[Qgis-user] Question On "BallPark Transform"

[Michael.Dodd]

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"


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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
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