One of the most common forms of join operation is the equijoin, where the values in one table must match the values in another. SQL is a common language employed for performing join operations using relational databases. A textual description of a geographic feature or object placed on a map. A usable subdivision of a dataset, generally containing objects of certain classes, for example rivers, roads or buildings. A graphic representation of features of the earth's surface or other geographically distributed phenomena.
Examples are topographic maps, road maps, weather maps.
This normally involves a mathematical model that transforms the locations of features on the earth's surface to locations on a two-dimensional surface. Because the earth is three-dimensional, some method must be used to depict the map in two dimensions. Therefore such representations distort some parameter of the earth's surface, be it distance, area, shape, or direction. The coordinate units in which a geographic data set is stored. Map units can be inches, centimeters, feet, meters, or decimal degrees.