Chartmaking in Chile

Up and down every coast the bays, harbors, and coves that are well-charted are usually well-crowded. The mariners that seek out the gunk-holes, secret fishing spots, and hidden havens usually have to make do with small scale/large area charts that give little detailed information and may be way out of date.

However, modern electronic navigation instruments, along with the traditional tools found aboard, make creating reasonably accurate charts of a small area an easy and interesting project. Chartmaking is an exercise that can use many handsall that are available, in fact. The finished product can be a simple black-and-white line drawing showing just the shorelines, soundings, and prominent features, or it can be an elaborate, multicolored work complete with scanned-in or photocopied drawings and photographs.

Why go to the effort of making a chartlet when after a few visits it is easy enough to get in and out of a place with just local knowledge?

There are several answers to this question: 1) Entrances at night or in fog are surely safer when a large-scale chart is available. 2) If the place has promise as a “hurricane hole,” an intimate knowledge of the bottom and shoreline is essential to make a safe mooring, particularly when jockeying for position with other shelter-seeking vessels. 3) Research, sport, or commercial projects require more than the cursory information available on coastal charts.

The main reason to make your chart, though, is that the project gives everyone involved experience with the new, and time-honored, tools of the navigator.

How to begin

Pick a body of water not more than a mile across (one-half mile is about the limit of easy visual signaling between the small “survey boat” and the “mother ship”) in any direction; and that has, as much as possible, all-around visibility from the center and depths suitable for anchoring the mother ship.Ideally, on the rising tide, currents should not be too strong.

For comfortable working conditions, choose a day when low tide is finished, before afternoon breezes spring up, or the heat becomes intense. Avoid spring freshet periods or other times when unusual winds and waters make tide tables inaccurate.

Decide on the purpose for making the chartlet. In the case of our chart-making exercise, the chartlet we constructed was to determine the size of the area with at least two meters of depth and a mud or sand bottom in the estuary at the mouth of the Rio Coliumo that flows into Bahia Coliumo just north of the major Chilean city and port of Concepción/Talcahuano. The information will be useful in planning how many boats, of two meters or less, can be safely rafted up in the estuary after a regatta during the coming summer season.

Plan to anchor the mother ship (or mother ships) well before the morning low tide. If possible, position the vessel the night before. Depending on the depth, wind, and current, moor with at least two anchorsthree are preferred. Place the anchors with the idea of keeping the location of the radar antenna constant. Once the vessel or raft of vessels is in position, start to log GPS or loran positions every hour or so. For our chartmaking, only one anchor was used on our Vagabond 47 ketch Murielle, but we would have been more precise if we’d used two or three anchors.Take an azimuth of the sun or other celestial body or bearings of well-charted distant objects to determine compass error of the main steering compass. Then, standing under the radar antenna, compare a round of bearings with the handheld compass to the same bearings taken with the steering compass. Make a table of corrections, if necessary, to correct any deviation of the handbearing compass compared to the steering compass. Once the vessel is in position, the compass error of any bearing taken with the main steering compass will remain constant. This is not necessarily true with bearings taken with the handbearing compass.

Have clipboards and forms ready for those crewmembers who will be gathering data in the small boat or boats and for those remaining aboard.Choose a sounding pattern

Decide on the pattern of soundings

For our chart, the object was to locate the extent of bottom with at least two-meters depth at chart datum. Thus, the zigzag pattern of soundings was most efficient, as previous examinations had shown the center of the area to have a regular bottom. Otherwise, the traditional “Ferris wheel” pattern of soundings makes sense.

In either case, if an unknown sunken wreck, treacherous reef, or other such danger is discovered, additional soundings and measurements should be made to be sure the extent of the hazard is well known.

Depending on the number of people and extent of equipment available, make up the teams. A boat party ideally consists of: 1) coxswain or boat operator; 2) recorder and handheld compass operator; and 3) leadsman to handle the leadline and/or graduated pole. One of the members must display the portable radar reflector when an accurate sounding is taken.

The ideal motherhship requires a radar operator, a handheld compass operator (if more than one boat is working, a handheld compass operator is needed for each boat to avoid confusion), and a recorder and signalman for each handheld compass operator.

A photographer and/or video camera operator can record the event, both for use in constructing the chartlet and for future pleasure.

Set up a system of signals

If portable VHF radios or walkie-talkies are available for each boat (with battery power for two or more hours of operation) communication is easy. Otherwise flag signals will have to be set up. One signal from the mother ship is needed to indicate that a bearing and range have been taken of a sounding so the boat can proceed. Another signal is needed for a recall.

Synchronize all watches, as both the number of the sounding and the time of the sounding is needed to ensure there are no mix-ups in bearings, ranges, and soundings.

Just before low tide, take a round of compass bearings and radar ranges of the general shorelinenote in a remarks column, set aside on the clipboard, prominent features and other observations. If a sandbar or islet prevents the main shoreline from being observed, this should be noted.

Ideally, a boat party can mark the far shore of the obstruction and the hidden mainshore line by two methods. First, a party can display a portable radar reflector on a boat hook so it is visible above the obstruction by the radar aboard the ship. Sandbars, mangrove swamps, and other similar features are not good radar targets, so adjustment of gain will make the radar reflector a strong “blip” while the near shoreline’s return signal is weak. Second, one can use a handheld compass and a measured length of line. (If the obstruction is a strong radar target, the first method may not be practical.) Soundings and main shoreline features behind the island obstruction may have to be located by hand-bearing compass angles taken by a boat party.

Send off the boat parties to take soundings. When a good sounding is taken, the boat party displays the radar reflector and a radar range and bearing are taken by the mother ship. When the observations are noted on the clipboard forms, the mother ship signals the boat that they are ready to observe the next sounding. The process continues until all soundings have been taken.

If a Ferris wheel pattern of soundings is taken, signals must be set up to make sure the boat makes the turns at the right interval, such as at every 20°.

Two hours is probably the maximum duration of everyone’s accurate and enjoyable effort on the project. If the soundings are not completed in this time, they should be broken off and finished on another day.

Now it is the time to crunch the numbers and wait for the time of high tide.

Check each clipboard to make sure that the numbers and times of soundings agree with the numbers and times of the applicable range and bearing. Enter the tide tables and calculate the corrections necessary to adjust each sounding to the chart datum.

Correct each bearing for compass error and deviation between the main steering compass and the handheld compass (or compasses) used.

Review the GPS or loran positions from the mother ship taken since it was anchored. Eliminate any positions that are not close to the majority of the readings. From the majority of the positions, calculate the most reasonable position of the mother ship for the survey. This system isn’t accurate for a professional survey, but it is accurate enough for this project.

Set up a universal plotting sheet (UPS) with the mother ship’s position at the center. If the area surveyed is a mile across at the widest dimension, then each parallel will be a quarter of a mile apart. Thus, each “tick” will equal 25 feet, or 4.16667 thousands of a nautical mile.

Plot the ranges and bearings taken off the land, and note any features from the remarks column. Fill in between each plotted position to make a continuous shoreline.

Then, with the pair of clipboards pertaining to each boat party at hand, plot each sounding’s position and note the depth of the sounding on the plotting sheet.

At high tide, heights of various landmarks can be determined by vertical sextant angle. Distance off can be determined directly by radar in most instances. If necessary, the portable radar reflector can be carried to the landmark (the base of a water tower, power pylon, or tall tree, or the summit in the case of cliffs or hillocks). When the distance off is known, take a vertical sextant angle between the high tide line, or the bottom of the object, and the top of the object; correct it for index error and the dip correction in table 22 in Bowditch. Compute, via calculator with trig functions or by table 31 in Bowditch, the height of the object. In many cases, table 9 in Bowditch can be used to find the height of the objectjust use it backwards. Enter with the observed sextant angle (Ho) and the distance off, then read upwards to find the height of the object. When complicated interpolation is required, using the straight trig functions is the easier method. The height of the object is the “side opposite” of the triangle. If the range was measured from the base of the object, the distance is the “side adjacent,” so the tangent of the observed triangle is needed to find the height of the object. However, if the range was measured from the summit, then the distance is the hypotenuseso the sine of the observed angle is needed to calculate the height of the object.

Plot these features and their heights on the UPS.

At this point the project can be considered complete. Photocopies can be made of the UPS and given to all who want one. However, much more can be done with the raw data collected.

The surveyed portion of the coastal chart can be enlarged to the point that the active section of the UPS can be superimposed on it and photocopies can be made of the result. Use available corrections for adjusting GPS positions with charted positions.

Color in the plotting sheet, following the protocols of chart symbols as shown on the Chart No. 1 chart symbols or in Chapman’s.

A diorama of the area can be made using the data collected plus photographs and videos. This is an ambitious project that might take a whole winter of effort by a lot of people, but the result could be an outstanding work of art and technology.

No matter how simple or how complex the work is at the end, everybody involved will have learned new skills and sharpened old ones and enjoyed themselves. If someday a boat is saved from the ravages of a major storm that is an extra benefit of producing a detailed chartlet aboard the modern small vessel.

Knick and Lyn Pyles are voyaging along South America’s west coast. Their boat Murielle is now in Chile.

By Ocean Navigator