When is GPS Dest Distance Recalculated?

[PandDLong]

With the start of winter I am back to working in iMatch.  My first task is to clean up a bit of a GPS mess that I managed to create in May (https://www.photools.com/community/index.php/topic,14303.msg100550.html#msg100550). 

I have hundreds (maybe thousands) of photos where I have different Created and Shown GPS co-ordinates and only the Created GPS is the right one.  I need to copy the Created GPS co-ordinates to the Shown and do a write-back to sync all the other related GPS tags.

A metadata template is needed so I am not doing each by hand.

My challenge is that I also have many photos where I have intentionally created "destination / shown" gps co-ordinates and I want to keep them.  So I need to identify only the photos which should have identical GPS co-ordinates in order to apply the template correctly.

I thought I would filter on the XMP:Exif GPSDestDistance tag as the wrong Shown co-ordinates seem to be physically close to the Created co-ordinates (less than 100 meters) and my intentionally different co-ordinates are further apart than that.

But the values in the GPSDestDistance tag seem quite inconsistent - and I noticed that they often stay the same even if I move the co-ordinates on the map (and do a write-back).  So, I am not sure I can rely on it as a filter.

I did determine that if I delete the contents of the tag and do a write-back, it is then calculated and repopulated and seems about right.

My questions:

1. Is the GPSDestDistance tag calculated by iMatch or ExifTool?

2. What is the trigger to force a recalculate?   ie. Can I avoid making them all null and doing a write-back?

3. Is this field a bit unreliable in any case and I should find a different way to filter?


Thanks for the advice.

Michael