An Overview on How a Chart is Updated
The coastal waters of the U.S. are in a constant state of change. Channels are dredged and sometimes re-routed; new aids to navigation are established or deleted; new wrecks and obstructions are discovered; natural shoaling occurs in many areas; and new berthing facilities are built along the shoreline. In order for the mariner to transit safely, it is imperative that these changes be reflected on nautical charts as soon as practicable.

Chart Update Publications Affecting NOAA Nautical Charts
In U.S. waters, the mariner using paper charts relies on three weekly publications to provide update information; the USCG Local Notice to Mariners (LNM), National Geospatial-Intelligence Agency (NGA) Notice to Mariners (NM); and in the Great Lakes and St. Lawrence Seaway, the Canadian Coast Guard Notice to Mariners. The USCG LNM is published weekly by each of 9 individual Coast Guard Districts. This publication is the mariner’s primary source of information for most detailed chart updates. The NGA NM, for the most part, contains the same information published in the USCG LNM. However, the NGA NM is primarily intended for deep ocean vessels and typically does not carry LNM chart updates that affect waters shallower than 18 feet. Occasionally, an NGA NM will contain information that was not originally published in the USCG LNM, therefore, it is imperative that the mariner check both publications routinely. Since some U.S. and Canadian charts overlap, the Canadian Coast Guard’s Notice to Mariners will sometimes contain unique information not found in the USCG LNM.

Applying Chart Updates to Traditional Charts
If a mariner purchases a traditional paper chart that has an edition date one year prior to the purchase date, it is quite likely that important new changes affecting the chart have been published in the USCG LNM, NGA NM, or the Canadian Coast Guard NM. It is imperative that these changes be applied to the chart so the chart can be brought up to date. Mariners using traditional paper charts apply chart corrections manually by using pencils, erasers, rulers, compasses, dividers, and protractors. Applying updates to a suite of paper charts can be time consuming, tedious, and error prone.

Updated Print on Demand (POD) Charts
NOAA has developed a paper product that addresses the problem of manual updates to paper charts. A POD chart is a paper chart that is printed at the time of purchase and contains chart updates up until the time of purchase. For example, a mariner may have the choice of purchasing a new traditional paper chart that has an edition date one year prior to the purchase date. In doing so, this traditional chart must be brought up to date by manual corrections. If the mariner chooses to purchase a POD chart, all updates that have been published in the one year between the new edition date and the purchase date have already been applied.

NOAA cartographers maintain a master electronic copy of every new chart edition. When the USCG, NGA, or Canadian Coast Guard publishes chart update information, NOAA cartographers immediately access this information electronically and apply it to its master electronic chart versions. These updated electronic master charts are made available to NOAA’s private partner Oceangrafix for download. When a participating POD chart agent anywhere in the country orders a chart, Oceangrafix prints out an updated POD chart immediately, and ships it to the agent within one day. Eighteen agents world-wide have their own print capability.  POD charts will show the dates of the latest LNM and NMs applied to the chart. Once a mariner owns a POD chart, that chart must be updated from thereon manually.

Updating Raster Charts
Since Raster Navigational Charts (RNC) are simply electronic images of NOAA’s paper chart, the same update system described for POD charts also applies to RNCs.  Once NOAA cartographers update the master electronic versions, the files are geo-referenced in .BSB format, a compressed raster format, and the digital differences between the updated chart and the base edition are isolated into an update or patch file.  NOAA posts these updated files on the Internet where they are available for free download. Electronic chart companies offer an update service for mariners.  The update or patch files (considerably smaller that the entire chart file) can more easily be sent by e-mail or downloaded by subscribers.  Software is available to apply the update patch to the base version of the raster chart.

Updating Electronic Navigational Charts (ENC)
Currently, NOAA has over 600 ENCs available for free download. Updates for these ENCs are also available for free download. ENCs are a vector-based product and are not constructed from the same pipeline as paper charts or RNCs. Each LNM and NM chart correction must be applied independently to the electronic master version of the ENC. NOAA’s goal is to post ENC updates weekly for its entire suite of charts.

Updating the U.S. Coast Pilot®
Like charts, corrections to the Coast Pilot are published in the USCG LNM and NGA NM. These corrections can be conveniently found with the Coast Pilot downloads.

Where does Chart Update Information Come From?
Chart update information that is published in the LNM and NMs are derived from a variety of sources. Aid to navigation additions, deletions, and changes usually originate within the local Coast Guard District. Therefore, this information is typically transmitted internally between the Aids to Navigation unit and the LNM unit in a particular Coast Guard District. Since the Coast Guard maintains a network of stations throughout the country, chart update information may be reported directly to these units by a variety of local sources. For example, a Port Authority or a local boating group might bring a chart update item to the attention of the local Coast Guard. The Coast Guard LNM unit will then make a decision concerning whether a chart update should be published.

Over 95 percent of the chart update information that appears in NGA’s NM and the Canadian Notice to Mariners has already appeared in a Coast Guard LNM. Occasionally, a military unit will report a chart discrepancy directly to NGA and not to the USCG or NOAA. In this case, NOAA may first learn about the chart update by examining the NM. Likewise, it is only occasionally, that the Canadian Notice to Mariners contains unique chart update information that pertains to NOAA charts.

The majority of the chart update information that appears in the LNM and NMs first enter the charting pipeline through NOAA. The U.S. Army Corps of Engineers (USACE) is responsible for providing NOAA with survey information that pertains to federal channels and waterways. The Corps routinely conducts pre-dredge, condition, and post-dredge surveys in channel areas that serve as the main arteries for our nation’s ports. The latest information concerning depths in these federal channels is extremely important to deep draft vessels. This information is shown on NOAA charts through channel legends, tabulations, and by other means.

Over 25 Corps districts throughout the country provide NOAA with survey information concerning federal channels. NOAA cartographers analyze surveys and other information provided by the Corps in order to determine whether a chart update is required. In most cases, an update is required to notify mariners of the latest channel depths, channel re-configuration, or changes in adjacent depths. NOAA cartographers prepare these chart updates in an electronic form that is readily usable by the local Coast Guard unit responsible for publishing the LNM. NOAA cartographers also provide this information to NGA’s NM and the Canadian Coast Guard if applicable.

Other types of chart update information also enter the charting pipeline through NOAA. The USACE is responsible for issuing construction permits in coastal areas. NOAA receives these permits and monitors construction that ultimately may result in a change to the nautical chart. If the resulting construction is deemed “critical” to the chart, then this information will be provided to the LNM and NMs for inclusion. If the information is not critical, it will not be issued as a LNM or NM, but will appear on the next new edition of the chart. NOAA hydrographic field units and contractors conduct surveys in coastal areas throughout the country. If these field survey units find a critical chart discrepancy, they will report it directly to NOAA cartographers for application to the chart. This information will then be provided expeditiously to the appropriate LNM and NM publications for dissemination to the general public.

NOAA also receives chart update information directly from the recreational boating community. NOAA and the United States Power Squadrons (USPS) have had a partnership for over 40 years. The Cooperative Charting Program enables USPS members to report chart discrepancies by Internet throughout the country. NOAA conducts a similar program with the U.S. Coast Guard Auxiliary. If a chart discrepancy report is judged by NOAA cartographers to be “critical,” it is applied to the chart and furnished to the U.S. Coast Guard and NGA for inclusion in LNM and NMs.

It should be noted that not all chart update information in NOAA’s possession is made available to the public immediately through the LNM and NMs. If new information is deemed “critical”, it is always made available to the public through the LNM and NMs in an expeditious manner. However, if new information is not deemed “critical,” it will appear only on the next new edition of the chart.