HOW TO GET FREE, UPDATED DIGITAL CHARTS

Nautical Charts

There is a difference between the terms "map" and "chart." A map shows land areas. It also shows the physical features of the land, cities, towns, roads, political boundaries, and other geographic information. A chart, specifically a nautical chart, shows primarily areas of navigable waters. It also shows coastlines and harbors, depths of water, aids to navigation, channels, and obstructions. A chart provides a means of describing position in terms of latitude and longitude.

 

Latitude is a measurement of Distance

1Degree of Latitude = 60 nm

1 minute of Latitude = 1 nm

1 nm = 2000 yds = 6000 ft = 1.136 statute miles

 

Longitude is a measurement of Time

15 degrees of Longitude = 1 hour

Greenwich mean Time is the Zero meridian (UTC or Zulu Time)

The international date line is 180 degrees after/before the 0 degrees meridian

180 degrees = 12 hours (1/2 way round the Earth AND ½ way through the day) The date changes at 180 degrees.  All longitude are read as either east or west of the prime meridian, example:  Moscow, Russia approx 037 degrees East, New York is approx 074 degree West.

 

 

Most of us will use nautical charts created by Mercator projection, common for inland and coastal navigation.  The nautical chart is a two dimensional image of Earth’s surface. Transferring the information from a round surface to a flat representation is called a projection.  This results in various distortions, but as long as two requirements are met we can use this image for navigational purposes. First, the angles are true representations of actual angles between the real objects, which they represent. Second, a straight course should appear as a straight line on the chart.

 

The Mercator projection fulfills these requirements with parallels & meridians, which are straight and parallel; the meridians & parallels will be perpendicular to each other.

 

 The Mercator projection is constructed by wrapping a cylinder around the planet so that it touches the equator. On this cylinder the surface of the earth is projected and finally the cylinder is cut open to yield our chart. But where the meridians converge on the globe they run parallel in the projection, indicating the distortion. Look, for example, at a high parallel. The length of such a parallel on the globe is much smaller than the equator. Yet, on the chart they have exactly the same length creating a distortion, which gets bigger nearer to the poles.  To compensate for this distortion, measurements of distance on a Mercator chart are always measured along Lines of Latitude closest to the object or area being measured.

 

 

BASIC INFORMATION SHOWN ON CHART

The chart’s legend will show the title of the chart, which describes the waters covered; type of projection used; the scale; unit of measurement used for water depths (feet, fathoms, or meters); and the datum plane for the soundings.

 

Also on the chart (where space is available) other useful information can be listed. This information could include the meanings to special abbreviations, cautions, special markings, and any other information that may be of value to the mariners. Boxes and notes may be printed in the margins or on the face of the chart at locations where it will not block out other navigational information.

 

EDITIONS AND REVISIONS

The edition number and the date of the last revision will always appear in the lower, left-hand corner of the chart along with the chart number.

 

COLOR AND LETTERING

Land areas are shown in a buff or yellowish color. On DMAHTC charts, land areas are shown in a gray tint. Shallow or shoal waters are shown in blue, and deep-water areas are shown in white. Areas that may be submerged at some tidal stage, such as sandbars, mud flats, and marshes are shown in green. The color "magenta" is used for most information listed on the chart because it is easier to read under red nightlights.

 

By knowing the type of lettering being used, you can more easily and quickly interpret the type of information being presented.

 

Upright or Roman Lettering - Identifies features that are dry at high water.

 

Slanting or Italic Lettering - Identifies submerged or floating hydrographic features (this does not include sounding figures, showing depth).

   

You should have a copy of Chart 1, handy at your Nav Station.  This is the key for all symbols and abbreviations used on the chart. 

 

Chart Corrections

In a perfect world you should check your weekly Notice to mariners and make chart corrections on your charts.  I highly recommend that you periodically review and correct you charts through out the season and definitely before entering unfamiliar waters or areas which you have not cruised recently. 

 

At the beginning of the boating season, it may be prudent to purchase POD  (Print on Demand) charts which are corrected to time of printing (when ordered), especially for high traffic and areas of multiple corrections.

 

 

CHART BOOKS & GUIDES

Chart Books and cruising guides, remember that chart books and cruising guides have not been corrected to recent Notice To Mariners.  Same may be true for digital charts!  Some digital chart providers, such as Maptech and Nobeltec, offer update service for their digital charts.  I always recommend that you print a copy of the corrections from your local notice to mariners for any charts in your operating area.  Review them each cruise.  Make corrections and notes (temporary operating area activity and restrictions, fishing notices, etc) on your paper chart as a reference along with your electronic chart plotter!  The suspenders and belt system may keep you safe and worry free.

 

 

FREE UPDATED DIGITAL NOAA CHARTS

Another item of interest – Free Digital and Vector Charts!  You may download updated digital Raster (RNC) and Vector (ENC) directly from NOAA .

Raster Charts -   http://nauticalcharts.noaa.gov/mcd/Raster/Index.htm

Vector Charts - http://www.nauticalcharts.noaa.gov/MCD/enc/index.htm   

 

As the new boating season begins, please Take a little time to review you navigation charts and publications.  Update as required, get you current Notice to mariner, tide & current tables, look over the rules of the road and enjoy a safe boating season!