Longitude and Latitude Inconsistancies - SailNet Community
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post #1 of 12 Old 11-15-2009 Thread Starter
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Longitude and Latitude Inconsistancies

I recently became a GPS user. I trust the positions my GPS tells me but I'm still somewhat skeptical about some of the sources of data. I went to the following sources and pinpointed the slip in my marina. There are significant differences. Can anyone please help me explain this?
Thanks in advance. Note - I tried real hard to get the table to format properly but gave up. Sorry.
harbin2
Islander 30 Bahama

Source North West
Garmin etrex gps 39.18.03.80 76.23.08.20
Smart phone using Active Captain 39.18.06.30 76.23.13.90
NOAA online charts 39.18.07.18 76.23.13.22
Maptech using NOAA chart 39.18.07.90 76.23.13.40
Microsoft Streets and Trips 2009 39.30.01.02 76.38.57.80
Google Maps 39.30.09.80 76.38.54.61
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post #2 of 12 Old 11-15-2009
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Nigel Calder's book "Cruising Handbook" talks exhaustively about a source of apparent GPS errors that are due to the GPS and chart using different asumptions. It seems that the earth isn't really round but has different bumps and out of roundness. There are various standatds cartographers use to account for this; e.g. World Geodetic System 1984, North American Datum 1927 and North American Datum 1983 can all be used on a chart in North America. While the differences are normalli insignificant (i.e. 30 meters on the Easf Coast.) in some places on the West coast and Carribean the error can be 100 meters. In the West Pac 900 meters. To prevent this the GPS must be set to the datum standard as the chart reference.
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post #3 of 12 Old 11-15-2009
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And in some parts of the world, such as where we are now, the charts are constructed from data collected by the Conquistadors.

Outside the "first world," it's best to use GPS to get into the right general neighborhood, and then bearing and range from other sources (radar, range finder, depth sounder) for precision navigation ("I know from the chart that there's a shoal that extends 1/2 mile NW from the island. The island is in the wrong place re absolute lat/lon, but my radar tells me where it really is, and how far I am away, so I can use it to avoid the shoal, just by staying more than 1/2 mile off.)

It is unwise to demand more of a technology than it is capable of delivering.

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post #4 of 12 Old 11-15-2009
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Quote:
Originally Posted by harbin2 View Post
I recently became a GPS user. I trust the positions my GPS tells me but I'm still somewhat skeptical about some of the sources of data. I went to the following sources and pinpointed the slip in my marina. There are significant differences. Can anyone please help me explain this?
Thanks in advance. Note - I tried real hard to get the table to format properly but gave up. Sorry.
harbin2
Islander 30 Bahama

Source North West
Garmin etrex gps 39.18.03.80 76.23.08.20
Smart phone using Active Captain 39.18.06.30 76.23.13.90
NOAA online charts 39.18.07.18 76.23.13.22
Maptech using NOAA chart 39.18.07.90 76.23.13.40
Microsoft Streets and Trips 2009 39.30.01.02 76.38.57.80
Google Maps 39.30.09.80 76.38.54.61
The way you wrote the degrees is a little bit of a shorthand which frankly you shouldn't get in the habit of because it is subject to interpretation.

If you write it like this:
N41 12.52'
W72 06.06'

Your meaning is clear

You can have big variations because
N41.1252
N41 12' 52"
N41 12.52'

Are all very different latitudes even though the numbers are the same.

In order to find the on the keyboard just hold the alt-key and press 0176 on the keypad (not the numbers on the top of the screen). If you are on a notebook you have to turn on NmLk first.
You only have to get it once because you can copy and paste it to use again.
Of course there is nothing wrong with:
N41 Degrees 12.52 Minutes
W72 Degrees 06.06 Minutes

My guess is that the "errors" you are seeing is you do not have the display set the same on all your devices. There are several websites that let you convert from one display to another but I would recommend just changing all the devices to show degrees and decimal minutes if that is what is shown on your charts.

PS.
All GPS are supposed to use the same datum per wikipedia.

Last edited by davidpm; 11-15-2009 at 10:03 PM.
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post #5 of 12 Old 11-15-2009
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That might explain why the best navigational tools is your eyes; especially when used with a paper chart, handbearing compass, plotter, dividers and pencil.

Even with a "properly" set chartplotter, it shows me on land when I am anchored stern-to in Princess Bay on Wallace Island. The leadline says I have 20 feet of depth off the stern.

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post #6 of 12 Old 11-15-2009
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I sail the Sea of Cortez in Mexico. Longitude is off consistantly by `1.5 miles....it is well known by most. Charts are old data....:-)

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post #7 of 12 Old 11-16-2009
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The OP is complaining about several different GPS devices giving him different numbers. Most likely he is not reading them properly. If the GPS and the chart do not agree that is a different problem and most likely the chart is wrong. GPS is usually accurate to a few feet. If it is off by 100' that is a really big deal. Charts on the other hand may be very old and not accurate at all.
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post #8 of 12 Old 11-16-2009
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Sorry but have had GPS showing me a couple of hundred yards westward on the beach when I was smack dab in the center of Freshwater bayou and it is only about 60 yards wide in most places.
When you can do Coastal Piloting... Do it and not depend on the GPS. It is government operated and how much do you trust your government??
Please note my avatar.... It is a Sextant and for obvious reasons I own and use one.

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post #9 of 12 Old 11-16-2009
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That is why GPS is a tool and not an all in one solution. When used with good charts, local information, and common sense... it is a good thing. The "GPS Assisted Grounding" is a common occurance becasue we become dependant on the technology and don't use our heads.

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post #10 of 12 Old 01-21-2010
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As other have said, the GPS and chart datums are probably incorrect.
The good news is that for charts of the scale you use for navigation, the poition offset can be assumed to be fixed. So measure the offset E and N between your GPS and plotted positions, and use that to get your GPS fix to match the chart.
Some GPS receivers allow you to enter this offset, which makes things easy as long as you remember to change it when changing chart!
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