MAPC2MAPC Help pages : Datums, Grids and Projections 


There are three things to know about any map. The Datum is the mathematical model of the world on which the map is based. The international standard is known as WGS84 but this is a result of accurate satellitebased measurement. Before satellites, measurement was difficult and the results not accurate. Each country often used its own measurements. In the UK, for example, the Airy 1830 Datums is used. A large set of datums is distributed with MAPC2MAPC and the program will convert as needed. The Projection is the mathematical rules used to display a representation of the curved surface of the earth on a flat screen (or piece of paper). Different projections have different advantages and disadvantages. Mercator, for example preserves directions but not distances; Plate Carree is simple but only accurate for small areas. The program understands the following : Plate Carree (Latitude Longitude), Mercator, Universal Transverse Mercator, Transverse Mercator (including British, Irish and Swiss), Lambert Conformal Conic, Albers Equal Area, Polyconic and Oblique Styereographic. The Grid is the coordinate system used on the map. This interacts with the projection : a map of California, for example may have an Albers projection but the grid defines the base points for the projection, the measurement units (usually metres) and the point from which the units are measured. Grids are held in a file M2GRIDS.TXT and new entries can be added to this file up to a limit of about 100 entries. Grids are currently supported as Transverse Mercator, Lambert Conformal Conic, Albers Equal area, Polyconic and Oblique Stereographic. Comments in M2GRIDS.TXT describe the fields necessary  note that commas (,) are used as field separators and periods (.) as the decimal point. 