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Shape of the Earth

The earth is approximately a sphere, but with a varied and irregular surface. One of the basic aims of the science of geodesy is to determine the position of points...

Geoid Model

So what is the true shape of the earth? The geoid is defined as a surface on which the earth's attractive (i.e. gravitational) forces are everywhere equal...

Reference Ellipsoid

Because the ellipsoid is a good approximation to the shape of the geoid, and it is simple to define mathematically, it has been used in classical geodesy for over 200 years...

Geodetic Datum

There are many different ellipsoids on which positions may be expressed. The size, shape and positioning of the ellipsoidal reference system with respect to the area of interest is largely arbitrary...

Height Datum

Because the geoid is an irregular shape, its surface is not generally parallel to the ellipsoidal surface. Therefore it is usual to fix the geoid at one location, usually some reference mark...

Cartesian Co-ordinates

Positions may be given in absolute terms, relative to the earth's centre of mass, or an assumed centre (as implied by a geodetic datum)...

Geodetic (Ellipsoidal) Co-ordinates

With Geodetic (Ellipsoidal) co-ordinates the position of a given point (P) may be measured on the earth's surface and given a latitude (F) and a longitude (?) in terms of the reference ellipsoid...

Plane (Grid) Co-ordinates

Having established a 3-Dimensional co-ordinate reference system, it remains to depict the position of points of interest onto a flat surface...

Irish Grid Reference System

The Irish Grid is a plane co-ordinate system based on a modified Transverse Mercator Projection. Map positions expressed in this system are based on...

GPS Reference Systems

The adoption of regional or local reference ellipsoids results in different positions for the same point along common boundaries between two different regions...