[Qgis-user] Question On "BallPark Transform"

[Randal Hale]

Nyall had requested I submit a bug report...which I forgot 
about....until now soooo -> https://github.com/qgis/QGIS/issues/34983


I added your explanation as a comment to the original email.


Randy


I added your message as a comment

On 3/10/20 2:09 PM, Michael.Dodd wrote:
> Is there any more news on this 'bug' as it appears to be serious or it 
> might simply be the wording.
>
> Example:
> Project created in old version of qgis and opened in recent version. 
> It comes up with the message about "Ballpark Transformations" and 
> suggests installing proj-datumgrid-europe-1.5.zip to enable a better 
> transformatoin between epsg:27700 and epsg:4326. The project contains 
> the OS GB1936 system and WGS84.
>
> So I installed the relevant .gsb files in the correct directory with 
> the other .gsb files, restarted and tried again to open the project.  
> This time it did not come up with the message suggesting the download 
> but instead just suggested using one of the other transformations 
> which are less accurate than the proper OSTN15_NTv2 transformation.
>
> Need to know why it is doing the less accurate transformation and how 
> to get it to do the correct transformation instead.
> ------------------------------------------------------------------------
> *From:* Qgis-user <qgis-user-bounces at lists.osgeo.org> on behalf of 
> Randal Hale <rjhale at northrivergeographic.com>
> *Sent:* 27 February 2020 17:56
> *To:* qgis-user at lists.osgeo.org <qgis-user at lists.osgeo.org>
> *Subject:* [Qgis-user] Question On "BallPark Transform"
>
> CAUTION: This mail comes from outside the University. Please consider 
> this before opening attachments, clicking links, or acting on the 
> content.
>
> Greetings,
>
> I received an email from two users with 3.12 asking about "Ballpark 
> Transformations" - the exact phrase was "Used a Ballpark 
> transformation from EPSG:4326 to EPSG: 2274".
>
> I've been able to generate the message twice by:
>
>   * changing the projection of a new project with no data from 4326 to
>     2274
>   * adding data with an incorrect spatial extent that I believe was in
>     4326 and applied to a project in 2274 (I think - I currently don't
>     have access to that data to confirm)
>
> I've found mention of it here: 
> https://qgis.org/api/classQgsCoordinateTransform.html#a16adb051fafc25058c0040a310f1606b 
> <https://qgis.org/api/classQgsCoordinateTransform.html#a16adb051fafc25058c0040a310f1606b>
>
> The expanded message in QGIS is:
>
> /*An alternative, ballpark-only transform was used when transforming 
> coordinates between *//*EPSG:4326 - WGS 84*//*and *//*EPSG:2274 - 
> NAD83 / Tennessee (ftUS)*//*. The results may not match those obtained 
> by using the preferred operation:*//**/
>
> /*Possibly an incorrect choice of operation was made for 
> transformations between these reference systems. Check the Project 
> Properties and ensure that the selected transform operations are 
> applicable over the whole extent of the current project.*/
>
> If I check the project properties there is no Datum Transformation 
> defined after all this happens.
>
> I'm assuming that if a project has no data and one jumps datums by 
> changing the projection it's saying "Hey since there is no data this 
> transformation is a "ballpark" i.e. not exact". Is there a preferred 
> way to dealing with this after it happens?
>
> Randy
>
> --
> Randal Hale
> rjhale at northrivergeographic.com  <mailto:rjhale at northrivergeographic.com>
> (p) 423.653.3611
> http://www.northrivergeographic.com

-- 
-----------
Randal Hale
rjhale at northrivergeographic.com
https://www.northrivergeographic.com
(423)653-3611

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