Seascape Alaska 6: Gulf of Alaska Transit Mapping


Seafloor Mapping the Inside Passage: Story of a Self-Fulfilling Prophecy

Stormy seas forced NOAA Ship Okeanos Explorer to take refuge in the Clarence Strait of southeast Alaska’s Inside Passage during the Seascape Alaska 6 expedition. Okeanos Explorer is the only federal vessel solely dedicated to ocean exploration, and during mapping expeditions like those conducted during the Seascape Alaska series, seafloor mapping is a 24/7 operation. Mapping in the Inside Passage was not a part of the original expedition plan, but it became a historic venture. One of the last hydrographic surveys– surveys that map features underwater–in this location was conducted in 1920.

More than a century ago, this area was mapped using lead lines. This technique involved dropping a line with a lead weight at the end of it off a ship until the weight hit the seafloor. The length of line it took to reach the bottom was measured to estimate depth. These measurements are known as soundings, and were recorded in fathoms (one fathom is equal to around six feet). Leadlining was a lengthy, difficult process, but essential for creating nautical charts to ensure safe navigation by ships.

Lead lining was established in the early 1800s and was the primary seafloor mapping method until the development of single beam sonar in the 1920s. These sonars sent one pulse of sound to the seafloor and returned one measurement. Multibeam sonars, which send out many sound pulses and can map a much larger swath, were developed in the 1960s.

Okeanos Explorer is equipped with an EM 304 MKII multibeam sonar that emits 512 beams in one swath, able to produce up to 1,600 soundings in one “ping” of sound. These swaths can be up to approximately 10,000 meters (6.2 miles) wide. With lead lining expeditions only able to collect a few hundred soundings per expedition, to Okeanos Explorer multibeam sonar collecting hundreds soundings every few seconds, the advancement in seafloor mapping technology in the last 100 years is astounding.

The 1920 expedition report discusses the difficulties of mapping near exposed rocks and the gaps in data avoiding them would create: “...gaps of this character are likely to remain unsurveyed for years and render a chart incomplete.” These seafloor mappers were quite prophetic, as it would be more than 100 years until this area was mapped again.

Original chart from the 1920 expedition to map Alaska’s Inside Passage showing about 140 soundings collected using lead lines
Original chart from the 1920 expedition to map Alaska’s Inside Passage showing about 140 soundings collected using lead lines.
Image courtesy of NOAA Download largest version (jpg, 1.9 MB).
Bathymetric map produced from multibeam sonar data.
Bathymetric map produced from multibeam sonar data collected from NOAA Ship Okeanos Explorer during the Seascape Alaska 6 expedition with over 100 million soundings.
Image courtesy of NOAA Ocean Exploration, Seascape Alaska Download largest version (jpg, 2.0 MB).


By Ingrid Martinson, NOAA Ocean Exploration Explorer-in-Training

Published November 29, 2023