by Joan Conover
Over the past several years, governments have moved away from marine charts in paper to an electronic format. A Caribbean Compass article (2021) described the U.S. National Oceanic and Atmospheric Agency (NOAA) change from paper to an electronic delivery format. Now per NOAA, RNCs (Raster Nautical Charts/vector charts) are planned to be canceled by December 2024. NOAA continues to work on rescheming charts; see project progress online (distribution.charts.noaa.gov/ENC/rescheme). This year, the British Admiralty approved the migration from paper to electronic formats as well.
With the migration to electronic formats, there is also interest in the addition of satellite data to expand electronic chart detail. This is specifically useful for areas with limited or no recent surveys and for locations without the funding to expand or perform new bathymetric studies. This designation—limited or old survey data—covers many areas of the world, specifically, in the tropics.
Where the work on Satellite Derived Bathymetric (SDB) becomes of interest to Caribbean Compass readers is on occasions that files are available for the Caribbean, or if sailors are heading to the Pacific Ocean. New efforts to use satellite data to assist in marine chart development for Caribbean waters include downloadable files developed for the Dominican Republic by Bruce Balan of S/V Migration as MB Tiles. Chart Locker, Balan’s website (chartlocker.brucebalan.com), also includes MB Tiles for the San Blas, Turks and Caicos, and Bahamas.
Discussion and links to the files are also described (see sidebar). All files and an application (OPENCPN, www.opencpn.org) are open source/free to download and use.
The process for Satellite Derived Bathymetic (SDB) can include automated assigning of a depth derived from values from a recent survey or surveys to individual pixels within a processed set of imagery. (Note: Processing the imagery involves doing some math functions to the Green, Blue, and Infrared wavelengths from an image). These values are impacted by currents, debris and weather impacts on water surfaces. NOAA has recently updated the Nautical Chart Manual (which can be considered the NOAA “rule book” on any number of topics); SDB data received from U.S. government hydrographic processing teams can be used. Prior to this rule change, cartographers had to chart all SDB derived contours and shoreline as approximate. For safety, a caution area, labeled comments, has been added around data based on SDB.
The problem of limited availability of updated marine charts in the Caribbean is exacerbated if governments do not actively survey waters. Currently used are old charts, paper or vector, which have not been officially revised. In some cases, if no government has taken responsibility for validating the data, they may be many years out of date. Some just show a rough shoreline without detail. It’s up to the captain to navigate with limited information.
Well known offshore mariner John Kretschmer, who recently completed a transatlantic crossing via a far northern route, used electronic charts and paper charts, then compared them with satellite imagery, for course planning. Accurate charts, electronic or paper, for his passages in the far north weren’t available.
Cruisers in the Pacific have also addressed this problem by creating charts from satellite imagery, thanks to bluewater voyaging-sourced inputs. These Pacific charts are available at no charge, as MB Tile formats, usable in some electronic charting applications, such as OPENCPN. They must be used with care; they are not the same as official government released charts used for navigation. SDB-derived charts must be considered a supporting application to compare with existing government or commercial products.
Dave McCampbell (S/V Soggy Paws) has published a detailed article in the SSCA Cruiser Bulletin (March 2023) with discussion of commercial vs. satellite imagery development. He writes: “Several cruisers, including Terry Sargent on S/V Valhalla, Sherry McCampbell on S/V Soggy Paws, Jon Hacking on S/V Ocelot, Balan, and Mike on S/V Zen Again maintain large collections of satellite charts that are shared with other cruisers.
“The image collections number in the thousands, cover from the Eastern Pacific through the Indian Ocean, plus the Atlantic Ocean and the Caribbean, and are continually being enlarged. Tracks in and out of many of these anchorages are also available. These five cruisers also maintain huge collections of anchorages in the same areas with information details included. All this has been made possible by Paul Higgins, who years ago developed a program which includes options for converting a satellite image to a navigation chart. Now there are also multiple sources for other satellite imagery besides just Google Earth images. Cruisers can now choose from multiple images if there are clouds or sun reflections degrading the desired image.”
Hacking’s website includes Pacific and Caribbean files, with great detail on the processing and software downloads (svocelot.com/Cruise_Info/Equipment/Chart_Downloads.htm).
Scroll down for both Eastern Caribbean files as well as the Galapagos. As previously stated, these are created from satellite imagery and are to be used with the understanding that they are a support to the captain’s existing navigation tools and charts.
The current marine charts (paper or electronic) for the Dominican Republic require updates to reflect changes to depths, markers and coastlines; most charts are derived from surveys from the late 1960-90s. Coral areas are known for alterations, with sometime rapid changes due to storms. Harbors are modified due to human activity, marine markers and shoals. The “under construction” cruise terminal development
and public anchorage/dock changes at Samana, Dominican Republic, is an example.
For the northern Caribbean, current charts exist for the Bahamas, Puerto Rico, and the Virgin Islands. All have been updated recently via government projects. With the Dominican Republic, the most accurate charts are considered by locals to be Navionics with a cruiser-based updating system, as well as Balan’s. Some paper charts are available from commercial vendors based on British Admiralty Charts.
Balan, in discussion with Seven Seas Cruising Association, has suggested creation of satellite imagery charts for more areas of the Caribbean. He proposed images/charts similar to those done in the Pacific.
As an example, SSCA compared the coastline of the northern Dominican Republic to both the satellite and a chart with good correlations. We also made a comparison with data contained in Bruce Van Sant’s book, The Gentleman’s Guide to Passages South. The north coast area, which includes Ocean World Marina, shows a mapped entry, buoys and depths. Images successfully replicate the facility, the bathymetics and the channel markers. This particular area was chosen due to its location to the west of Puerto Plata (which no longer has an anchorage), and the Socha area, now full of charter boat day moorings.
In sum, MB Tiles do not replace any government marine charts. These satellite images, while useful, are not to be used for navigation. Captains must use validated charts as well as be aware of possible changes; however, satellite images (SDB) do offer a good look for comparison and may help identify possible changes to markers or harbor entries/structures. These sample files are usable on Electronic Navigational Chart applications which can upload MB Tiles such as the free application OPENCPN. If these charts assist cruisers along the northern Caribbean, and the file download/comments offer encouragement, other areas can be considered on request to the author of this article.
Many thanks to NOAA Office of Coastal Survey, SSCA’s McCampbell, and Balan for assistance with this ongoing issue. The Seven Seas Cruising Association will continue to monitor this topic to improve cruisers knowledge and safety.
Commodore Joan Conover is president and cruising host coordinator of the Seven Seas Cruising Association. She also serves as SSCA’s Clean Wake project coordinator.
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