Designing a new SR 520 bridge requires an accurate three dimensional topographic and bathymetric map. Bathymetry is the study of underwater depth, the underwater equivalent to altimetry.

A motor boat equipped with sonar and a global positioning system (GPS) is used in the deep water of Lake Washington, but something else is required for maneuvering through the environmentally sensitive marshlands near the Seattle Arboretum, Foster Island and the south shore of Portage Bay.

This oar-powered pontoon boat rigged with a GPS and total station tracking was used to conduct bathymetric soundings of the marsh.

The inflatable one-person raft was selected as the best survey platform to allow taking depth measurements and recovery of real time GPS locations.

The boat traveled unobtrusively through lilies and reeds, past herons and beavers, collecting depth and location information.