6.12 Altimetry

6.12.2 Define: 

(a) height;

The vertical distance of a level, point, or object (considered as a point) measured from a specified datum.

The term is also used to describe the vertical dimension of an object, such as a tower, building, or other obstacle.

(b) altitude;

The vertical distance of a level, point, or object above Mean Sea Level (AMSL), expressed in feet.

It is indicated by a pressure altimeter with the sub-scale set to the current QNH for the location of the aircraft.

(c) mean sea level (MSL);

The average height of the sea surface, determined from long-term observations of tide levels at a particular location.

(d) AMSL;

A term used to describe the vertical distance above Mean Sea Level (MSL).

When MSL is used as the datum, the vertical distance above it is called Altitude, and is normally measured in feet.

(e) ground level;

The surface of the land or water directly beneath a point or object.

(f) AGL;

The vertical distance of an aircraft above the ground directly beneath it.

(g) elevation;

The vertical distance of a point above Mean Sea Level (MSL).

For example, the elevation of an aerodrome is its height above MSL.

(h) pressure altitude (PA);

The height indicated by a pressure altimeter when its subscale is set to 1013.25 hPa.