CHARTS and CHARTING

[GordMay]

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

[captjohn360]

ok gord explain the difference between rastor, vector, and pilot charts ...jt

[GordMay]

I have little personal practical experience using Electronic Charts - so please excuise (but correct) any iinaccuracies.

Electronic Charts:
There are two different technologies used in electronic charting.

Raster charts:
A raster chart is an exact replica - an actual picture - of a paper chart. To create a raster chart, the mylar films are “scanned.” Raster charts look exactly like a photograph of a paper chart, and the data cannot be manipulated.
Raster charts are produced by Maptech and SoftChart in the United States, NDI in Canada, British Admiralty in England, and by official hydrographic offices in a number of other countries, and are distributed on CD-ROM.

Vector Charts:
Vector charts are distillations of paper charts, but their presentation is more innovative (selective display, always sharp, always "heads up"). Vector charts are akin to CAD drawings, with all the data is described in digital form (for example a buoy will be represented by a data item describing position, color , type, lights, top mark etc). Vector displays allow you to view all or select "layers" of information, to reduce clutter or add detail to your chart. And the depth soundings and other information are always very readable - no matter the scale or rotation of your chart. The advantage here is that it is easy to change data with updates, the charts take less space, and you can turn off unwanted data.

In the past vector charts were displayed on dedicated plotters, but today vector charts can be used with both computer software and plotters. The most popular vector charts (for plotters) are made by C-Map and Navionics, and they come on proprietary cartridges. Vector charts are also now available for computer-based electronic charting systems. Nobeltec, Transas Marine and MaxSea are all programs that use vector charts

Generally, raster and vector charts that run on computers provide much more coverage for the dollar than do charts for dedicated plotters. However, chart plotters can be less expensive (and if so, less capable) than the laptop computers used for electronic charting. The total cost includes the computer or plotter, the software, and the electronic charts, and any installation costs.

Because vector charts occupy a fraction of the disk space of raster charts, they re-draw on screen very rapidly, even on a modest computer, and it's also possible to supply them on floppy disks or over Inmarsat C or the Internet.

More ...
Raster vs. Vector Electronic Charts - Canadian Hydrographic Service:
http://www.charts.gc.ca/pub/en/products/raster.asp

Electronic Charting 101 - by Rob Handley (Bluewater Charts)
http://www.bluewaterweb.com/Electron...Echarts101.asp

HTH,
Gord May

[GordMay]

Pilot Charts:
The Pilot chart documents prevailing weather patterns (by month) for the North Atlantic, South Atlantic, North Pacific, South Pacific, or Indian Oceans, including:
~ wind (direction & speed)
~ currents
~ wave heights
~ visibility
~ surface pressure
~ sea surface temperature
~ and ice caps
~ shipping routes and distances
A pilot chart contains graphic representations of averages obtained from data gathered over many years in meteorology and oceanography to aid the navigator in selecting the quickest and safest routes. Each atlas contains a chart for each month of the year. Explanations of how to use each type of information depicted are included.

NGA Digital Navigation Publications - Atlas of Pilot Charts:
http://164.214.12.145/pubs/pubs_j_apc_list.html

HTH,
Gord May

please explain the difference between a rhumb line and a great circle, and wjhen i would use them.
thanks

[GordMay]

A Great Circle is the path traced by the intersection of a plane passing through the center of a sphere and the surface of the sphere, and is represented by part of a sign wave (*1) on a Mercator chart. The great circle track appears as a straight line on Gnomonic (great circle) charts. It is the shortest distance between two points on a sphere (*2).

A Rhumb Line is the path traced on the surface of the Earth by a vessel that maintains a constant track (crossing all meridians at the same angle), and is represented by a straight line on a Mercator chart. It is the longer distance of the two paths discussed between two points on the Earth (*2).

Generally; for longer voyages you’d use the shorter Great Circle route with its continuous change of compass bearing, and for shorter voyages use the the longer but simpler Rhumb-Line with its constant compass bearing. I've never sailed a true Great Circle.

In common practice, the Great Circle route is a series of Rhumb-Lines (Chords) between waypoints laid out on a mercator chart, which is why one needs to make frequent course changes to follow the course.

(*1) Provided it does not lie along either a meridian or the equator. The equator is a great circle, and all lines of longitude are great circles.

(*2) Unless the two points are both on the same meridian. If the points are nearby, say within 50 miles of each other, the difference between the two paths is inconsequential.

Most (all ?) GPS units use great circle calculations for distance and desired track. If you create a route with only two points (start & finish waypoint), the map (if available) will show a great circle route, and the initial course to steer (CYS) will be the initial great circle course. Following the GPS (CTS) to the finish waypoint will require periodic heading changes (about every degree), resulting in a Great Circle voyage. Most GPS programs will create a Great Circle route, divided into the desired number of way points, by entering departure and destination only.

Of course, neither a Rhumb-Line nor a Great Circle plot can stand uncorrected.
Set & Drift (leeway) are caused by tidal currents, windage, and helmsman error. The Course to Steer, from one waypoint to another, must be corrected to account for set (direction) & drift (speed).
Magnetic Variation changes over longer distances, so you must redo your TVMDC corrections occasionally (at each waypoint).
C = T ± V (+ West and - East)
T = true north; V = variation in degrees between true north and magnetic north; M = the magnetic compass reading after adjustment is made between true north and magnetic north; D = deviation of the magnetic compass as it may be affected by the magnetic forces on the boat such as electronic devices and/or metal parts located near the magnetic compass, and C = Course to Steer.

For more information Goto:

“The American Practical Navigator ” ~ by Nathaniel Bowditch
The American Practical Navigator or "Bowditch", as it is commonly known, is the most comprehensive manual ever compiled on the art and science of navigation. This version of the Bowditch is in PDF format in individual chapters.
http://pollux.nss.nima.mil/pubs/pubs...s.html?rid=187
Specifically:
Chapter 24 - The Sailings
Chapter 21 - Navigational Mathematics

See also: “How to get from here to there?” ~ by Tony Phillips http://www.math.sunysb.edu/~tony/wha...vigation1.html

On-Line Calculator:
http://www.stormy.ca/technology/grea...avascript.html

HTH,
Gord May

and still, I keep bumping into islands ...