  In addition to the intrinsic clustering of galaxies themselves, the spatial
distribution of galaxies observed in surveys is modulated by the presence of
weak lensing due to matter in the foreground. This effect, known as
magnification bias, is a significant contaminant to analyses of galaxy-lensing
cross-correlations and must be carefully modelled. We present a method to
estimate the magnification bias in spectroscopically confirmed galaxy samples
based on finite differences of galaxy catalogues while marginalizing over
errors due to finite step size. We use our estimator to measure the
magnification biases of the CMASS and LOWZ samples in the SDSS BOSS galaxy
survey, analytically taking into account the dependence on galaxy shape for
fiber and PSF magnitudes, finding $\alpha_{\rm CMASS} = 2.71 \pm 0.02$ and
$\alpha_{\rm LOWZ} = 2.45 \pm 0.02$ and quantify modelling uncertainties in
these measurements. Finally, we quantify the redshift evolution of the
magnification bias within the CMASS and LOWZ samples, finding a difference of
up to a factor of three between the lower and upper redshift bounds for the
former. We discuss how to account for this evolution in modelling and its
interaction with commonly applied redshift-dependent weights. Our method should
be readily-applicable to upcoming surveys and we make our code publicly
available as part of this work.
