Information on chart formats
What is the difference between raster & vector?
A raster chart is a facsimile reproduction of a paper chart, with the same appearance as the source paper
chart. There is a difference between official raster chart production, and unofficial raster chart
production.
Official raster charts such as ARCS and Seafarer originate from government Hydrographic Offices direct
from the printing masters used to create paper charts. They are therefore identical to the paper
charts. These formats are maintained up to date continuously and all have an official correction service.
Unofficial raster charts such as MapTech are created by scanning official paper charts to create their
own raster images of them. Correction services are variable - you need to check with the manufacturer
how frequently their charts are corrected and whether there is a correction service.
Vector charts consist of a database containing details of the objects on the source paper
charts. Their appearance is different to paper charts and the chart data can be selected
on or off in layers.
Official vector charts, known internationally as ENCs, are created from the source hydrographic data
collected by hydrographic offices. Currently there are limited numbers of ENCs available and their
application is mainly to the shipping industry.
Unofficial vector charts, such as C-Map CM93/3, are created by scanning paper charts to create a raster
image, and then vectorising so that the data is stored in layers. This means that the data is very
compact and can be stored on just one CD. Correction services vary between manufacturers.
What charts should I buy?
It's really a matter of personal preference whether you like the look and feel of a raster chart or a
vector chart. Depending upon which software program you buy, both chart types can be zoomed in and out
and automatically changed once you are underway. It's worth pointing out that zooming doesn't produce
any more information than was on the original paper chart. With vector charts, it may look as if there's
more information, but in fact you have zoomed in to a larger scale chart. As always, you need to make sure
you've bought charts of scales appropriate to your route.
Of importance for navigation is the question of corrections. Are the charts you intend to buy kept
up-to-date by the manufacturer, and can you buy updates for them later on?
We will gladly discuss chart formats with you in more detail - just give us a call: +44(0)1752 254205.
