NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Lu Abel
Date: 2013 Mar 26, 09:46 -0700
ECDIS is really an international standard for digital charts.
There are two types of digital charts: "Vector" charts draw things like shorelines and depth contours by "connecting the dots" between data points. They are used in chartplotters. "Raster" charts which are simply image scans of paper charts. Because of this they use maybe ten times as much memory as vector charts. Nevertheless many computer-based chart display systems such as The Cap'n use raster charts.
If you go to the NOAA/NOS web site you can download any US chart in raster-scan format.
Many but not all are also available in this ECDIS format. It is a "vector" format but not the same as the one used in today's recreational chartplotters. NOS calls these "ENC" (which I believe stands for Electronic Nautical Chart) format.
If you want to play with ENC/ECDIS format charts on your computer, try OpenCPN (www.opencpn.org), an organization that is developing a open-source, free, PC-based navigation system that uses ECDIS format charts.
BTW, ECDIS seems to be a "bucket of bits" format where it is up to the display program to decide how to display the information. Let's take a red nun buoy for example. If you look at the raw data for that buoy, it will include the fact that it's red, the buoy's number, and the buoy's shape (nun), its latitude and longitude, and a host of other information. It's up to the display program how to display that -- will it show a standard buoy and note "nun" next to it, or will it try to actually show the shape?
The great advantage touted for vector charts is that you can selectively turn off various types of items. So, for example, you can turn off shoreline features. Or depth contours. Or buoys. (not sure why you'd want to do either of those). Most important, you can change display parameters, eg, show soundings in feet, meters, or fathoms -- just change the setting on your chart display.
I'm amazed how quickly ECDIS has moved into the commercial maritime world. It used to be that a ship traveling in, say, US waters had to have an up-to-date version of each and every chart for the area in which is was traveling. A lot of paper and a huge cost. Now ships are authorized to use ECDIS charts.
For recreational boating chart-plotters, charts are produced by two different companies in two incompatible data formats. The charts come on take-away memory chips like the chips used in digital cameras, etc. The charts are produced by separate companies from those who produce the display units.
When I go to boat shows, you see displays by the plotter companies such as Raymarine or Garmin, but there are no booths staffed by the chart chip companies. The chip companies take paper charts and "digitize" them. I have been unsuccessful in finding out exactly how that happens, but I fear it's done in some sweatshop in Asia by people with absolutely no nautical knowledge. Worse, these companies do not describe how they check the accuracy of their digitized charts.
And that leads me to a story of why I will never trust a chart-plotter. Before I moved to the San Francisco Bay area, I lived in the suburbs of Boston and sailed in Narragansett Bay and Buzzards Bay. A few months before I switched coasts I went to the Newport Boat Show. Several companies were displaying their latest and greatest chartplotters. I played with one, scrolling the display from Newport to Cuttyhunk, a tiny island with a large harbor that is a popular overnight anchorage for recreational boats. The approach to Cuttyhunk is simple, but it does have several significant but well buoyed reefs that must be avoided. The reefs and buoys were not shown on the chartplotter!! (I suspect this happened because the 1:80,000 coastal chart of the area does not show them, only a more detailed harbor chart and the chip company never bothered to digitize the information from the harbor chart). I caught the attention of the most senior looking person in the booth and pointed out to him that someone could buy that chartplotter, walk down to their boat, sail a few miles, and rip the bottom out of his boat trying to get into Cuttyhunk. He shrugged and said "we don't make the chips"
If someone were to offer an ECDIS-based chartplotter, I'd trust it though. NOAA actually creates a database of information used in a chart (interestingly, both current and historical) and a computer program then draws the chart. Or creates an ECDIS database. Same data, so ECDIS will be as complete and accurate as any paper chart produced by NOAA.
Nice boat, the Ericson 35. Almost bought one before I bought my CS36
Lu
Ooo ECDIS, where can I get one for my Erickson 35?
gl
--- On Tue, 3/26/13, Jeremy C <jcaoy---com> wrote:
From: Jeremy C <jcaoy---com>
Subject: [NavList] Re: US. Navy fix accuracy and Interval.
To: garylapook---net
Date: Tuesday, March 26, 2013, 7:13 AM
Bryon, you Navy guys and your yards! I love it when I hear on the radio: "US NAVY WARSHIP to the ship 1200 yards on my port bow." It is funny because most sailors in the world don't know what a yard is. We tend to talk meters, cables (1 cable = 0.1 nm), and NM's.
But, back to the matter at hand: Navigational accuracy and timing. Electronic Chart Display and informations Systems (ECDIS) are now required on a sliding scale for the merchant ships of the world. Just last week I was in a class for this system. We spoke extensively about navigational accuracy.
There is no regulation or standard for absolute cross track error or positional accuracy in the regulations. The key is to be within your pre-determined (by the master) "safe areas of navigation." These areas should be clear of dangerous navigational hazards and deviation from the plan will sure to be a finding in any accident investigation. In the end, the Master decides how much error from the track line he can allow.
In some cases more allowable XTE is better since you often have to deviate from your intended track due to traffic, especially in the termination of traffic schemes. Fishing boats and yachts love to hang out right on your track line. If you allow yourself only 50 meters, you will most likely be in waters you haven't properly vetted and that will get you in trouble during an investigation.
The next question is how close can we get? If you are using DGPS, like nearly all ships, your accuracy is going to be within 10 meters. You can say that your positional accuracy is within a ship width. DGPS is good to about 200nm from the transmitter site. Pure GPS will get you within 30 meters for "deep sea" navigation. With the introduction of "L2" recievers using both GPS and GLONASS, accuracy is within a meter. So with properly set up and equipped ECDIS systems, you can know your position within a shipwidth at all times.
As far as timing is concerned. There is a regulation for this. The ECDIS shall log the position (and other data) at least once per minute. Typically we set the ECDIS to drop a position on the chart every minute but log position every 10 seconds or so. Standard GPS and DGPS receivers calculate position every second. My latest L2 reciever has a 10 Hz position rate so it gives me a GPS/GLONASS position every 0.1 seconds which can be logged as well.
Positional checks are also part of the equation. This is done near land, mostly via radar overlay or manually comparing a range and bearing of a radar conspicious object to it's range and bearing on the ECDIS with it's GPS position. We also look at the GPS receivers HDOP to judge the confidence of our position.
I will note that there is even a visual bearing device available that interfaces with the ECDIS and gyro systems so that you can take visual LOP's and have them plotted directly on the ECDIS. Sadly I doubt I will ever see one on my ship.
Quite frankly, these days knowing where I am isn't the issue. I basically know where I am at all times. My biggest issue is that the charts aren't accurate or detailed enough. Truely, the weakest link in navigation these days is the hydrographic data available to me.
Jeremy
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