Raster and vector electronic files are made up in different ways to represent and image. In its most basic form, raster images are made up of a number of pixels, all different colours. At the appropriate density or scale these pixels make up an image.
Vector files are made up of paths made up by mathematical formulas. Unlike raster files, vector files are infinitely scalable, are relatively small files but do not blend colours well, without rasterizing. Raster files are usually larger, dependent on their ppi (the number of dots in a given area).
The advantages of vector charts includes:
- Less electronic storage room is required
- Zooming in and out makes movement between small- and large-scale charts unnoticeable and seamless.
- Vector charts have a "clean" look because they present less information on any one screen.
- Programs and apps using vector charts often build in additional features or layers including information about marinas, social features, etc.
Vector charts though generally present less information upfront and some training is probably advisable so that one can properly understand how to zoom in and out for more information.
A raster chart is effectively an electronic picture of a paper chart obtained through a detailed scanning process and therefore you see exactly what is represented on a paper chart.
The advantages of raster charts includes:
- All paper chart information is retained, including every depth sounding, buoy number and description, and land features, etc.
- It is easy to work back and forth between the wide view of large paper charts and the small section seen on an electronic screen.
- There is no learning curve for mariners already familiar with paper charts.
Unfortunately, raster charts require a lot more electronic storage space although, as memory sticks and cards become cheaper this becomes less of an issue. Also, the type size (font size) is fixed, so that letters and numbers become large when you zoom in and small when you zoom out, becoming unreadable at either extreme.
Famously, Team Vestas, a competitor in the Volvo Ocean Race hit the well-charted reef Cargados Carajos, first surveyed by Captain Sir Edward Belcher CB of HMS Samarang in 1846. So how could this happen?
Well, if one relies on vector charts (layered with more and more information at different zooms) and then you do not check on the appropriate zoom or correct scale paper chart (or raster chart) then you might well believe that there is no reef to avoid when in reality this is far from the truth!
Passage planning should allow for reefs and rocks by choosing the appropriate scales and when actually underway regular checks at maximum zoom would be prudent. Of course, a comprehensive paper chart folio solves these issues - but this can be an expensive endeavour.