A raster chart can be thought of as a scan of an equivalent paper chart. It works within the same boundaries and has a similar serial number. Most raster charts are based on those issued by the national authority. This is the UKHO in Britain, and the charts are known colloquially as Admiralty charts.

Raster charts have the benefit of looking just the same as a user-friendly paper chart. All their data are visible while they are up on the screen. This means that unlike their vector equivalents, the operator doesn’t have to work at a series of buttons or touch-screen systems to reveal everything they have to tell him. As you zoom in and out on a given raster chart, nothing changes except the size of the image. What you see is what you get. There are no surprises. To find more information beyond the scale of the chart on the screen, which may be a passage chart, is closed and a new one with more detail is opened. The process continues right down to harbour-chart scale.

At all scales, any buoys, lights or fixed navigation aids are shown complete with their characteristics, as they are on a paper chart. No 'interrogation' is required. Also, because raster charts and their paper equivalent are prepared by professional 'human' cartographers, any serious dangers are shown clearly and cannot be missed by being 'zoomed out' as they can be with the vector charts found in most commercially available plotters.

Unlike vector charts, rasters are not naturally sewn together to cover the whole part of the planet you have paid for. They are individual units. Using them, it is often necessary to switch from one to the next. AngelNav makes this easy by offering the choice of a larger or smaller scale, together with a name and a thumbnail. Switching takes the length of time needed to tap an iPad. It's like having a drawer full of charts with the big difference for most of us that you have every one and they are all up to date.