Vector left and Raster right

Vector left and Raster right


In many respects, navigation has been the black art of seamanship.  The Coast Guard operators of Fir were conversant in navigation by the stars and dead reckoning.  In her last refit she had two tripods on the wing decks of the Wheelhouse to support large binoculars. These tripods linked to a Sperry gyrocompass such that the bearings taken by the binoculars were very accurate. She navigated predominately by dead reckoning.

Today, GNSS can place the location of a boat within several feet, and paper charts have been converted to a digital format.  Even the most basic equipment can locate a boat on an electronic chart.   The more advanced systems combine GNSS, electronic charts, radar, and AIS to give crew precise location relative to the globe and other objects.  However, it is important to understand the limitations.  First, all charts are dated to some extent, particularly sounding data.  Second, electronic charts are generally based on printed charts created by a government department charged with this duty.  For example, the coastal waters of the United States are charted by NOAA. Since the Cuban government does not produce charts, there is no reliable  information for this area.    

There are essentially two types of electronic charts: vector and raster charts.  Vector charts store chart data as a series of mathematical points. They can zoom in with accuracy and store data in layers so that you can display all or part of the chart information  Private companies may also add data to the government chart. 

Raster charts are essentially electronic copies of a government chart.  Raster files are more limited because they cannot be parsed or manipulated, similar to a PDF version of a Word document.  Labels on the chart cannot be decreased or eliminated, and zooming in eventually creates a grainy image.  The resolution of a raster chart is limited to the initial resolution of the printed source chart.

As a general rule, multiple charts should be consulted when building a course.  This coincides with our decision to have redundant systems.  Chartplotters are generally driven by vector charts, while navigation software can display both raster and vector charts.  

Charts are not a real time navigation system because the data illustrates history. We will use radar to identify the current location of land masses. Fir draws 11.5 feet which makes her a tight fit in much of the world.  With the environment constantly changing, this is a serious concern.  Monitoring our depth gauge can ensure our stay in deep water.  When it becomes necessary to enter shallow water, the sonar on our tender can create soundings that can be fed into our main charting systems.