Adam Clark accepted answer staff Oct. 10, 2013, 10:45 a.m.
USArray TA station coordinates are measured by handheld GPS and reported to five decimal places of precision.
The IRIS DMC internally can store up to six decimal places of precision for latitude and longitude (this is the SEED specification). Many of our web-based tools only display up to two decimal places, though, especially mapping tools. The rationale for this is to minimize the ease with which individuals could locate an installation and cause damage to the site and its equipment. We try to leave the high precision representations to the delivered data files.