StephenL wrote on Jun 7^th, 2011 at 3:17pm:

I guess it's the range for the radar to look. :-?

I think so. It goes to show that I have never read up on it's function in the manual either! :-[

Mark
From the ‘C-130 X-PERIENCE’ FLIGHT MANUAL Part II –Systems and Equipment, pages 108 and 109:

MAP 2.5 Mode
The 2.5 mode selects 2.5 nautical miles (NM) range displayed on the radar indicator and 0.5 NM distance between fixed range markers.
The MAP 2.5 position is recommended for short-range (up to 2.5NM), air-to-air operation

MAP 15 Mode
The 15 mode selects 15 nautical miles (NM) range displayed on the radar indicator and 3 NM distance between fixed range markers.
The MAP 15 (NM) position is recommended for medium-range (up to 15NM), air-to-air operation