Coordinate Reference System Type TypeOfCRSType extCRSType refinedCRSType refinedExtCRSType POSC 1.0 geographic The CRS is a lat/long based system, tied to the earth with a stated datum. projection The CRS is obtained from a geographic CRS using a map projection conversion. The conversion is formula used to convert a curved surface into a flattened surface. The coordinates are usually labelled as (easting, northing) or as (westing, southing). vertical The coordinate system is used for determining vertical positions - elevations or depths - and is based on a standard vertical datum. The datum usually represents a type of sea level. engineering The CRS is based on a local datum, which is not necessarily tied to a standard datum. Points are generally offsets from a locally defined origin, with axes directions defined, but not necessarily defined by geographic directions.