A map element used to graphically represent the scale of a map. A scale bar is typically a line marked like a ruler in units proportional to the map's scale.

This is something that happens which is unplanned, most often with bad results, such as damage to property, injury or death.

The condition of a substance which contains a sufficient amount of acid substances to lower the pH below 7.0.

The mixing or agitation of wastewater, allowing for the mixture of air (oxygen) with the wastewater.

A type of simple aquatic plant.

An extensive growth of algae in a body of water (river, lake, dam), usually as a result of high nutrient levels in the water.

Sub-samples; or a number of smaller samples taken from a sample area which together give results for the whole.

The condition of water or soil which contains a sufficient amount of alkali substances to raise the pH above 7.0.

A grid of numbered rows and lettered columns (or vice versa) superimposed on a map, used to find and identify features. Alphanumeric grids are commonly used as a reference system on local street maps.

An underground reservoir.