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Old 16-02-2012, 10:43   #1
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GPS Nav Question

Here is the question. It is kind of basic, but I couldn't find it spelled out.

Given that the chart being used has positional errors:

If I manually enter a waypoint, I can expect it to be in the wrong location on the chart.

MY QUESTION: will my boat arrive exactly at my waypoint, regardless of the chart error? (excluding any gps position variation)

Tnx
Art
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Old 16-02-2012, 11:16   #2
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Re: GPS nav question

The built-in chart is simply a human interface to navigating by lat/long, it's there to make the human feel good. The GPS will take you to the exact waypoint you ask it to.
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Old 16-02-2012, 11:17   #3
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Re: GPS nav question

Quote:
Originally Posted by cptratbag View Post
Here is the question. It is kind of basic, but I couldn't find it spelled out.

Given that the chart being used has positional errors:

If I manually enter a waypoint, I can expect it to be in the wrong location on the chart.

MY QUESTION: will my boat arrive exactly at my waypoint, regardless of the chart error? (excluding any gps position variation)

Tnx
Art
Art,

GPS positions on the earth's surface are repeatable....within the error tolerance of the GPS system itself. Generally, the errors with a modern multi-channel GPS unit will be within the tens of feet. Very close.

Charts are an entirely different thing. There may be charting errors, as you mentioned, in which charted objects of shorelines are shown quite far from their actual location on the earth's surface. Some charts date back 100 years or more, when positioning wasn't very precise.

Other chart-related errors occur because charts are a flat representation of a round earth -- a globe -- and there are numerous ways of representing charted features. These are called "projections". The Mercator projection is the most common, but there are numerous others. There are also a number of "datums" in use. These attempt to locate precise geographic positions on the flat chart. The one in almost universal use is the NAD-83 AKA WGS-84. But, there are others and it's important to know which datum was used by the chart maker on the specific chart you're using.

The correspondence between GPS derived locations (Latitude and Longitude) and those shown on charts depends very much on the accuracy of the chart in use. For much of the U.S., charts made by NOAA are pretty accurate, i.e., there is pretty good agreement between the Lat/Lons shown on the chart and those derived from a GPS signal.

This is very elementary stuff, and doesn't address differences in types of charts (paper and electronic), chart maker skills and craftsmanship, etc., etc.

Hope this helps a bit.

Bill
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Old 16-02-2012, 15:35   #4
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Re: GPS nav question

Both those posts clear up a few issues. Thanks

Is the distance measuring function (Distance to the Mark) accurate on areas not covered by a WGS or Mercator projection chart?

(For example, the black areas on OpenCPN)
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Old 18-02-2012, 19:39   #5
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Re: GPS nav question

cptratbag, Yes. Your computer doesn't need charts. It's all a wgs84 grid (by default on OPCN ) unless you can find a way tell it otherwise. Translated to x and y pixel numbers of a screen. It can't see. It just tells the graphic card to show colours because you, the chart or the gps tell it to.
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Old 18-02-2012, 20:44   #6
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Re: GPS nav question

With charts you can have two different types of errors. You can have geodetic errors caused by using a different geodetic system for your GPS than the charts geodetic system. WGS-84 is the most commonly used geodetic system now.

There are also survey errors caused by the survey team not knowing exactly where they are with respect to a reference position.
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Old 18-02-2012, 21:24   #7
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Re: GPS nav question

Pretty good explanation here:
Navigating With GPS, Charts, and Eyeballs
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Old 19-02-2012, 06:49   #8
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Re: GPS nav question

Quote:
Originally Posted by DrivinSteve View Post
Pretty good explanation here:
Navigating With GPS, Charts, and Eyeballs
At a quick glance, that looks good. I bookmarked it and will read it later.

Here's what I wrote about the subject:

=>> Inland Waters Resources - Chart Errors

-dan
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Old 19-02-2012, 20:47   #9
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Re: GPS Nav Question

Thanks everyone for your input. I was playing around with GE2KAP and Open CPN and got confused when a waypoint I plotted didn't exactly fit in the appropriate location on either the GE image .kap or the NOAA chart.
Since the error is different on both, I'll use caution and not trust either one in this area.
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Old 20-02-2012, 20:37   #10
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Thumbs up Re: GPS nav question

Quote:
Originally Posted by dacust View Post
At a quick glance, that looks good. I bookmarked it and will read it later.

Here's what I wrote about the subject:

=>> Inland Waters Resources - Chart Errors

-dan
Dan,
This should be required reading for anyone who buys a GPS before they just take off on the magenta line!
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Old 20-02-2012, 21:35   #11
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Re: GPS Nav Question

Is it possible with any of the mapcal or similar programs to select and calibrate an area of a chart instead of the entire chart?

In other words, if an island or other feature was in the wrong location, and I had known gps coordinates, would it be possible to adjust it's position on my chart?

Art
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Old 22-02-2012, 11:54   #12
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Re: GPS Nav Question

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Originally Posted by cptratbag View Post
Is it possible with any of the mapcal or similar programs to select and calibrate an area of a chart instead of the entire chart?

In other words, if an island or other feature was in the wrong location, and I had known gps coordinates, would it be possible to adjust it's position on my chart?

Art
yes.
Use the forum search.
Search "imgkap" and tunnel through a long thread!(My favourite that answers your particular request) You can do this in several ways though. Search "Mapcal", and Google Earth to BSB / KAP
OpenCPN thread has a ton of stuff on the subject.Search "Charts" search kap bsb etc
Don't fear the cmd line.
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Old 22-02-2012, 15:06   #13
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Re: GPS Nav Question

Just to add some clarity here.

This is a photo of my chart plotter display.

It shows my track, black line, while coming into Francois Bay in Newfoundland.

You can see the track passes through land, 500' high hills to be more exact.

The curve of the track matches the entrance to the harbor, it is off set to the NNE by about 0.2 miles.

The circumstances were dense fog. I could see the buoy on radar, but could not confirm that it was not just a rock (or "sunker" as they are called there.) I slowly motored up to the bouy, once I made it out then I was sure of where I was (right crack in the rock) and ran under radar into the harbor.

The GPS is right, but it does not match the charts.

Just hoping a visual will help make my point.
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Old 22-02-2012, 20:36   #14
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Re: GPS nav question

Quote:
Originally Posted by DrivinSteve View Post
Dan,
This should be required reading for anyone who buys a GPS before they just take off on the magenta line!
Thanks!

Quote:
Originally Posted by DrivinSteve View Post
Pretty good explanation here:
Navigating With GPS, Charts, and Eyeballs
I finally went back and read this. Very well written. They cover chart survey errors in depth and describe it in multiple ways. Next time I update my page, I'll add a link to it.

I cover some additional types of errors, but mine is more cryptic and may be harder to understand for some people.

I recommend people read both and see what they learn from each.

-dan
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Old 22-02-2012, 21:03   #15
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Re: GPS Nav Question

Quote:
Originally Posted by hpeer View Post
.

The GPS is right, but it does not match the charts.

Just hoping a visual will help make my point.
More precisely, the GPS is right but the charts do not match the GPS.

As was noted and as you probably have read, many of the charts that we stilluse today were generated a very long time ago. Many by British sailors using a sextant. Often, the chart for a piece of land is an accurate depiction of its shape but displaced with respect to latitude and longitude. The farther from England, the greater the error and where it has not been critical to shipping, the less likely that it will be resurveyed. There are some famously great examples along the coast of Mexico there your GPS will show you 1/4 mile inland. I understand that much of the northern areas are not well mapped at all and may never have been surveyed.

The obvious potential here is that you might be operating on auto helm and run afoul of a reef that is not charted or is well off from where you might think it is.

Think of the GPS lat-long lines drawn on a sheet of glass. Place the glass over a bare map of a land mass but scoot the paper off station.

In Open CPN and Seaclear you can take a scanned in map and "scale" it into the program by defining various points.

For a while, there was a pilot program where boaters were given recording devices that would log GPS position time and depth. At internet hot spots the data would be uploaded and added to the charts. The mapping folks eventually decided to abandon the project. I think the reason given was the potential uncertainty of calibration of depth meters, liability etc. I think a cloud of "maybe" data is better than none. Test maps overlayed on NOAA charts made for some fantastic detail in well traveled areas. This essentially increased the number of soundings by over a hundred-fold.
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