How are they made?

The first stage in preparing a traditional paper chart for use with computers is to run it through a scanning machine. The scanner rasterizes the chart, i.e. it makes a computer file which contains the visual information from the chart stored as a series of dots of varying color and density. This raster image is much like a photograph in a newspaper, the quality of the image depends upon the resolution that the chart was scanned at. To preserve all the detail of the original chart requires scanning at a very high resolution. This has two disadvantages – firstly the files are large and require a powerful computer for them to display quickly on screen; and secondly, like a photograph in a newspaper, the moment one zooms in to look closely at an area of the image, one sees that the image is indeed just a series of dots – and the closer one zooms in, the bigger the dots get!

To create vector charts, things are taken one stage further. Digitizing technicians accurately trace the outlines and exact positions of all the features on the scanned raster chart into a series of mathematical representations (vectors). Because the vectorization is performed by experienced humans, rather than a machine, features on the chart may be stored onto many separate ‘layers’ of information, e.g. one for spot depths, one for navigational marks, one for isolated dangers, and so on.

Because the information is stored in layers, it is possible to switch the layers on or off depending upon how much information the user wants to see at any one time. For example, if we are passage planning with a Livechart on screen, the chances are that we don’t want to see every charted feature all the way around the island – they merely clutter-up and confuse the screen. The solution is to switch them off when using the chart as an overview and to progressively switch them on again as we zoom further into the chart.

In reality, this switching on and off is done by the charting software, and an appropriate amount of information is automatically shown at any one time. Livecharts support seven levels of zoom to give a sensible amount of information for a wide range of purposes.

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, it's also possible to supply them on floppy disks or over Inmarsat C or the Internet.

Features and drawbacks of raster charts

• They look very familiar to the navigator
• They're here now
• Cost is relatively low
• Large files mean a powerful computer and fast CD-ROM is needed
• Compared to vector, they can no be interrogated for information
• Cannot be used in 3D or automatic grounding predictions etc.

Conclusion

AAllformats and types have their place, if you need 100% (or as close as can be) quality and ability to have weekly updates, then either ARCS (with navigator service) or S57 is the way to go. For the leisure market then choose Livecharts (3d capable) if you want vector data or ARCS skipper service for area not covered by livecharts.

The charts you purchase will also be influenced on the area to be covered, for example in the USA you can obtain BSB raster charts at a good price or better still S57 data by downloading for free. In Australia you are best served by the seafarer raster charts and in New Zealand by NZ mariner (all charts on a single CD).

Unlike some other system seaPro does not restrict you to one particular type or make of chart format, you can choose from a range, and even better they all work seamlessly together to give you the best coverage of the area you ar interested in.

Most importantly, if you are considering getting an electronic charting system, ask for a demonstration system which functions and which includes sample charts of the different types – don't be fooled by a "demonstration" which is simply a selection of screen-shots that the manufacturer has chosen to show you!