"USE GPSAlt" option on geotagging - what does it do?

The Mission Planner geotag utility has an option: “Use GPSAlt”.

I’ve not used this option except once or twice to explore. But as I advance into RTK navigation for photogrammetry, I should understand it’s function so I can use it - or not - properly.

Without this option, my geotags have an average variance of 9 meters too high according to Pix4D. I need to improve on that.

It would help if I knew which altitude Mission Planner uses with or without the “Use GPSAlt” option selected.

For example - without “Use GPSAlt” does Mission Planner calculate the altitude based on the GPS location of the Home location - and the adjust it with the barometric data?

The geotag utility has another input field: “Rel Alt base.” How exactly does this field contribute? Is it perhaps a way to input an accurate surveyed altitude of the Home point to be used in the barometric calculation of altitude?

The wiki for Mission Planner geotagging is quite old - and doesn’t mention these options:

The Mission Planner geotag tool is a really wonderful feature - and I’m surprised at how few people use it. Maybe once I fully understand these options I can help out by updating the documentation.

by default it uses relative alt.
use gps alt uses gps alt

having user AMSL alt however as other way go get gps alt

Thank you Michael - I appreciate your input on this.

A fair amount of this can be figured out by trial and error. But if I were to write up something about it, I’d want to be sure of the facts. (In fact - I tried exactly what you suggested before I got your response - and it worked.)

I still have quite a few more questions about Mission Planner geotagging. Is this the best place to ask those questions? If not - would you please direct me to the proper forum?

One example of a question is about the messages that appear when doing the pre-process step:

question for Michael

What does it mean when it says “AMSL alt NOT found” and “0ms shutter lag”?

Many thanks!