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Chart Formats

Raster

A raster image is a regular grid of coloured dots called pixels (short for Picture Elements). Most computer images are like this. A raster chart is a raster image and other information about the image, as a minimum this extra information must relate the grid of the image to its position on the world. This is essential for navigation systems as now the position of the ship (using GPS) can be overlayed onto the chart (and much more).

Vector

Vector chart information consists of series of geographic features. Broadly these describe Points, Lines and Areas. For example a buoy would be a point feature - this feature contains geographic position and details allowing the buoy to be drawn correctly.

Some potential advantages of supplying information in this way are:

• Can be zoomed to arbitrary scale.
• Can choose which features to display - which can make a view less cluttered.
• Searchable - you could search through some vector data and list all the Light features in it, for example.
• Safety features - because features like depth contours can be understood by the display system extra safety features can be included, like an audible warning if the ship is heading into shallow waters and liable to ground.

And some problems are:

• When zoomed to a scale more detailed than that at which the data was produced the viewer could be fooled into thinking the information was more detailed than it really is. This could be unsafe. The ECDIS standard requires a warning be displayed in this case.
• The computer must decide for any given view of vector information how to label the information (where to put the labels). The labeling of a raster chart will have been done by a skilled human being and at present humans do a much better job. The result of this is that typically the density of information at any scale is higher on a raster chart. At CherSoft we find that ENC vector charts are normally more readable at twice the compilation detail.
• Just because you can turn the display of features off does not mean they are not there. An uncluttered display is little consolation when the craft collides with something.

The greatest problem with vector data for Navigation is that very little data exists.

Updates

Some chart formats support Updates, when a chart is updated it is not necessary to get an entire new chart only the changed portions need be sent. For raster charts this involves supply a number of small 'patches' to replace the changed portions. Vector charts are supplied as a number of 'features' and only the changed features need be supplied in an update. This can work well as localised chart details, for example the placement of a buoy, frequently change. The aim is to make the updates take up much less disk space than a complete chart or vector chart cell. The advantage of updates being small is that many can be collected on a CD and the time taken to download them is more likely to be acceptable over an expensive satellite link

ARCS

The United Kingdom Hydrographic Office has been providing global electronic chart coverage of the world's shipping routes commercially since 1996.

• Raster format
• ARCS charts are digitised versions of UKHO paper charts and are as such of extremely high quality.
• Support updating
• Supplied on CDs as area updates
• Almost all UKHO paper charts are also supplied digitally giving almost worldwide coverage. ARCS charts cannot be made for Polar regions for technical reasons. The other paper charts for which ARCS charts are not available is for countries which do not permit the UKHO to supply digital charts under copyright law, these countries are Canada, Japan and Brazil
• CherSoft was one of the first companies to develop a viewer for ARCS charts for a PC

NOAA/BSB/Maptech

A format used for American NOAA and Canadian produced charts. BSB Electronic Charts was a company that held the exclusive license from the U.S. Department of Commerce to develop and license electronic versions of NOAA nautical charts. Maptech now holds this license and refers to the BSB charts as the Chart Kit format.

• Raster format
• Latest version (version 3) supports updating
• CherSoft can support display of version 3 charts and updating support is to be added

ENC

ENC is a vector format based on S57. Sometimes the two terms are used to mean the same thing. ENC is a particular use of the S57 standard, an S57 product, there are other S57 products like AML. ENC is designed for (commercial) Marine Navigation.

The availability of commercial data suitable for navigation is limited and improving slowly. A Navigation System only able to show ENC data would be useless. Typically navigation systems also support ARCS at least.

CherSoft supports ENC charts.

VPF

VPF is a standard format, structure, and organization for large geographic databases that are based on a georelational data model. VPF is primarily used for organizing and encapsulating such digital geographic databases for transmission. VPF is used as a standard way for two chart creating organisations to exchange data. CherSoft have developed VPF viewing software for demonstration.

Proprietary Vector Formats

Some Electronic Navigation System suppliers produced their own proprietary vector formats. A great problem with this is that the data is not (directly) supplied by one of the world's Hydrographic Offices. Many people consider this is to render the information less trustworthy and certainly the chart information will not be backed by the guarantees provided by an 'Official' data provider.

Other Raster Formats

There are a number of other raster formats and ways of geo-referencing raster information. Examples are CRP, ASRP, GEOTIFF and some proprietary formats used in GIS applications. Some of these formats are supported by the CherSoft product ChartGen which converts them into the required format for display using CherSoft's navigation software (Henry).