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post #1 of 10 Old 04-09-2009 Thread Starter
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SeaClear Accuracy

I'm just getting SeaClear up to speed and will be using it to view some charts that I may not have paper versions, and to provide coordinates for my handheld GPS. I've learned, so far, how to make a route with a series of waypoints, which leads to my question: How accurate is the waypoint coordinate? For instance, if I make Buoy A a waypoint, and plug that waypoint into my GPS, will I get to Buoy A (notwithstanding the accuracy of my GPS, of course)?
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post #2 of 10 Old 04-09-2009
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If you're using the NOAA charts, then SeaClear will be as accurate as the charts are... But, the NOAA charts don't always have the latest Notices to Mariners added to them, so you really need to look those up for your area and make a note about any significant changes. Buoys get moved, replaced, etc. So do sandbars.

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post #3 of 10 Old 04-09-2009
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Quote:
Originally Posted by h2osnowfan View Post
I'm just getting SeaClear up to speed and will be using it to view some charts that I may not have paper versions, and to provide coordinates for my handheld GPS. I've learned, so far, how to make a route with a series of waypoints, which leads to my question: How accurate is the waypoint coordinate? For instance, if I make Buoy A a waypoint, and plug that waypoint into my GPS, will I get to Buoy A (notwithstanding the accuracy of my GPS, of course)?
I've found both my helm mounted GPS and Seaclear have been accurate for getting me within sight of ATONS I've plugged in. Where I find Seaclear a real aid, is orienting myself when entering a creek or anchorage. The large screen and detail of the NOAA charts really helps when you are trying to identify landmarks at an unfamiliar anchorage in the real world.

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One other thing... make sure that the chart datum on the SeaClear program is set properly. If it isn't the lat/long coordinates won't be accurate when you transfer them to the GPS unit. Most modern charts use WGS84 as a chart datum, but you need to check that they're set properly.

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post #5 of 10 Old 04-09-2009
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The WGS 84 is true around North America, but there are other places in the world where NAD27 or even less obscure datums are in play. Given that it is more problematic to scan and import charts into Seaclear from those sources, that may not be such an issue.

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I've been very happy with SeaClear. I have it on board and integrated with my GPS, speed, depth, and wind transducers. I added an AIS receiver last season. It all works great. At the beginning of the season, I just download all the current NOAA charts for the area I plan to be sailing in. I've never had an accuracy issue.

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post #7 of 10 Old 04-11-2009 Thread Starter
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Quote:
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One other thing... make sure that the chart datum on the SeaClear program is set properly. If it isn't the lat/long coordinates won't be accurate when you transfer them to the GPS unit. Most modern charts use WGS84 as a chart datum, but you need to check that they're set properly.
How do I check that?
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post #8 of 10 Old 04-13-2009
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The datum is always in the text block of the paper chart, and should therefore be in the scan of said chart.

World Geodetic System - Wikipedia, the free encyclopedia

How you reset this in Seaclear, I don't know off the top of my head, but as it's usually set in the GPS device as a menu choice, would that not translate directly into Seaclear?

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post #9 of 10 Old 06-15-2011
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when Seaclear II opens, right click. Then:

tools---properties---comm----
scroll down to gps datum settings. Set to WGS84.


Good luck,

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post #10 of 10 Old 06-17-2011
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From the Seaclear help file:
"Calculations and accuracy
Heading and distance calculations are done using the rumbline. All calculations are done with double precision eliminating math errors. On greater distances rumbline calculation is less accurate. In normal use, such as in arcepelago navigation, the errors are not notable, as the GPS position and the charts are far less accurate.
SeaClear does not work with charts and routes spanning the dateline (180 E/W)."

Rhumbline Sailing may be OK for your requirements. Bowditch says don't use Rhumblines over a few hundred miles, what ever that is (300 maybe?). The accuracy depends on mid latitude and how much east west component there is to a route. The waypoints should be at the right position, but with Plane trig, the distance and direction may be a little off.

From San Francisco to Hawaii the difference is only about 20 NM out of 2000, but the course difference is about 8 degrees. That is huge. You will be way off course when you leave. As another example from Jacksonville Florida to Bermuda, I get a course difference of 4 degrees over a distance of 858 miles.

Another example at a higher latitude is from Hobart Tasmania to Resolution Island in New Zealand. Using Plane trig, it shows the distance and direction from Hobart to New Zealand (Resolution Island) as 828 miles and 106 degrees True. The actual distance and direction is 843 miles and 110 degrees True. That's about the same course descrepancy (4 degrees) as the Florida to Bermuda example, but the distance descrepancy is greater.

For an east-west course, you can estimate the descrepancy by using the cosine of the latitude. For example the cosine of 45 is about 0.7, so where a minute is a mile on a great circle, a minute on the 45th parallel is 0.7 miles. That is a big descrepancy.

If your waypoints are 200 mi or less apart then Plane Sailing would be OK, but for trans-ocean sailing you would probably only want 1 waypoint at your destination so Great Circle Sailing would be better; just depends on what your requirements are.
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