Maps for Mariners
Charts for Marylanders
Chartmaking I
Chartmaking II
The Language of Charts

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Charting the Chesapeake:  1590 -1990

Compass Rose
Chartmaking II: Surveying the Seen and Unseen

A chart is only as good at the information upon which it is based.  One of the major challenges of chartmaking is the collection of detailed measurements and data about something that cannot be seen, the bed of a body of water. The process of obtaining this data for charts is called surveying.
Aerial Photography and Charting

The greatest advance in topographic technology was the development of photogrammetry.  This method of surveying uses photographs to provide the data upon which a map or chart is based.

In 1928 the multiple lens camera was invented permitting large area land surveying. There have been many improvements in the instruments and methods used for mapping. In 1937 a nine-lens camera was designed by Lt. Commander O.S. Reading of the U.S. Coast and Geodetic Survey and shortly thereafter photogrammetric mapping along the coasts of the United States began.

Aerial photography increased the accuracy of charts and made possible much faster revisions of data where important changes -- both natural and man-made -- had occurred. After World War II, the U.S. Coast and Geodetic Survey making extensive use of color photogrammetry. A color photograph provides more detailed information that can be interpreted as charts are produced. Photogrammetry is used in nautical charting to position landmarks and navigational aids, controls for inshore hydrographic surveys, to map the coastline and along shore features, and to maintain information on charts by monitoring the coastline for changes.

As electronic methods of navigation came into use, many of these topographic features on nautical charts of the bay were dropped in favor of greater clarity of the water areas. With present navigating aids, detailed topography is no longer of such major significance to a vessel's captain.

Surveying the Seen

Topographic surverying describes features seen on the land. In the early days of the U.S. Coast Survey, this type of surveying was an important part of the bureau's work. Detailed topography along the shorelines was included on nautical charts then because coastal piloting was the principal method of navigating in the bay.

During the second half of the nineteenth century, most of the U.S. Coast Survey charts contained complete and detailed topography for one-half to four miles inland, depicting buildings, woods, roads, cropland, docks, towers, and the like. The work required a large crew of men who observed, measured, and traversed the land. Since shoreline erosion was occurring at rates as high as eight to nine feet a year, results of these painstaking efforts were quickly out of date. The high labor cost and outdated information led the agency to discontinue such topographic surveys

The U.S. Coast Survey

The need for accurate charting of American coastal waters was evident well before the Revolutionary War. Existing charts were often inaccurate and unreliable because they were based on ad hoc surveys conducted by naval or merchantmen of areas they happened to visit.

In 1807 Congress authorized the government's first chartmaking agency, the Survey of the Coast. The agency's work was delayed by many things including the War of 1812, transfer to and from the military's control, and a simple lack of funding.  Survey operations in the Chesapeake began in the 1840s, after the agency had been renamed the U.S. Coast Survey. Under the guidance of its first superintendent, Rudolph Hassler, charts produced by the U.S. Coast Survey set a standard for accuracy by the mid-1800s.

Aerial photograph of Annapolis
Courtesy of the National Oceanic 
and Atmospheric Administration.
This photograph of Annapolis was taken with a nine-lens camera on December 4, 1942 .
 U. S. Coast and Geodetic Survey.
Aerial photograph of Cape Charles, VA
Courtesy of the National Oceanic and Atmospheric Administration.
Aerial photograph of Cape Charles, Virginia
April 27, 1962. 
U. S. Coast and Geodetic Survey.
Surveying the Unseen

Surveying below the surface of the water is called hydrographic surveying. The purpose of hydrographic surveying for navigational charting is to measure the depth of water and to locate shoals, channels, banks, and all dangers to navigation.

Early surveyors for the U.S. Coast Survey measured the depth of water with a weighted line, called a "lead line," which was marked to indicate feet or fathoms.  The weight was lowered overboard and when it hit bottom, the surveryors then read the markings on the line.

Some lead lines were also used to survey the type of bottom deposits in a given area. These lead lines had an indented lower end into which a lump of tallow was pressed. If the bottom was sandy or muddy, bits of sand or mud stuck to the tallow. If the bottom was rocky, the tallow came up clean.

Hydrographic surveying also involves measuring the tide and tidal currents. "Tide" is defined as the vertical change in the surface of the water, and "tidal current" as the horizontal motion of the water that accompanies the rising and falling of the tide. The earliest tidal observations by the U.S. Coast Survey for the bay began in Annapolis in 1844.

Sketch D. Illustrating the Progress of the Survey of the Coast in Chesapeake Bay and  Its Vicinity in 1844, U.S. Coast Survey, 1844, MSA SC 1399-1-469
Sketch D. Illustrating the Progress of the Survey of the Coast in Chesapeake Bay & Its Vicinity in 1844
U.S. Coast Survey
Washington, DC, 1844
Huntingfield Corporation Map Collection, MSA SC 1399-1-469

Triangulation to fix the relative positions of locations in the Chesapeake Bay began in 1842 with the careful construction of a base line on Kent Island. This sketch was included in the first progress report to Congress by the U.S. Coast Survey. Triangulation provides a rigid, cohesive network of positions from which charting can be accomplished.

Lead line, CMM 74.76
Lead Line
Courtesy of the Calvert Marine Museum, 74.76

A lead line is a weighted line with graduated markings used to measure water depths, or "soundings." This particular lead line was used by waterman Ruby Dixon for locating lumps of oysters.

Photograph of the Matchless
Early surveys by the U.S. Coast and Geodetic Survey were carried out in vessels like the Matchless, a schooner built in Key West, Florida, in 1859. The Matchless was used for the resurvey of the Patuxent River conducted in 1908 to revise and verify the charts. She is shown here on May 31, 1909 at the pier of the Solomons fish and guano factory on Mill Creek.
Coast Chart No. 33, Chesapeake Bay from Choptank River to Potomac River, U.S. Coast Survey, 1862 Coast Chart No. 33, Chesapeake Bay from Choptank River to Potomac River
U.S. Coast Survey
Washington, DC, 1862
Courtesy of Carl Fleischhauer

An example of the early U.S. Coast Survey charts, this chart reflects the painstaking topographic and hydrographic work carried out by the agency. Shoals are indicated with progressively lighter shading as depth increases. The topography of the shorelines is indicated and provides valuable information about land use at the time. For example, you can see that Solomons did not exist as a community in 1862.

Diagram of a boat using echo sounding.
Courtesy of the National Ocean Service, 
National Oceanic and Atmospheric Administration
Echo sounding replaced the lead line for hydrographic surveying. By using a vertical sonar pulse and measuring the time between emission of the signal and the receipt of the echo off the bottom, the depth of water can be accurately calculated.

Echo sounding was developed shortly after the end of the World War I when the underwater sound-ranging apparatus was introduced for locating the positions of submerged submarines. This technology was quickly adapted for use in marine surveys.

Small half model of a boat called the Drift.
Half model, Drift
Courtesy of the Maryland Historical Society

The Drift was a 76-foot schooner built in Baltimore in 1876. She was used as a survey vessel by the U.S. Coast and Geodetic Survey. Around the turn of the century she was also employed by the Lighthouse Board.

Photograph of the Rude, one of the National Ocean Service's vessels
Courtesy of the National Oceanic and Atmospheric Administration
The National Ocean Service now operates the Atlantic Marine Center (AMC) in Norfolk, Virginia, as the East Coast base for the NOAA Fleet. Under the command of a rear admiral in the NOAA Corps, one of the Nation's seven uniform services, the AMC provides logistical and engineering support for the vessels, and processes and compiles hydrographic and photogrammetric survey data. One present-day NOS survey ship which sometimes works in the Chesapeake is the Rude, shown here underway.
United States--East Coast, Maryland and Virginia: Chesapeake Bay, Northern Part, National Ocean Service, 1984, MSA SC 1399-1-736
United States--East Coast, Maryland and Virginia: Chesapeake Bay, Northern Part
National Ocean Service
Washington, DC, 1984
Huntingfield Corporation Map Collection, MSA SC 1399-1-736

This chart includes LORAN "C" lines, which were added to Chesapeake Bay charts beginning in August 1975. LORAN is an electronic navigation system, the name derived from the initial letters of LOng RAnge Navigation. LORAN receives use pulsed signals from two or more stations to permit the navigator to find his position with great accuracy.

Today's mariners have a variety of navigation tools available to them. Highly accurate charts provide a wealth of information, while electronic instruments reinforce charted data, help plot a course, and further ensure safety.
Image of LORAN C keypad
Courtesy of Coast Navigation.
(Long Range Navigation)

LORAN C indicates the exact location of a vessel in terms of latitude and longitude. This allows the navigator to plot his location on a chart and find his way back to harbor or some other designation.

LORAN C also helps plot a course. By entering a desired location (WAYPOINT) into LORAN C's memory, it can show what direction the waypoint is relative to location (BEARING), the distance to the waypoint (RANGE), how far off course the skipper is steering (CROSS TRACK ERROR), how fast the vessel is traveling and in what direction (SPEED AND COURSE OVER GROUND). By following from one waypoint to another, a skipper can easily navigate where he or she could not rely on other navigation methods. Most LORAN C sets will store up to 100 waypoints.

Image of VHF radio
Courtesy of Coast Navigation.
VHF Radio
(Very High Frequency Radio)

VHR radios have a number of channels which are used by mariners to talk to each other, listen to weather reports, receive navigation information, send distress calls, and for other communication purposes.

Image of a modern compass
Courtesy of Coast Navigation.

The compass is the most critical of all navigational tools. It issues a constant report on the boat's heading relative to the direction of magnetic north. A compass can also be used as a sighting instrument to determine the directional relationship of the boat to some mark on the shore or on the water.

Image of knotmeter
Courtesy of Coast Navigation.

The knotmeter measures the boat's speed, important information for plotting a course.

Image of depth sounder
Courtesy of Coast Navigation.
Depth Sounder

The depth sounder indicates the depth of water beneath the vessel's hull. Some depth sounders are equipped with an alarm which alerts the skipper that he or she is in danger of running aground.

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