How do you calculate reduce level in surveying?

You first have to have a reference point, beacon or bench mark - this has a known elelvation, or reduced level in metres above mean sea level (AMSL). (Where MSL = 0). When your helper places the staff (graduated measuring staff like a big ruler) on the reference point, read off the value as observed by you, through the machine you are using (level, theodolite, other electronic instruments) and add this to the level of the reference point. This is called the Collimation height and is basically, the height of your instrument, above the reference point (and in turn, above mean sea level). If you helper then holds the staff on another point of unknown reduced level, you takes the reading of the staff through your instrument, and subtract that from your Collimation height. This will then give you the REDUCED LEVEL of the point. Thus, the REDUCED LEVEL IS A LEVEL, ABOVE MEAN SEA LEVEL, OF A POINT AS COMPARED AGAINST A REFERENCE POINT OF KNOWN REDUCED LEVEL. Different places will derive their own AMSL system based upon, obviously, their own local, average sea level.