cmParabolicIp 
Parabolic Interpolation: this kind of calibration is derived from a minimum of 4 parabolic approximations of parallels and meridians. In order to calibrate a map using this model you need to know at least 2 parallels and 2 meridians (the more the better). The advantage of this model is that it can be applied to maps up to scales of about 1:20 million. It cannot be used in polar regions.

cmBiquadraticPn 
Biquadratic Polynomial Grid: this kind of calibration model is ideally suited for situations where the map is only slightly distorted (i.e. the scale of the map is 1:1,000,000 or lower). The biquadratic polynomial can be calculated from an irregular set of geographical locations. Thus it can be used to calibrate maps with known positions (i.e. obtained from a GPS receiver). Please note that this kind of calibration requires at least 6 known positions and is quite sensitive to errors in the calibration data. Thus we recommend to use about 10 calibration points which are distributed over the entire region of the map.

cmConicConformal 
Conic Conformal Grid: Conic conformal models are typically used in highscale projections of continents or large countries. The conic conformal projection shows linear meridians and curved parallels. You need at least 6 known positions on the map for successful calibration.

cmEquirect 
Equirectangular Projection: This projection maps meridians to equally spaced vertical straight lines, and circles of latitude to evenly spread horizontal straight lines. The projection introduces considerable distortion and is neither equal area nor conformal. It is therefore most often used in thematic mapping. However, it is often used in computer applications such as Celestia and NASA World Wind because of its simple relationship between the geographic location and the corresponding position of an image pixel on the map.

cmMillerCylindrical 
Miller Cylindrical Projection: The Miller cylindrical projection is often used for displaying the entire world (such as in maps showing the time zones) at scales around 1:10 Mill. The grid of this projection is strictly rectangular, however with uneven spacing between the parallels. The scale is valid only along the equator. A minimum of 6 calibration points are recommended. Please be careful to spread the calibration points over the entire map.

cmUTM 
Universal Transverse Mercator Projection: The UTM coordinate system is a gridbased method of specifying locations on the surface of the Earth, but it is not a single map projection. The system instead employs a series of 60 zones, each of which is based on a specifically defined secant transverse Mercator projection. Thus, when using UTM one has always to specify for which zone and which hemisphere the UTM coordinates are specified.
