# Relative, metric coordinate system

For some applications, it is not useful to specify positions in terms of decimal degrees of longitude and latitude. In this case it would be more helpful to specify positions in meters, for example, to indicate braking distances, radii of steady state circular tests and track deviations.
If the radius of action on the earth's surface is limited (10-50 km, depending on the required accuracy), the earth's curvature and its effects can be neglected within this range, permitting the establishment of a planar, metric coordinate system relative to a reference point. This reference point (specified in decimal degrees) constitutes the zero-point of the metric coordinate system used as a basis by conversion formulae (decimal degrees to meters).

In this case:

```SFN = North-scale factor for conversion (m/decimal degrees) at the reference point
SFE = East-scale factor for conversion (m/decimal degrees) at the reference point
L = Degree of latitude
L0 = Reference degree of latitude
λ = Longitude
λ0 = Reference degree of longitude
RN = North curvature radius (WGS84) at the reference point (refer to article WGS84)
RE = East curvature radius (WGS84) at the reference point (refer to article WGS84)
PosX = Relative position in meters northward
PosY = Relative position in meters eastward``` 