1. A method of determining a geometrical measurement of a retina of an eye, comprising: obtaining a two dimensional representation of at least a portion of the retina of the eye, deriving a geometrical remapping which converts the two dimensional representation of the retinal portion to a three dimensional representation of the retinal portion, wherein deriving the geometrical remapping comprises determining a correction for distortion introduced into the two dimensional representation of the retinal portion in production thereof, the distortion being introduced into the two dimensional representation of the retinal portion by optical and mechanical properties of an imaging device used to produce the two dimensional representation, and wherein determining the correction for distortion in the two dimensional representation comprises modelling the optical and mechanical properties of the imaging device by: (i) constructing an optical description of an imaging system comprising the imaging device and a model eye, (ii) passing a ray through the imaging system onto a retinal surface of the model eye, (iii) calculating an actual measurement of the ray at the retinal surface, (iv) determining a horizontal scanning angle and a vertical scanning angle of the imaging system for the ray, (v) calculating an expected measurement of the ray at the retinal surface using the horizontal scanning angle and vertical scanning angle of the imaging system, (vi) repeating steps (ii) to (v) for a plurality of further rays, and (vii) comparing the actual measurements of the rays at the retinal surface with corresponding expected measurements of the rays at the retinal surface to determine the correction for distortion in eye representations of the imaging device, using one or more coordinates of the two dimensional representation of the retinal portion to define the geometrical measurement to be taken of the retina on the two dimensional representation, using the geometrical remapping to convert the or each coordinate of the two dimensional representation of the retinal portion to an equivalent coordinate of the three dimensional representation of the retinal portion, and using the or each equivalent coordinate of the three dimensional representation of the retinal portion to determine the geometrical measurement of the retina of the eye.