  In image processing, source detections are inevitably affected by the
presence of the geometric boundaries in the images, including the physical
boundaries of the CCD, and the boundaries of masked regions due to column
defects, bright diffraction spikes, etc.. These boundary conditions make the
source detection process not statistically isotropic. It can lead to additive
shear bias near the boundaries. We build a phenomenological model to understand
the bias, and propose a simple method to effectively eliminate the influence of
geometric boundaries on shear measurement. We demonstrate the accuracy and
efficiency of this method using both simulations and the z-band imaging data
from the third data release of the DECam Legacy Survey.
