What's the difference between AG and IG heater???

We have an 28 ft AG pool with a "dished out" bottom, so our total water volume is somewhere around 22,000 gallons according to my guesstimates. We are wanting to add a gas heater. But the heaters I have seen advertised for AG applications are only 100,000 btu. I found a formula sometime back (have to dig for it now) that indicated I would need more btu than that to work efficiently. I would rather have too large than too small. But my problem is the heaters with more btu are specified for in-ground pools. I don't mind spending the extra money to make something right, but I'm worried that something will come up that prevents it from working on AG application. Practically speaking, what is the difference between IG and AG heaters?

Guess I could re-state my question. I'm looking for a 150,000 btu gas pool heater that can be used for AG pool. All I can find with that output are classified for IG pools. What's the difference?

My guess is simply larger capacity and heavier dutier.

I've often wondered the same thing about AG vs IG automatic vacuums. Your AG pool has the capacity of my IG.

I want to buy a heavy-duty heater and like I said, I don't mind spending a little more money if I get something that is really good quality. But what I don't want is to pay big bucks for something that I get home and then find out can't be installed on my AG pool. But everything that I see for AG is about 100,000 btu. And according to my calculations I would want at 150,000 btu minimum.... all of which are called "in-ground pool heaters." I will have to talk to someone that sells them at the pool store I guess, but that is always a last resort because I get such bad information from them so often. That's why I posted the question here first to see if someone might know the answer.