Make your own charts

After he nearly lost HMS Resolution on the Great Barrier Reef, James Cook must have regretted bitterly not running a survey before nosing into uncharted waters. After all, he was already famous as a marine cartographer. Beginning in the 18th century and continuing into the 20th, the European powers — Britain, Spain, Portugal, Holland and France — sent supreme navigators like Cook to map sea routes, first for traders and later for navy gunboats. Marine cartography peaked in the early 20th century, and this old work provides the framework for even the latest representation of ocean and coastal regions.

The latest, evermore-expensive charts conspicuously lack detailed information that is of particular interest to small-boat sailors in foreign waters. Even the fanciest digital-chart cartridges and monitors cannot make up for the lack of data. Due to the explosion of recreational boating, the charts for domestic waters of developed nations have retained high quality. However, the small-boat navigator sailing away to those enticing islands scattered between the tropics of Cancer and Capricorn or in very high latitudes better stay on guard.

Absurdly, as the GPS system perfected electronic positioning, the nautical charts covering foreign destinations now glare with empty blanks outside the main ship channels. The British Admiralty and the U.S. Defense Mapping Agency have chosen to shrug off detail for remote waters. In other words, you may not be able to refer your GPS fixes, though they are accurate within feet, to any charted features in the area. You are on your own unless you go only where a cruising guide will lead.

These guides vary in the amount of information delivered and often show only a recommended course surrounded by empty paper, assuming that today’s navigator can always motor along in a straight line to the recommended anchorage. Factors like engine breakdowns and unexpected weather phenomena often play havoc with such rigid plans. In real life, a mariner must know locations of both deep water and hazards to navigation. Fortunately, the latest navigation tools allow you to survey and chart a hazardous area in record time &mdash a big difference from the days when surveyors worked with leadlines, magnetic compasses subject to magnetic variation errors and celestial navigation &mdash impossible to use when skies are overcast and skewed by chronometer errors.

Needed charts in Panama

We discovered a need for self-made accurate charts while cruising Panama’s Caribbean coast. There, at the extreme eastern end of the San Blas territory, we could anchor and dinghy ashore in order to explore tropical coastal forests quite unspoiled by human activities. It was well worth the effort. Within a few miles of this coast began the hills of the continental divide, crossed in 1513 by the Spanish conquistador Vasco de Balboa who, led by indigenous allies, became the first European to see the Pacific Ocean. The forests have changed very little since, and unusual in this crowded world, far fewer people live there today than in the 16th century. The winds and seas in this part of the Caribbean stay mild except for occasional disturbances from which we needed to escape to a protected harbor. British Admiralty chart 1278 (scale 1:200,000, on which one nautical mile is only 3/8 of an inch), the only one for the area, shows an almost landlocked Puerto Perme but lacks soundings in it or anywhere near. The outline of the harbor is based on a Spanish government chart from 1817.

We approached cautiously through deep water between scattered reef patches and found perfect shelter inside. We piloted in by eye, but even by casual glances at the compass, we could see that the correct approach led from the south and not from the east as charted. We confirmed this by circling the shores inside and outside in our inflatable, equipped with a hand-held GPS and a portable depth sounder ticking on the stern. We wrote down the latitude and longitude of fixes at several points, together with depths, and all we had to do to create our own chart was to convey this data to a paper chart.

Image Credit: Tom Zydler
A Chart the author constructed for the Panama Coast. He found the Admirality chart for Puerto Perme showed the area incorrectly.

The new chart had to be in Mercator projection, like the standard nautical chart, which allows taking courses and direction from any meridian drawn on it. Mercator projection allows meridians and parallels to cross each other at a constant 90�, which aids plotting fixes and, in our case, plotting the GPS readings. In Mercator projection, one minute of latitude equals one nautical mile, which is why to measure distance, you always have to apply your dividers to the side of the chart. The length of a minute of longitude at the bottom of the chart relates to the scale of a chart and is chosen by the mapmaker who decides on the accuracy of his product. This predetermined length of a minute of longitude affects the size of a minute of latitude and the length of a nautical mile on that particular chart.

Minutes of latitude and longitude

With the Mercator projection, minutes of latitude and longitude are equal only on the equator. As you move away north or south, the graphic representation of one minute of latitude begins to become larger than one minute of longitude, and the difference between the two grows more pronounced with increasing latitude. The result of this attempt to represent a piece of round earth on a flat sheet of paper causes some distortion in drawing the outlines of very large landmasses. This distortion, insignificant while portraying small areas, makes the Mercator projection unsuitable for mapping large areas, and you will notice that world atlases use other projection systems.

In order to create your own chart, you must first decide what length your minute of longitude should be. It depends on how accurate you want to make your chart.

If you start with a minute of longitude represented by 32 millimeters (which equals 1 1/4 inches) you will have ample space to plot your GPS fixes. First draw a baseline along the bottom of a sheet of paper. Next, from the left end draw a perpendicular line that represents the first meridian. Mark a 1-1/4-inch length along the base, and then draw a vertical line from it &mdash the second meridian of your chart.

Now you need to establish the length of your minute of latitude. A simple procedure referring to a median latitude of your area will let you find this length graphically. Let us assume we are mapping a bay on an island lying somewhere between 32� N and 33� N. Using navigational triangles or a course plotter, draw a diagonal line at a 32.5� angle (the median latitude of the area) from the corner where the baseline and the first meridian meet. From the point where this line intersects the second meridian, swing your dividers until it hits the first meridian of your chart. The distance from the baseline to this intersection point is one minute of latitude as well as one nautical mile. Now you can mark minutes of latitude, and after marking the rest of the minutes of longitude, you will have a blank chart ready for plotting. On this chart, your latitudes must range between 32? and 33?, but you can mark any desired longitude along the baseline parallel. In the Western Hemisphere, the values of longitude will decrease from left to right, while in the Eastern Hemisphere, they will increase from left to right.

You will probably find that in making charts for yourself, you will cover areas under five square nautical miles &mdash so mark as many minutes on both the longitude and latitude sides as you need. It helps to work on graph paper, which facilitates plotting fractions of minutes of your GPS fixes. You can tell your GPS unit to represent these fractions in decimals of a minute or in seconds (60ths) of a minute. Decimals may be easier to plot if you work on graph paper. Otherwise, divide one length of a minute of longitude and one length of a minute of latitude on your base chart into 10 parts, each representing six seconds, and use them for reference with your dividers. For all practical purposes, you can simplify this plotting by dividing these two reference minutes (one of latitude and one of longitude) on your base chart into four parts, each representing 15 seconds, and then interpolate the smaller fractions by eye when plotting the GPS fixes.

Enter the data points

Now you can whip out the notebook with the GPS points you wrote down during the survey in your dinghy and begin plotting. After you have them all on paper, connect them to create an approximate outline of a bay or an island. Next, plot the GPS fixes of the soundings you read off your portable depth sounder, and you end up with a pretty decent chart of the place. Satellite navigation allows us to make such a chart in record time without losing any accuracy.

Our Panamanian cruises evolved into producing a cruising guide, and charting the channel into Puerto Perme revealed an error made by a British Admiralty cartographer, the orientation of the chart was 90� in error. He was working with an old Spanish survey and somehow failed to notice that his source was drawn with the east at the top of the chart, as many nautical charts were in previous centuries. This huge mistake is an exception, though. Other old charts for the isthmus show admirable veracity of shape, soundings and distances. When we sailed through Laguna de Chiriqui and Bahia Almirante on the Caribbean coast of Panama, with charts made by U.S. Navy surveyors in the late 1920s, we admired the accuracy of detail presented for the entire area, both for deep trenches and the shallow reef waters, as well as prominent land features. The 1930 price stamped on them read 15 cents! The recent DMA charts for the same area (costing more than 100 times more) show the deep channels leading to large ship terminals, while the surrounding waters are blank &mdash no soundings at all. The new charts offer one improvement &mdash the latitude and longitude are plotted correctly, while our old favorites are often off by a half mile, especially in longitude.

Hurricane changes

Although the charts of domestic U.S. waters represent the best of marine cartography, they lag in updating serious topographical changes. So you may find your charting skills very useful even in domestic waters &mdash a hurricane or a winter storm may erase whole islands and reshape channels and shallow banks. During just one of our years in Georgia waters, we witnessed how a hurricane swell wiped out Pelican Spit, a small barrier island with dunes and stands of sea oats, which served as a breeding ground for shore birds at the mouth of Hampton River flowing between Little St. Simons and St. Simons islands. It will be years before the charts show this breach in the coastline, so we decided to survey and apply the changes to the chartlet included in our guide to coastal Georgia.

Only a little later, still in Georgia, we were navigating on the chart showing Delegal Creek on Skidaway Island and expected to sail through Steamboat Cut, keeping a charted marsh island to starboard. Instead, on our starboard hand, we got an unobstructed view of muddy waters chopped by strong tidal flow. As an important destination with complete shelter, Delegal Creek merited its own updated chart.

Several private markers along the edge of the shallows leading to the marina located in deep water up Delegal Creek simplified the survey work. However, strong currents and the proximity of a wide sound open toward the ocean made for a wet job in a small jumping boat. We wrote down the GPS positions in the centerline of the recommended approach channel, then found the GPS fixes of the markers and finally made several passes outside the channel and over the area of the erstwhile island &mdash it was more than a foot deep, even at the lowest spring tides. We did not need to draw our own Mercator grid to plot the updated data for this part of the coast. We simply copied the grid from the existing NOAA chart, scaled at 1:40,000 with a minute of longitude 1 1/2 inches long &mdash plenty for adequate plotting.

The ability to create charts will serve a navigator well in many popular cruising waters. In Georgia, the low coastline of sandy banks and barrier islands exposed to the Atlantic may change dramatically: Islands vanish, and rivers cut new courses for themselves, as we found out on Wolf Island. Meanwhile, your own charts of the waters of your immediate cruising area will keep ahead of the delayed updates of the official aids to navigation. You will need to improve on existing information even in cruising areas covered by many cruising guides, such as the Bahamas. Over there, you must rely on piloting by eye with the sun high behind you, but only your own chart with self-plotted GPS positions will let you run for shelter in overcast conditions.

Tom and Nancy Zydler live aboard their 1961 Pearson Invicta Mollymawk and are the authors of The Georgia Coast: Waterways and Islands, published by Seaworthy Publications.

By Ocean Navigator