Dictionary:Rms positional error
The circle whose radius is the square root of the mean of the sum of the squares of the distances of measured positions from a point. If the uncertainties in x and y directions are equal and normal (that is, a Rayleigh distribution), the probability of a measurement lying within the rms positional-error circle is 41 percent. Called one-sigma circle because this radius is equal to the standard deviation when taken about the mean of the observations. Compare CEP.