Hi Eddie: Raster scan charts are simply straight scans of the chart in question–the same approach as when you scan something with your home scanner. As such, the data is all on one layer, and takes a lot of computer space.

The alternate approach is vector based charts, where the data is digitally encoded as opposed to the simple scan. The advantage here is that it is easy to change data with updates, the charts take less space, and you can turn off unwanted data.

The popular Maptech charts are the Raster type. I understand Nobeltec is now marketing the UK Admiralty chart catalog which are the vector type but we have no direct experience with these.