[Qgis-user] Question On "BallPark Transform"

[Randal Hale]

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 


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
(p) 423.653.3611
http://www.northrivergeographic.com

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