What decides MDA?

[axmiha]

The title is my question. How MDA is decided / defined in each airport? I ask because I thought it was about terrain surrounding the field, but I'm landing on SBMQ (Macapá) which is sea level with nothing around it and the MDA there is 640 feet - or 640'(593') as it is in the chart - when generally it's 200 feet. So that's my question for pilots.

[ilias.tselios]

When MDA is used, is referred to a non-precision approach, which does not provide vertical  guidance. This is barometric altitude, above mean sea level. Those kind of approaches have, usually, minimums around 500-600 feet. 

Min. around 200 feet or lower, are usually for precision approaches, like ILS, where you have vertical guidance, and always is height above touchdown, designated as DH (Decision Height).

[axmiha]

I see. Ok, thank you very much.

Sorry to waste a whole topic for such a simple question.

[Ben Russell]

It's ok, we recycle 100% of the electrons used to run the site.

[axmiha]

Ha!