Altitude

Vertical elevation of a place or object relative to a basic level. Altitude is also a value that expresses a difference between a given point and an average level. On land, this level is usually the sea level (or "zero level"). Specifically, the altitude of the body of water is considered as average for natural water bodies and as normal operating level or normal water retention level (NR) for man-made water bodies. The normal level, as well as the average level, is defined as the usual level of the water line of the body of water. Unless levelling, the elevation of the water line or elevation of the water body is that of the closest altimeter indication obtained on site (landmark, etc.) or on a 1:25,000 map. The altitude is indicated by using the applicable local benchmark (French Surveying and Levelling or NGF), accurate to the metre. It can exceed 1,000 metres for water bodies in the mountains but can also be negative for stations located in areas below sea level.