account different
parameters: accuracy advertized, area of use, availability of grids,
supersession information, ... and when all those criteria are equal, it just
sorts by name... Here EPSG:4833 (Amersfoort to WGS 84 (4)) and EPSG:15934
(Amersfoort to WGS 84 (3)) have the same accuracy, area of use, don't require
grids and ... are reported as being both current ! The text remark that EPSG:
4833 replaces EPSG:15934 isn't translated in a record in the
'epsg_supersession' table (whereas EPSG:11125 and EPSG:1672 are clearly marked
as superseded by other codes).
So for that particular case, PROJ probably makes the "wrong" (see final
comment at end of email) choice based on incomplete information from the EPSG
dataset. You should probably file a change request to IOGP so that they add an
entry in the supersession table. Unless this is intended by them to keep only
the last 2 valid entries (perhaps "replaced by" doesn't equate to "is
superseded by"...), and just tag as supersed the previous ones.

If one wanted to improve the situation from the available information, there
could be two choices:
- analyze text remarks to guess extra supersession information. That is look
for pattern like "Replaces FOO (code BAR)"
- take into account the " (number)" trailing information, to put first
transformation with higher number, but I'm wondering if that wouldn't be
fragile in other situations.
Not sure which one I dislike the less :-)

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.

> 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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Re: Meaning of (x) numbers in proj transformations in QGIS

Nyall Dawson
On Fri, 28 Jun 2019 at 19:03, Even Rouault <[hidden email]> 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
>
> --
> Spatialys - Geospatial professional services
> http://www.spatialys.com
> _______________________________________________
> PROJ mailing list
> [hidden email]
> https://lists.osgeo.org/mailman/listinfo/proj
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Re: Meaning of (x) numbers in proj transformations in QGIS

Even Rouault-2
On samedi 29 juin 2019 16:06:47 CEST Nyall Dawson wrote:
> On Fri, 28 Jun 2019 at 19:03, Even Rouault <[hidden email]>
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.

OK I've addressed this per
https://github.com/OSGeo/PROJ/pull/1537

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Re: Meaning of (x) numbers in proj transformations in QGIS

jmckenna
Administrator
In reply to this post by Nyall Dawson
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 <[hidden email]> 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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