NOAA announces change in channel depths on raster nautical chart products

NOAA’s Office of Coast Survey recently announced plans to change the U.S. Army Corps of Engineers (USACE) maintained channel depth values on raster nautical chart products, which include paper nautical charts and the corresponding digital raster navigational charts (NOAA RNC®). Minimum depths (also called controlling depths) are collected during periodic USACE sonar surveys of channels. In the past, these depths were provided on raster charts, but controlling depths will now be replaced with the original channel design dredging depths used by the USACE (called project depths). Standardizing depth presentation on these products will improve data consistency and overall safety. Implementation begins in early 2019.

NOAA’s suite of electronic navigational charts (NOAA ENC®) are not affected by these changes. Mariners are encouraged to use NOAA ENCs for critical safety information as these products are typically updated up to one month ahead of raster products.

Why the change?

The USACE makes the depth information from recent surveys publicly available on their website before NOAA nautical products are updated and published. NOAA prioritizes making updates to ENC over RNC products. This often results in RNC products not accurately reflecting the most current controlling depth values (as represented on NOAA ENC and the USACE website). To eliminate inconsistencies among controlling depths, NOAA will only show project depths on raster chart products in the future. The Initial implementation of this change will focus on deep draft shipping channels where the primary product used for navigation is the ENC.

How will mariners be notified?

NOAA first publicized the concept of charting project depths in the National Charting Plan released in February 2017. As the changes are made on individual charts, NOAA will include a note on each chart directing mariners to review the USACE website and use NOAA ENC to access the latest controlling depths. Additionally, a statement drafted jointly by Coast Survey and the U.S. Coast Guard (shown below) is being published weekly in the Coast Guard’s Local Notice to Mariners.

NOAA recommends that mariners take advantage of the most recent 
chart updates by using the NOAA Electronic Navigational Chart (ENC)
for navigation in U.S. waters. ENCs provide the most up to date
information, whereas paper and raster nautical chart updates may
be up to one month behind the corresponding ENC coverage. Over the
next few years, mariners will see continued improvement in the
extent and detail of ENC coverage, while there will be a reduction
in RNC and paper chart coverage and service. ENCs will include
routine changes between editions that are not published through
notices to mariners. One significant change to the RNC and paper
charts will be the removal of controlling (minimum) depth
information from many maintained channels. Controlling channel
depths will still be provided on ENCs.

Comments or concerns can be addressed through ASSIST, NOAA’s
Nautical Inquiry and Comment System.
https://www.nauticalcharts.noaa.gov/customer-service/assist/

Project depths and controlling depths defined

Federally maintained channels are broken into a series of individually named sections called “reaches.” There are two different depths associated with each reach that are reported by the USACE, the project depth and the controlling depth. Project depths are the original design dredging depths of a channel reach constructed by the USACE. They may or may not be maintained by dredging after completion of the channel. In other words, the actual depth of the channel may be shoaler than the project depth (for example, Reach B in image below). Controlling depths, or minimum depths, are the least depths within the limits of a channel reach. These depths are updated with each new USACE survey. Minimum depths restrict the safe use of a channel to ships with drafts less than the minimum.

diagram explaining controlling depths and project depths
Example of a federally maintained channel with a project depth of 30 feet. Reach A has a controlling depth of 32 feet and Reach B has a controlling depth of 28 feet.

Displaying project depths on raster charts

Controlling depths are depicted on raster chart products by channel tabulations, depth legends, and hydrography (individual depth soundings). Here are a few examples of how depiction of channel depths will change on raster charts:

Channel tabulation to project depth legend:

Depth legend to project depth legend:

Hydrography to project depth legend:

Controlling depth channel tabulation to project tabulation:

Project Depth Note:

The following note will be added to raster charts as controlling depths are replaced with project depths:

PROJECT DEPTHS

  Channel legends and tabulations, where indicated, reflect the
U.S. Army Corps of Engineers (USACE) project depths. The channel
may be significantly shoaler, particularly at the edges. For
detailed channel information and minimum depths as reported by
USACE, use NOAA Electronic Navigational Charts. USACE surveys
and channel condition reports are available at
http://navigation.usace.army.mil/Survey/Hydro.

The change from showing – often outdated – controlling depths to showing channel project depths on raster nautical chart products, such as paper nautical charts and RNCs, will provide greater clarity and safety for mariners. The project depths shown on raster charts will give users an idea of the original channel design dimensions. The up-to-date controlling (minimum) depths provided on NOAA ENCs and on the USACE website (referenced on raster chart products) will give users the latest information on the safe depth in which ships may transit through federally maintained channels. Comments or concerns about these changes can be addressed through NOAA ASSIST.

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