Posted: August 28, 2015

We all use charts for navigation (either paper or electronic), but on my recent World Circumnavigation, I found that charts come in varying degrees of accuracy. The further you get from ports used by ships, especially in third-world countries, the less accurate the charts become. This is independent of whether you choose electronic or paper charts as the electronic charts are usually based on the paper charts. In far-flung areas, there is usually only a single underlying marine survey (except in the Bahamas, see the "Experience Example" below).

Common wisdom is: “The prudent mariner will not rely solely on any single aid to navigation.” Sometimes the charts themselves are the “aids” not to be relied upon.

In areas of lower-quality charts, Google Maps can be a valuable addition to your arsenal of navigational tools by showing the exact locations of reefs and other near-surface features.

An Example

In Fiji, charts are reasonably accurate when entering major harbors used by commercial ships. When going to less-travelled islands, the locations of land areas are represented reasonably accurately but the positions of reefs were only somewhat related to their actual positions. Musket Cove is a popular anchorage in Fiji and while we were anchored there, a large catamaran grounded hard on a reef. They were lucky and sailed free on the next high tide. While it is recommended that you only sail reef-strewn areas in times of good lighting, this is not always possible.

Figure 1 shows the electronic chart of Musket Cove overlaid with the Google Maps satellite/aerial photograph. Notice that while the land is represented accurately on the chart, the reefs are only marginally accurate, particularly in the center where my anchor position is marked on a green reef. In actuality, I was in over 40 ft. of water in the center between the two reefs as shown by Google. In this case, Google is the more accurate position source.

Useful Techniques

For this application, go to Google Maps and use only the satellite image mode. You can change from street maps to satellite image with an icon in the lower left corner of Figure 2.

If you already have a waypoint or lat/lon information from a chart or chartplotter and want to see it in Google Maps, enter the coordinates directly into the Google Maps search box (“1” in the figure). Google Maps is quite forgiving about the format for input: A decimal-degrees input is shown but you can use degrees-minutes and put in a space for the degrees symbol like this: “17 46.623s 177 11.038e” and Maps takes you to that location and puts a marker there.

To find the true position of a feature you see in google maps (like a reef), put your cursor on the feature, right-click, and select “What’s Here?” (“2” in Figure 3) A popup will appear showing the coordinates (“3” in the Figure):

Google maps displays a decimal-degrees system (rather than minutes) and uses minus signs to indicate the Southern or Western hemispheres. Conversion to decimal minutes positions is pretty straightforward. To get the minutes, take the decimal part of the coordinate and multiply it by 60.

So the lat/lon is:

These coordinates can be keyed into a chartplotter or plotted on a paper chart. TIP: if you click on the coordinates in the popup, Google Maps will center on that position and the coordinates will appear in the search box. You can copy-paste from there to your calculator app. Google Maps also does a conversion to Lat/Lon but it is converted to degree-minutes-seconds which most chartplotters won’t accept.

PLAN AHEAD: When you most need the Google Maps data, you probably won’t have internet access. Before starting a passage, go to Google Maps and save the information you need about your destination. An easy way (in Windows) is to use your browser to display the map you want to save. Alt -PrSc (“Print Screen”). This copies the map data from your browser to the clipboard. Open your favorite image program and paste the image in. I paste images directly into my word-processing app because it handles images well and I can easily add notes. You can make the images small in the word processor and still zoom in when you need more detail.

KEEP IN MIND: Even though Google Maps can show you the locations of dangers, it doesn’t have depth information and it can never show you safe locations. Always check your chart to see water depths.

Experience Examples

Our chartplotter uses Navionics charts, which are based on government surveys and are less accurate than the Explorer Charts (which I recommend for anyone sailing in the Bahamas). To reach the Exuma Land and Sea Park from the northwest, a Navionics-charted course will put you on the wrong side of a sand-bar and you’ll have to backtrack to get in if you're careful enough to avoid running aground. The sand-bar’s location is clearly visible in Google Maps and with preparation, you can take a few positions, create waypoints, and sail into the Park without hazard.

Similarly, entering Port Resoluation on the island of Tanna (Vanuatu), the charts are sketchy on the location of rocks off the point you must round to enter the bay. But Google Maps shows the extent of the rocks because the breakers you'll need to avoid are clearly visible. In some areas, the Google images will be at high tide and/or on a calm day and may not be as useful as the examples here.

All-in-all, when you're navigating in areas of marginal nautical charts, Google Maps is worth a look.