Coastwise navigation like a local

Oct 19, 2010

To the editor: Most boaters have looked at a chart on approaching a new harbor and squinted a bit trying to visualize the few features shown shoreside. A tower here, a tank there, a jetty obscured in the haze and lights. As anyone who has ever sailed off the New Jersey shoreline can attest, after a while, every water tank looks the same. We have GPS, radar, depth sounders, and chartplotters, but still, getting a mental picture of the harbor approach and anchorage takes patience and skill. With shifting and shoaling channels, the challenge is more complex. What the mariner needs on approach to a new port is local knowledge.

We’ve often looked at those crisp satellite images on Google Earth and wondered how to interpret what we see. Aids to navigation are too small to see in such images, and trying to align the image on the computer screen with our charts is both cumbersome and potentially dangerous. Now, however, a company in France has taken nautical charts and Google Maps, and has created a clever and useful “mashup” that gives local knowledge an entirely new spin.

Our home port is Passe-A-Grille, Fla., just north of the entrance to Tampa Bay. The channel is well-marked and fairly deep, but even so, shifting sand bars make finding day anchorages a challenge. The chart is of little help, but South Channel looks promising.

We can compare the chart to Google Maps, and begin to notice a problem with the chart.

GeoGarage.com’s wonderful Web site makes it easy. Using the slide control on the right side, under NOAA, we can mix the chart with the satellite hybrid view. By moving the control right and left, we can have all chart, all satellite, or a mix…a bit of chart with lots of satellite.
Or a bit of satellite with lots of chart.

Note that on the chart, South Channel’s northern entrance is shown as carrying a depth of more than 12 feet. The satellite view reveals that this has been silted in and is no longer navigable (unless you happen to own an amphibious craft!). Another advantage of the satellite view is that often (though not always) the image is newer than the chart data. Close inspection of these images shows that the copyright date is 2010, while the chart is significantly older.

GeoGarage.com has very good coverage. U.S. coverage is extensive.
Other chart sources are also available including New Zealand, Brazil, the U.K., Canada, Argentina and Australia. Some of these provide far-ranging coverage (the U.K. covers wide areas of the Pacific, for instance.) The site also exports information about waypoints and tracks to common navigational file formats, and even directly interacts with Garmin’s plugin to directly load track and waypoint data into a compatible GPS unit (though, sadly, not the hybrid chart views themselves.) GeoGarage has a family of apps for the Apple iPhone, iPod Touch and iPad that access its database of marine charts (the U.S., Brazil and New Zealand are now available via iTunes at http://itunes.apple.com/us/app/marine-us/id376844755?mt=8). We can hope that some day, the ability to combine the chart with satellite views will be directly accessible in that app, along with some way to save the combined chart data for use when we go beyond the mobile telephone coverage area.

As with any electronic charting system on the Internet, GeoGarage warns that its service is to be used only as a planning tool, and not as an aid to navigation. It’s a good idea to have a full set of charts with you. With this caveat firmly in mind, though, the service is an invaluable aid to mariners planning a cruise in unfamiliar waters.

Lawrence A. Husick is an intellectual property attorney and sailor based in Philadelphia and Florida.

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