Minimum Descent Altitude (MDA) and Required Visibility
The first number is the Minimum Descent Altitude (MDA) in BIG BOLD Numbers. It is also called the Decision Height (DH) when describing a Precision Approach (PAR or ILS). This value is an Altitude given in Mean Sea Level (MSL). It is the LOWEST altitude you may fly after reaching the Final Approach Fix. You can NOT descend below this altitude unless you have VISUAL on the airfield and are in good position to land.
If you reach the missed approach point while flying this altitude, and can not see the airfield, you MUST execute a missed approach.
The next value is visibility required and can be given in statute miles or RVR (runway visibility range). RVR is given in meters so the numbers will be 24, 48, etc.. and are assumed to be 2400, 4800 meters etc.. When the numbers are 5 , 1, 2 etc.. that is STATUTE MILES of visibility. These merely indicate the visibility that MUST exist at the time you begin your approach. If the reported visibility from ATIS, FSS, or ATC is below the published amount, you CAN NOT begin this particular approach.
Using these two numbers together, we know that our MDA is 920 ft. MSL and we must have a visibility of 1 ¼ sm. to begin the S-25 approach.
If this were a Precision approach like an ILS, the MDA is called the Decision Height (DH) and is the altitude that we maintain until we intercept the glideslope guidance. NOTE: BIG BOLD numbers (except for visibilities) are FEET IN MSL (mean sea level). Smaller numbers are Feet in AGL (above ground level).