Marine Computers and Electronic Charts

Many 'medium-tech' yachtsmen are using general purpose laptop computers to run chart programs, but within a few years, dedicated navigation computers of the type developed by Diverse Marine of Hamble will become the norm. Amongst other talents, these compact, powerful tools can be programmed to carry all the tidal information that appertains to the charts they run. Thus, a tidal stream atlas for any hour of any day can be overlaid onto the active chart. Set and drift can be shown at the punch of a button and a course shaped to suit your ship speed. Even weather maps can be superimposed onto ocean charts. The met data is processed by the computer to give the best course to steer for speed or comfort, while 'what if' scenarios allow at least a modicum of intuition to be applied to this powerful flow of information. The Two Types of Electronic Charts - "Raster" and "Vector"  RASTER CHARTS Raster Charts are basically Electronic Photographs of the original official paper charts which are stored in a computer readable form. The image is built up of a large grid of tiny coloured dots (pixels), normally at resolutions of 100 by 100 dots per inch (or more). This is the fastest, easiest and least expensive way to create electronic charts and they are created by simply scanning the paper chart. Various reference information is then added so that latitude and longitude positions can be calculated. The resulting raster charts can then be reproduced on a computer screen with the appropriate navigation software. Because raster charts are basically a photograph of the paper chart all the information is shown on the computer screen exactly as it is on the paper, i.e. all the symbology is the same, the text is the same, the white border of the chart is present, etc. This can be an advantage (the electronic chart is immediately familiar and understandable to the user) or a disadvantage (the chart can not be easily manipulated) depending on the users requirements and needs. Zooming and panning is also generally limited with a Raster Chart since when zooming in for greater detail, the photographic image is simply magnified and no more detail can or will appear. The advantages of raster charts are as follows: Faithfulness of reproduction. The electronic chart looks just like the familiar paper chart  They are generally cheaper than the Vector charts  The coverage is generally much wider than the coverage of Vector charts  The disadvantages are as follows: No information can be selectively removed or displayed at different zoom levels  Generally raster charts are not easy to update Raster charts are generally slower to display on the screen.  Raster chart systems are generally more memory intensive and can take many megabytes to store the image of a single nautical chart. They are generally distributed on CD-ROM and therefore Raster chart systems normally require a CD-ROM drive.  VECTOR CHARTS  Vector charts are created by scanning the paper charts to create raster files (as above) and then vectorising the data. Vectorisation converts the lines on the paper charts into coordinates that are then stored. These coordinates are then used by the Navigation systems to re-create the lines of the chart on the screen. These lines that are vectorised from the chart are stored in groups (layers) according to the type of line being vectorised. For example all the coastline is stored in one layer the drying line would be stored in another layer etc. Once stored the layers of coordinates can be compressed so that the resultant vector chart is stored very efficiently. A Vector chart can take less than 1/10th of the space of a Raster Chart to store the same amount of information The advantages of vector charts are as follows:   Smaller memory requirements and therefore the charts generally display faster and can be distributed in memory cartridges and on floppy disk rather than CD-ROM's  The displaying of Vector charts is very flexible since display different layers of information can be shown at different times. As you zoom in, increasing levels of detail can be available without any sacrifice in image resolution. Chart presentation can be changed to remove layers of information that may not be required for the task that is currently being executed.  Chart plotter manufacturers can build low-cost, water resistant, dedicated chart plotter units  The major hydrographic offices of the world, including the largest map maker in the world, the U.S. Defense Mapping Agency (DMA), have already announced that their electronic charts will be in vector format. The advantages and benefits of using vector charts are almost limitless and we can expect to see map makers of all types moving to the vector electronic chart in the years ahead. The world's most advanced hydrographic institutes will use seamless vector chart technology when they build electronic charts in the future and the International Hydrographic Office standards are for seamless vector cartography. Glossary ECDIS Electronic Chart Display and Information System  IMO International Maritime Organization  IHO International Hydrographic Organisation  ENC Electronic Navigational Chart  SENC System Electronic Navigational Chart  S57 IHO Transfer Standard for Digital Hydrographic Data  S52 Specifications for Chart Content and Display of ECDIS  DX90 Data coding description