Mind your ZOC

NOAA is helping boaters cope with inaccurate survey data.

Did you know that some areas of the U.S. coastal chart you’re using may only show land and bottom features accurate in position to +/1,600 feet? You shouldn’t have confidence in the depth soundings, either.

However, NOAA now has a new way to show us how old and potentially inaccurate much of its survey data are, and it will surely give pause to those who pay attention. Zones of Confidence (ZOC for short) is rolling out gradually as diagrams on paper-style raster charts, but the ZOC changes are coming much faster to NOAA’s ENC vector charts. It could be made easily accessible on the commercial vector charts you’re probably using.

NOAA is replacing the Source Diagram box that’s printed (over land) on most detailed charts (usually up to 1:80,000 scale) with a much more informative ZOC diagram in an effort

to, “help mariners assess hydrographic survey data and the associated level of risk to navigate in a particular area.” The Source Diagram does suggest how old the survey data are and includes cautions, like “partial bottom coverage,” but the ZOC really gets down to the nitty-gritty level of risk.

HUMBLING HUMBOLDT

very low confidence rating of Zone D.

Note also that while the Seafloor Coverage for Zone A1 is characterized as “All significant seafloor features detected,” in Zone D, “Large depth anomalies may be expected,” and neither Zone B or C has the “All features detected” confidence rating either.

NOAA’s blog entry about ZOCs explains that modern surveys using multibeam sonar that can see all bottom anomalies, but the old tools, such as singlebeam sonars and lead lines, were blind to wide swaths of bottom structure, and the surveyors weren’t quite sure where they were anyway.

ZOC ROLLOUT

So far, NOAA has issued only four paper/ raster charts with ZOC diagrams and the rest of the portfolio will be updated at the rate of about two per month. But, really,