CHART SCALES:
The Navigational Chart is one of the most fundamental tools available to the mariner.
Charts are available in various formats and scales as described below. A Nautical Chart is a graphic portrayal of the
marine environment showing the nature and form of the coast, the general configuration of the sea bottom including
water depths, locations of dangers to
navigation, locations and characteristics of man-made aids to
navigation and other features useful to the mariner. The Nautical Chart is essential for safe navigation. In conjunction with supplemental navigational aids, it is used by the mariner to lay out courses and navigate ships by the shortest and most economically safe
route.
The term "scale" refers to the relationship between distance on the map and distance on the ground. Generally, it is given as a fraction or ratio, such as 1:100,000 or 1:10,000. The first number represents the map distance, and is always 1. The second number represents the ground distance and is different for each scale.
For example, a scale of 1:10,000 means that one inch on a map corresponds to 10,000 inches on the ground (or one centimeter on the map corresponds to 10,000 centimeters on the ground). On a 1:100,000-scale map, one inch on a map corresponds to 100,000 inches on the ground.
The scale of a map is also related to its detail. A 1:100,000-scale map is usually much less detailed than a 1:10,000-scale map. This is because a one-inch-by-one-inch map square can only hold so much detail. Using a square inch of paper to depict a 100,000-square-inch ground area makes it necessary to omit some details of the terrain. Because of the size (on paper) of terrain features, such maps are known as "small-scale" maps. Generally speaking, small-scale maps cover a large area of land, at low detail.
When depicting a 10,000-square-inch ground area, the cartographer can "fit" more details in the square inch of paper. Such maps are called "large-scale" maps: they cover smaller land area, but at greater detail.
* The smaller the second scale number (e.g. :10,000), the "larger” the map scale, and the more detail shown.
* The larger the number (e.g. :100,000), the “smaller” the scale, and the less detail shown.
There are five distinct "classifications" of conventional nautical
charts produced by NOAA. You're likely to have a mix of these paper charts aboard your vessel, depending upon the type of
boating you do. The differences between these classifications primarily depend on scale and the level of detail depicted.
Harbour Charts:
The most detailed of these charts are scaled between 1:50,000 or larger and are classified as Harbor charts, which cover from 6 to 601 nautical square miles. A digital image of a harbor chart is what you would be seeing on your chart plotter when zoomed-in to the last level while entering or departing a port. They're also useful for locating a protected anchorage.
Coastal Charts:
Coastal charts are what you would likely find aboard most vessels as paper charts, as they usually cover the cruising area of the average boat. The scale of coastal charts is 1:50,000 to 1:150,000 with anywhere from 866 to 5,413 nautical square miles being represented. Coastal charts are primarily intended for near-shore navigation.
General Charts:
General charts have scales from 1:150,000 to 1:600,000, and cover anywhere from 9,623 to 86,605 nautical square miles. These charts are useful when navigating a coast, when your course is fixed well
offshore.
Sailing Charts:
Sailing charts have a scale of anywhere from 1:1,600,000 to 1:3,499,000. They can run thousands of miles
offshore and are intended for planning voyages or for fixing a position when approaching a coast from the open ocean. Sailing charts cover anywhere from 117,879 to 962,274 nautical square miles. Large ships and circumnavigators have these paper charts aboard and often utilize this scale on their navigation systems.
International Charts:
Lastly, NOAA produces what are known as International charts, that run in scale between 1:3,500,000 to 1:10,000,000. They cover somewhere between 2,946.965 to 24,056,854 nautical square miles and are used to create the generalized background maps you see when first starting your chart plotter or zooming out to the very last level. Having a paper copy of one of these aboard your vessel is most likely unnecessary.
Smaller chart boundaries, usually located around busy
ports, represent large-scale charts that cover small geographic areas in great detail. Conversely, the larger outlines, extending up the coast and offshore, are small-scale charts that cover large areas with much less detail.
Latitude Scale
No matter where you are on the planet, one degree of latitude equals 60 nautical miles. Each degree is divided up into 60 minutes, and each minute is further carved into either 60 seconds or decimal minutes. You can use this catchy phrase to help you glean distances from the edge of your chart: "A minute's a mile the world around."
Be careful: The same isn't true of a minute of longitude, which varies in distance depending on how close you are to the equator. The prime meridian, which passes through Greenwich, England, is the line of longitude (or meridian) that has been arbitrarily given the value of 0 degrees.
On-line References:
NGA - ATLAS of PILOT CHARTS:
The Pilot Chart contains meteorological and oceanographic information to aid the navigator in selecting the quickest and safest maritime
route. It is not intended to be used alone, but in conjunction with other navigation aides. Each chart focuses on a major ocean area which presents in graphic form averages obtained from winds, currents, ice, and other
marine data gathered over many years. The Atlas of the Pilot Charts is divided into monthly charts.
http://164.214.12.145/pubs/pubs_j_apc_list.html
NOAA's Electronic Navigational Chart (ENC) is a vector- based digital file containing marine features suitable for marine navigation. It is based on the International Hydrographic Organization (IHO) S- 57 standard.
http://nauticalcharts.noaa.gov/mcd/enc/index.htm
NOAA's Coast Pilot® consists of a series of nautical
books that supplements the nautical charts. They include a variety of information essential to marine navigation.
http://nauticalcharts.noaa.gov/nsd/coastpilot.htm
Water Level Tidal Predictions:
http://www.co-ops.nos.noaa.gov/tide_pred.html
Tidal
Current Predictions:
http://www.co-ops.nos.noaa.gov/currents04/currpred.html
HTH,
Gord May
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