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Longitude and Latitude Inconsistancies

2K views 11 replies 11 participants last post by  SailorNate 
#1 ·
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
 
#2 ·
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.
 
#3 ·
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.
 
#4 · (Edited)
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.
 
#5 ·
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.
 
#7 ·
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.
 
#8 ·
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.
 
#9 ·
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.
 
#10 ·
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!
Tim
 
#11 · (Edited)
1) your GPS is only accurate to anywhere from within a meter up to 100 meters or so, depending on a variety of factors (how many satellites you can see at once, radio signal propagation conditions, antenna type, chipset, software version, WAAS, DGPS, etc...).

2) The update rate on different GPS's is not necessarily the same, so two co-located GPS's may give slightly different positions, because they've each calculated those positions at different times.

3) As others have said, if the different GPS's are displaying the position using different "units," it may be hard to reconcile their locations. For many GPS's you can configure the units used to display coordinates.

4) "All GPS are supposed to use the same datum per wikipedia." Sure, but on many GPS's you can choose the datum that you want it to use to calculate your position (in which case choose the datum that matches the chart you are using). So it would be possible that different GPS's are giving different positions because they are set on different datums. But note that in some parts of the world, the charts have no datum! (and in some parts of the world, there are no charts!)

5) The online charts, microsoft street map, google maps, etc... you don't necessarily know what datum they are using, nor what the tolerance is for the "georeferencing" of their data. I wouldn't worry about small discrepancies here.

6) As others have said, think of GPS as a tool that puts you in the vicinity of where you are, but then use your Mark I eyeballs (and the rest of your head) to figure out exactly where you are.

Good Luck
 
#12 ·
I trust my life and that of others to GPS (WAAS enabled) coordinates on a daily basis. If you fly on a commercial aircraft, odds are you do too. There is no practical difference between the civil and military signal accuracy, i use both.

However, as mentioned earlier, there are several ways to induce human error into the system by those without thorough background knowledge, as well as the fact that many charts and maps are inaccurate, and/or hydrographic/geographic features have changed.

To those who believe that a sextant will provide superior accuracy to a GPS signal, you are sorely mistaken. Whether you need a sextant as a backup navigational source is a personal choice by the skipper, but it will alway be less accurate and prone to many more human errors than a GPS signal.

Rather than always using a chartplotter, try manually plotting your position on a paper chart, and finding your coordinates from a paper chart using resections and dead reckoning, this exercise will be helpful making you more comfortable with using coordinate formats and is a practical skill to be proficient with. A hand held GPS without a chartplotter attached is still a very powerful tool, and a great safety device to have aboard as a backup or two as many now cost less than 50 bucks. I would be sure to have a waterproof, hand held backup GPS with plenty of batteries aboard, long before I put a sextant in the locker.

There is no replacement for keeping your head on a swivel and piloting your craft, you are ultimately responsible for your fate and that of your vessel. I believe this level responsibility is a large part of what draws many to cruising.

Nate
 
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