Raster Navigational Chart FAQ
1. How do paper charts, RCDS and ECDIS compare as navigation tools?
2. How does the RCDS Performance Standard differ from the ECDIS Performance Standard?
3. . How does RCDS deal with charts of different map projections and on different datums?
4. How can raster charts replace paper charts when only a portion of a chart can be seen at any one time?
5. How can one plot a course and navigate that course when it crosses several charts which may be of different projection, scale and datum?
6. Does chart clutter make a RCDS unusable?
7. How are chart notes and legends found on a raster chart?
8. Can text and symbols be resized (rescaled) when one zooms in and out on a raster chart?
9. Can the mariner limit the information that is displayed on the RCDS?
10. Can data be interrogated with RCDS as it can with ECDIS?
11. What kind of automatic alarms and indicators does an RCDS support?
12. Is it possible to integrate a radar presentation with the RCDS display?
13. Can RNC's have daytime and nighttime colors like NOAA ENC®'s?
1. QUESTION: How do paper charts, RCDS and ECDIS compare as navigation tools?
ANSWER: The paper nautical chart is the fundamental tool of marine navigation. It has served for hundreds of years to convey information about the marine environment and for voyage planning and monitoring. The navigation features of the paper chart are well documented in IHO Publication MP-004, Chart Specifications of the IHO.
Raster chart display systems exactly reproduce the familiar paper chart. All chart information is available to mariners in a picture they are experienced at using. RCDS adds real time positioning, electronic tools for voyage planning and monitoring (way points, off-track alarms,...), improved nighttime visibility, access to chart notes, etc. In addition, RCDS provides for the automatic application of official updates issued by national hydrographic offices.
ECDIS does not display a paper chart but constructs a chart-like display which meets the standards adopted by the IMO. It provides all of the navigation functionality of RCDS and adds the capability of additional alarms triggered by the chart data (prohibited area, safety contour,...) and the ability to remove some chart information to simplify the display.
Both RCDS and ECDIS have the disadvantages of higher cost since they require a computer, limited display area, and some small chance of electronic failure. The risk of electronic failure should be no worse than for already accepted devices such as LORAN, GPS, radio, and radar. (Back to top)
2. QUESTION: How does the RCDS Performance Standard differ from the ECDIS Performance Standard?
ANSWER: The RCDS Performance Standard is identical to the ECDIS Performance Standard except where the difference between raster data and vector data requires a change.
RCDS does not have the automated alarms triggered by chart data that ECDIS provides nor could one reduce clutter by turning off chart objects. However, mariners would still have real time positioning, automatic updating and any alarms entered manually (off-track alarm, obstructions...). In addition, the familiar chart image would minimize training time and would allow mariners to use their existing navigation skills immediately. (Back to top)
3. QUESTION: How does RCDS deal with charts of different map projections and on different datums?
ANSWER: Metadata included with the RNC's identifies the projection and datum for each chart. The navigation software then converts positioning system information into the chart's projection and datum in real-time. If a chart's datum is unknown, the RNC metadata includes the parameters necessary to map geographic positions to RNC pixel locations calculated from the grid intersections printed on the chart. (Back to top)
4. QUESTION: How can raster charts replace paper charts when only a portion of a chart can be seen at any one time?
ANSWER: RCDS offers exactly the same field of view capability as ECDIS which has already been accepted by the IMO/IHO. Both RCDS and ECDIS specify a 270mm x 270mm screen size. Whether displaying a 1:40,000 scale RNC or 1:40,000 scale NOAA ENC® data, the width of the computer screen will only show approximately 5.8 nm.
RCDS and ECDIS deal with a computer's limited field of view in a similar way. RCDS users load the next smaller scale chart. ECDIS users zoom out and let the software suppress information and resize the remaining text and symbols -- in essence creating a smaller scale chart dynamically. Also, the ability to pan across a chart or to open multiple charts of different scales at the same time gives RCDS and ECDIS increased viewing capability. (Back to top)
5. QUESTION: How can one plot a course and navigate that course when it crosses several charts which may be of different projection, scale and datum?
ANSWER: Advanced RCDS software automatically switches charts when the vessel arrives at the edge of the present chart. The new chart is displayed in a scale as close to the previous chart as possible. The datum issue is addressed above. Projection differences are automatically accommodated by advanced RCDS software. (Back to top)
6. QUESTION: Does chart clutter make a RCDS unusable?
ANSWER: RNC's are exact copies of paper charts and contain all the detail of those original charts. Hydrographic offices had worried that this would appear "cluttered" when displayed on a computer monitor. However, users are reporting that "chart clutter" is not a serious problem with raster charts. (Back to top)
7. QUESTION: How are chart notes and legends found on a raster chart?
ANSWER: Hydrographic offices provide the coordinates of a rectangle surrounding each note, legend, tide box or channel depth table. They also provide reference point coordinates for each note. RCDS software places an unobtrusive (or invisible!) icon at the reference point. When the icon is "picked" by the mariner, a second small window on the chart opens, displaying the note. This method would allow notes to be displayed automatically as a mariner's ship approaches the coordinates of the reference points. (Back to top)
8. QUESTION: Can text and symbols be resized (rescaled) when one zooms in and out on a raster chart?
ANSWER: No. Text and symbols will be enlarged or reduced as one zooms in and out. If this results in text that is too small or too large to read, it means that you are operating out-of-scale and should switch to a different scale chart. (Back to top)
9. QUESTION: Can the mariner limit the information that is displayed on the RCDS?
ANSWER: No. RCDS displays a complete RNC which is a copy of a paper chart. The concern that displaying all chart information may be too cluttered is proving not to be the case. A benefit of this characteristic is that critical information is never accidentally turned off. (Back to top)
10. QUESTION: Can data be interrogated with RCDS as it can with ECDIS?
ANSWER: No. RCDS cannot directly interrogate an RNC which is merely an electronic picture of a paper chart. However, since the RNC displays all of the paper chart, most information is already displayed and the RNC does not need to be interrogated.
Additional data which may accompany an RNC, such as operator-entered items or other data bases can be interrogated in real-time and used to trigger alarms or perform other operations. For example, a mariner may designate hazards on or near his route. RCDS would then trigger alarms when these hazards were approached, even if the relevant chart was not being displayed at the time. (Back to top)
11. QUESTION: What kind of automatic alarms and indicators does an RCDS support?
ANSWER: RCDS issues automatic indicators when an RNC is being displayed over scale, when a larger scale chart is available and when the chart is on a different reference system from the positioning system. RCDS issues automatic alarms for exceeding an off-track limit, deviating from a route, approaching a critical point, using a chart on a different geodetic datum, approaching a critical point/line/area and for an RCDS malfunction.
RCDS allows mariners to enter points, lines and areas to designate other alarm items. A mariner may enter obstructions, navigable limits (rough safety contours), prohibited areas, etc. anywhere on a chart or on any chart. These are monitored by RCDS software and alarms can be triggered even though a particular chart is not being displayed at the time an alarm feature is approached. (Back to top)
12. QUESTION: Is it possible to integrate a radar presentation with the RCDS display?
ANSWER: Yes. Navigation software now filter the radar picture so that it and the raster display are legible. (Back to top)
13. QUESTION: Can RNC's have daytime and nighttime colors like NOAA ENC®'s?
ANSWER: Software can remap the RNC's colors and their brightness to any other colors and brightness available on the host computer. Thus dusk, twilight and nighttime colors can be specified. National hydrographic offices providing RNC's should specify a pallet for their charts which is most suitable for those different conditions. (Back to top)