  The common assumption that precision is the limit of accuracy in localization
microscopy and the typical absence of comprehensive calibration of optical
microscopes lead to a widespread issue - overconfidence in measurement results
with nanoscale statistical uncertainties that can be invalid due to microscale
systematic errors. In this article, we report a comprehensive solution to this
underappreciated problem. We develop arrays of subresolution apertures into the
first reference materials that enable localization errors approaching the
atomic scale across a submillimeter field. We present novel methods for
calibrating our microscope system using aperture arrays and develop aberration
corrections that reach the precision limit of our reference materials. We
correct and register localization data from multiple colors and test different
sources of light emission with equal accuracy, indicating the general
applicability of our reference materials and calibration methods. In a first
application of our new measurement capability, we introduce the concept of
critical dimension localization microscopy, facilitating tests of
nanofabrication processes and quality control of aperture arrays. In a second
application, we apply these stable reference materials to answer open questions
about the apparent instability of fluorescent nanoparticles that commonly serve
as fiducial markers. Our study establishes a foundation for subnanometer
localization accuracy in widefield optical microscopy.
