Velocity Measurement Error Estimate I have 2 position estimates (along with their measurement error) and a difference in time between estimates.  I estimate velocity using

Velocity = (PosA - PosB)/DeltaT


I am trying to estimate the error in my velocity estimate, but I can't seem to find any ways to calculate this.  I assume it has to use Sigma_PosA and Sigma_PosB.  I would also assume it's relative to DeltaT and/or abs(PosA - PosB).  What is the velocity measurement variance/standard deviation?
sigmav = sqrt((sigmaA)2 + (sigmaB)2)  / (DeltaT)

EDIT:

(Corrected an error above-- DeltaT should not be squared.)

It sounds as if the measurements are independent, so the errors are uncorrelated. We want the standard deviation of a linear combination of (two) variables:

$V = \frac{(B-A)}{\Delta_t} = \frac{1}{\Delta_t}B - \frac{1}{\Delta_t}A$

$\sigma_V^2= \sum_i^n a_i^2\sigma_i^2 = (\frac{1}{\Delta_t})^2\sigma_B^2 + (\frac{1}{\Delta_t})^2\sigma_A^2 = (\frac{1}{\Delta_t})^2(\sigma_A^2 + \sigma_B^2)$

$\sigma_V = \sqrt {(\frac{1}{\Delta_t})^2(\sigma_A^2 + \sigma_B^2)} = \frac{\sqrt{\sigma_A^2 + \sigma_B^2}}{\Delta_t}$

(I'm new to math.stackexchange-- gotta say I'm lovin' the MathJax.)
You want the propagation of error, or propagation of uncertainty. Since you're subtracting them, the formula is 

sqrt(sigma_posA * sigma_posA + sigma_posB * sigma_posB)