# Coordinate Reference System

A **Coordinate Reference System** is a method of *projecting* positions in the world onto screen/paper.

The surface of the Earth is curved but paper is flat. These two facts mean that paper maps will always be slightly distorted. Different Coordinate Reference Systems are designed to minimise the distortion in different ways.

Some **CRS** show orientation accurately, some show areas accurately etc. See here for a complete list.

**Coordinate Reference System** objects can be specified using Open Geospatial Consortium (OGC) Well-Known-Text strings.

## Implicit Coordinate Reference Systems

Cadcorp SIS Desktop lets you create **Coordinate Reference System** objects and save them in Named Object Libraries using names.

You can also use *implicit coordinate reference systems*. The DefineNolObject and GetImplicitNolObject API methods use *implicit coordinate reference systems* to create and query **CRS** objects. The format of the implicit string is **Open Geospatial Consortium (OGC)** Well-Known-Text. See the **OpenGIS Simple Features Specification** for more information.

You can see **Open Geospatial Consortium (OGC)** Well-Known-Text strings for **Coordinate Reference System** objects using CRS (Home > Map) to display the Coordinate Reference Systems dialog.

**Name**

A Geodetic Datum (sometimes called simply a Datum) models the sea-level surface more accurately than an ellipsoid. SIS contains equations for transforming points between Geodetic Datums, or you can make your own Geodetic Datums by applying seven conversion parameters to the WGS84 standard Geodetic Datum.

**Ellipsoid**

** Name**

Reference ellipsoid name, i.e.

Airy 1830in this example.

**Semi-major axis**

Distance from centre of ellipsoid to any point on the equator in metres.

**Semi-minor axis**

Distance from centre of ellipsoid to either pole in metres.

**Inverse flattening (derived)**

Inverse flattening value.

** WGS conversion parameters**

** Shift X/Y/Z**

Shift between WGS84 and Geodetic Datum.

** Rotate X/Y/Z**

Rotate between WGS84 and geoid, in seconds of an arc.

** Correction (parts per million)**

Correction scale in parts per million.

*Properties*

The **Properties** tab displays all properties of the coordinate reference system.

## Properties

**EPSG Code**: _epsg&

The matching European Petroleum Survey Group (EPSG) code number for the coordinate system, if found.

**Projection WKT**: _projection$

Coordinate reference system in Open Geospatial Consortium (OGC) Well-Known-Text format.