With e-charts becoming more and more popular with the affordability of apps such as Navionics, its worth being sure of what you're teaching your students, so here's a little reminder of the difference between Raster and Vector charts and why it matters what's what!
Vector charts, also called ENC charts, are a graphic format in which charts are presented in a more schematic manner. The screen presents less information about land and other features, and water depths are presented more by color layers than by soundings. As you zoom in, the information changes, however - it doesn't just get larger as on a raster chart zoom. You will see more depth soundings, for example, but the type used will always be the same. (If a number is hard to see on a small smartphone screen, it won't get any larger when you zoom in.)
The advantages of vector charts include:
- Less electronic storage room is required; most apps and programs using vector charts include all U.S. waters, so you don't have to get more charts when you go to a new area. (Some chartplotter companies using proprietary versions of vector charts may still charge you to get more regions.)
- 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.
The disadvantages of vector charts include:
- Less information is presented, depending on whose vector version you are using. As in the chart shown, land features and names are seldom included - this can be disorienting to mariners used to paper charts and road maps.
- Some specific information (such as descriptive buoy data) does not appear on the screen unless you select it or move a cursor to it - compared to always being there on a raster chart.
- It can be confusing to look back and forth between the wide view of large paper charts and the small section seen on an electronic screen.
- A learning curve is needed to use a vector chart accurately at the appropriate levels of zoom.
A raster chart is essentially an electronic picture of the familiar paper chart, obtained through an accurate, detailed scanning process. Raster charts therefore have exactly the same information as the paper chart. Depending on the software or app, the raster chart may even have the same NOAA chart number. Virtually all navigation programs join separate charts together, however, in a seamless, "quilted" electronic version, and in many programs zooming eventually takes you into a more detailed chart for that area.
The advantages of raster charts include:
- All paper chart information is retained, including every depth sounding, buoy number and description, and land features such as town names and streets bordering the water.
- Because raster charts perfectly match the paper charts, it is easy when underway 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.
The disadvantages of raster charts include:
- Charts require significantly more storage room (than vector charts) on electronic devices. The storage for all raster charts for a single region may exceed the space required by vector charts for the entire country.
- 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 less readable at both extremes.
- Zooming in too far to see your boat's position on a small scale chart may give a misleading impression - when a larger scale chart would be more accurate (vector charts automatically change scale).