Since 1980, MRJ Technology Solutions, Fairfax, Virginia, USA, has been developing and maintaining a geospatial database of maritime information to support ocean engineering applications and regional environmental studies. The need to understand maritime boundaries (e.g., territorial sea limits, exclusive economic zones) led MRJ to create coverages that depict and document international agreements and country claims, both undisputed and disputed. Some of these coverages will be published in December 1999 as the Global Maritime Boundaries Database (GMBD) CD-ROM. As part of the construction of the GMBD, MRJ developed a set of ArcView tools to improve the accuracy and repeatability of computationally derived median lines. These lines are used in areas where specific maritime boundaries between countries have not been declared. Countries may use an accurately computed median line as the basis to negotiate an agreed boundary. The ArcView tools will compute median lines given a set of attributed shoreline or baseline data and the reference ellipsoid. Users may also specify tolerances for the calculations and the output resolution. These new ArcView tools have other precision applications for similar types of computations on the ellipsoid.