Friday, May 8, 2015

MLS Seamanship Series - Electronic Charts (Raster vs. Vector)

Although there are many suppliers of electronic charts, the charts themselves can be divided into two groups: raster and vector. Raster charts can be regarded as electronic photocopies of paper charts, produced by scanning a master copy of a paper chart, in much the same way as a fax machine scans a document that is about to be sent. The chart is broken down into a vast number of tiny dots (pixels), and the position and color of each pixel is recorded. Instead of sending this information down a telephone line, as a fax machine does, the chart scanner stores it on the cartographer’s computer, from where it can be copied onto flash memory cards or CD-ROMS, and supplied to customers.

Raster charts are relatively cheap and simple to make, but each chart uses a lot of memory or disc space. Because they are electronically copied straight from the paper chart, they are familiar in appearance, and contain exactly the same information: nothing is added or taken away. The drawback is that they can only be used effectively at about the same scale as the original chart: if you zoom in, then letters and symbols become huge, but without any extra detail becoming visible; while if you zoom out, names and symbols become illegible.

Vector charts are produced by electronically tracing raster charts. The fundamental difference is that lines are not stored as strings of darkened pixels, but as lines. Vector charts originally became popular for small boat hardware plotters because although they are more expensive to produce, they occupy much less memory. The vector format also allows more flexibility in the way the chart is used: a vector chart can be zoomed in or out much further than a raster chart, but the letters and symbols always stay the same size.

On a raster based chart, a feature such as a buoy is represented by a cluster of colored pixels that make up the shape of a buoy symbol exactly as it appears on the original paper chart. On a vector chart, however, the buoy’s position is linked to a database of information about the buoy. The software can use this database in various ways. Some programs will represent all navigation aids by means of the same diamond-shaped symbol. When you select one (by ‘pointing at it’ with the cursor) the data is revealed in a text panel somewhere on the screen. Other systems use the database information to display a symbol showing the shape and color of the buoy itself.

More sophisticated versions of this are used on some electronic charts to provide graphic representations of the changing height of tide at particular places, to provide additional information such as lists of port facilities, or to superimpose arrows showing the tidal stream on top of the main chart.

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At August 24, 2015 at 12:48 PM , Blogger Philip Porteus said...

Etymology happens to be one of my passions (should have studied classics rather than science), so here is some relevant bumf:

Raster - an explanation

The term Raster was originally applied to a scanning pattern of parallel lines that formed the display of an image projected onto the cathode-ray tube of a television set or display screen. (from the Latin 'rastrum', meaning a rake (from the resemblance of the parallel lines to a rake's path). As each line was scanned across the display, individual dots (or pixels) were created, which together formed the overall displayed picture.
The term Raster has been retained to refer to any graphical image which is comprised of individual dots.

This is in contrast to Vector graphics, where images are not stored in the form of completed images, but as instructions as to how each individual image element should be drawn across the display by an appropriate computer program - from the Latin 'vehere', meaning to carry or convey (in this case, to convey instructions: such as magnitude and direction).

Hope this is of some passing interest ... (especially the 'rake' bit)

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