This paper studies the validity of horizontal dilution of precision (HDOP) measure to evaluate sensor network geometries when localizing impacts in structural health monitoring (SHM). First, HDOP is defined similarly to navigation applications. Even though several low-complexity closed-form solutions have been proposed recently, HDOP measure has been theoretically justified only for iterative least-squares approach. The localization errors of popular impact localization methods are experimentally collected and compared with HDOP data for validation. The experimental setup is also described. It is shown that HDOP, in general, correlates with the positioning error and can be used to characterize geometry.