Measuring the height of a place
Earth’s surface is uneven: high and low mountains, hills, plains, depressions – all together called relief. Height is of two kinds. Absolute height is a point’s height above sea (ocean) level; sea level is taken as 0 because still water is flat. For example the highest point in Uzbekistan is Hazrati Sulton peak (4643 m); Tashkent lies about 400–450 m above sea level. Places below sea level are written with a minus (–) sign: the Mingbuloq depression in the Kyzylkum desert is –12 m. The highest point on land is Everest (Chomolungma, about 8849 m) and the lowest land point is the shore of the Dead Sea (about 430 m below sea level). Relative height is the vertical height of one point above another: relative height = absolute height of the upper point − absolute height of the lower point. Height is measured with a level (levelling) and nowadays also with satellites.
Small investigation: take the table top as “sea level” and fix a longer and a shorter stick (pencil) in a lump of cork or plasticine. With a ruler measure each stick’s “absolute” height (from the table), then find the “relative” height of the long stick above the short one by subtraction. Safety: use blunt sticks or pencils rather than sharp-pointed ones; keep glue and paint out of mouths.