Maptech question for Capt. Stump

[Richard Formica]

Hi Dave,
I have never seen this web site so I appreciate you leading me to it, if unintentionally. I do have a question for you. In general I thought maps off the internet were not accurate enough to use in actual navigation. In your post did you mean to imply that these GPS coordinates could be used or are these maps just useful to get an idea of where you are going?
Thanks
rich
s/v Inerarity CD36

[D. Stump, Hanalei]

Rich,

I believe the MapTech site attaches a disclaimer to each map that states "Not for use for navigation". As far as the coordinates go, I compared those found at various points going into Little Narragansett Bay with what I had on my Loran and they weren't even close. I plan on electronically marking each bouy with my GPS this weekend, as I could have the wrong data in my Loran, I"ll then compare that data with what's on the MapTech site and let you know what I see.

So, I guess the answer is no, I wouldn't rely on the Lat/Longs you see on the MapTech site. Remember the old saw, "When navigating, log, lead and look out!" I remain your most humble servant....

Dave Stump
Captain Commanding
s/v Hanalei

[Murray Glue]

One needs to be mindful that the GPS system gives a position in the WGS84 datum, provided that no other datum has been selected internally in the receiver. Most nautical charts now are based on WGS84, but some may have a shift from WGS84 to the projection used, and this is usually given on the chart. Electronic charts should be verified before use to ensure that they are in WGS84, regardless of the source.
I note that the GPS system with SA now switched off, is giving raw GPS positions on a 12 channel Trimble 4000 receiver of less than 3 to 4 metres, with greater stbility than before, and a minmum of wild excursions, but it is still not unknown for the raw GPS position to take a wander, especially around sunrise and sunset.

Murray Glue
CD30 Dayspring
Nelson
New Zealand



gluebatts@yahoo.co.uk

[D. Stump, Hanalei]

Murray,

Whatever equipment is used, shouldn't the final Lat/Long plotted be the same for any individual point on earth? So, why would there be a difference between what my Loran or GPS says and what the MapTech site says? Stonington Point, CT does NOT move, so I would think its' Lat/Long would always come up the same. Explanation please, or should I "log, lead, and lookout!"???

How's winter????

Dave Stump
Captain Commanding
s/v Hanalei CD-30

[John Vigor]

Captn. Stump, Sir, with respect I am surprised that you should regard GPS as infallible. Latitude and longtitude plotted on paper charts from a GPS readout can be as much as a mile in error because of the different survey base references used over the years. That's why you have to be careful to correlate your GPS readout with the chart reference, if you can. Otherwise, the advice is to use coastal pilotage (soundings and bearings) as soon as you can identify marks ashore. This applies especially to some islands and reefs in the
deep oceans--but I don't suppose that's going to worry you on the way to Stonington.

John Vigor
Greaser's peggy
Aux. S.V. "Jabula."

Murray,

Whatever equipment is used, shouldn't the final Lat/Long plotted be the same for any individual point on earth? So, why would there be a difference between what my Loran or GPS says and what the MapTech site says? Stonington Point, CT does NOT move, so I would think its' Lat/Long would always come up the same. Explanation please, or should I "log, lead, and lookout!"???

How's winter????

Dave Stump
Captain Commanding
s/v Hanalei CD-30

jvigor@qwest.net

[D. Stump, Hanalei]

I would trust my GPS if I had actually gone to the point of interest and marked it electronically. But, the question is, why would there be a difference between that indicated Lat/Long. and that position you calculate off of a paper chart? I don't understand where datam has anything to do with the ultimate Lat/Long position of a place on the surface of the earth. Am I wrong, or should the Lat/Long for a position be the same no matter what you use to figure the position? If your answer is that you get different Lat/Long figures depending on the datam of the map, what good is the map? ie, doesn't Longitude start at the Greenwich Meridian? Doesn't 72 degrees W Longitude put me in or close to Connecticut?

THERE IS SOMETHING MAJOR HERE THAT I DON'T UNDERSTAND, PLEASE ASSIST....

Dave Stump
Captain Commanding
s/v Hanalei

p.s. I think I'll stay tied to the dock!!!!!

[D. Stump, Hanalei]

John,

I was thinking, if I remember my navigation and piloting training, when I was a fighting Captain aboard a Man-o-War for the British during the fight with Napolean, the Greenwich Meridian was established to provide a common reference for ALL navigation on the high seas. Prior to that time, in the 16th. and 17th. centuries, all nations had different reference datams. Hence, they could protect their favorite trade routes, and keep it secret from other "enemy" nations.

I mean, I know that Captain Jack Aubery and I knew where in the world we were! Am I missing something here????

Dave Stump

[Ken Coit]

Here is a link that may help explain what happens if the chart datum is not the same as the GPS.

Ken

I would trust my GPS if I had actually gone to the point of interest and marked it electronically. But, the question is, why would there be a difference between that indicated Lat/Long. and that position you calculate off of a paper chart? I don't understand where datam has anything to do with the ultimate Lat/Long position of a place on the surface of the earth. Am I wrong, or should the Lat/Long for a position be the same no matter what you use to figure the position? If your answer is that you get different Lat/Long figures depending on the datam of the map, what good is the map? ie, doesn't Longitude start at the Greenwich Meridian? Doesn't 72 degrees W Longitude put me in or close to Connecticut?

THERE IS SOMETHING MAJOR HERE THAT I DON'T UNDERSTAND, PLEASE ASSIST....

Dave Stump
Captain Commanding
s/v Hanalei

p.s. I think I'll stay tied to the dock!!!!!

Parfait@nc.rr.com

[Murray Glue]

I knew I was opening a can of worms, but what is this site for if not a frank and honest exchange of views.
The best thing for all to do to get a grasp of the problem is to read the Navionics site page recommended by Ken, which explains the problem without going into the complexities of adjusting from WGS84 to local spheroidal values. As the site says, for most purposes the difference is small and insignificant, but where charts from less reliable sources are used or these charts are based on local spheroids, or small scale chart is being used for approaches to harbours etc, it pays to know what one is dealing with. It is not unknown for satellite navigation positions ( especially from the earlier Transit system ) to be applied to locally produded charts with the result being a shiny yacht parked on a jagged reef.

Capt. Dave, if the concept still bothers you, I will try to find an explanation in my nav notes which makes sense. True, Greenwich is assumed to be Longtitude point of origin, but this only solves the problem for longtitude. The earth is assymetrical about the poles, so a system has to be adopted which flattens and distorts a true global shape into something which fits the earths real shape. The WGS84 system is the best fit; it has both x and y values, and a z value, such that at some points the speroid is above and at some below the actual mean sea level surface.



gluebatts@yahoo.co.uk

[Catherine Monaghan]

<a href="http://pollux.nss.nima.mil/index/uncon/ ... tml">Using Nautical Charts with Global Positioning System</a>

Make sure to scroll down the page on the web site indicated above and see the difference between the charted position of the island Farallon de Pajaros and the GPS position of the island.

Mariners managed to find their way to the island before the use of GPS, so I wouldn't dwell too much on the issue -- just be aware of it.


catherine_monaghan@merck.com
CD32 <a href="http://www.hometown.aol.com/bcomet/real ... ization</a>, #3
Rahway, NJ
Raritan Bay

Here is a link that may help explain what happens if the chart datum is not the same as the GPS.

Ken

I would trust my GPS if I had actually gone to the point of interest and marked it electronically. But, the question is, why would there be a difference between that indicated Lat/Long. and that position you calculate off of a paper chart? I don't understand where datam has anything to do with the ultimate Lat/Long position of a place on the surface of the earth. Am I wrong, or should the Lat/Long for a position be the same no matter what you use to figure the position? If your answer is that you get different Lat/Long figures depending on the datam of the map, what good is the map? ie, doesn't Longitude start at the Greenwich Meridian? Doesn't 72 degrees W Longitude put me in or close to Connecticut?

THERE IS SOMETHING MAJOR HERE THAT I DON'T UNDERSTAND, PLEASE ASSIST....

Dave Stump
Captain Commanding
s/v Hanalei

p.s. I think I'll stay tied to the dock!!!!!

catherine_monaghan@merck.com

[John Vigor]

Captn. Stump, sir, with all due respect, I have my doubts about what you were up to during the fight with Boney. You seem to have skipped some vital navigation lessons.

Yes, you are missing something important. Get out your GPS. Go to the Navigation page and select Map datum. Scroll through it, and you will see dozens of choices, including WGS 72, Wake-Eniwetok, Viti Levu
1916, and a host of others.

Now I happen to have in front of me at this moment a chart of the island of Hawaii. In the top right corner it says: "Old Hawaiian Datum." So if I want to enter Hilo without hitting anything, I scroll through the Datums on my trusty Garmin GPS 12 handheld, and select "Old Hawaiian." Now my chart and my GPS are talking the same language.

Murray Glue is correct when he says WGS 84 is the lingua franca, but it can't speak everybody's language all the time, so if you haven't correlated your GPS with your chart, WGS 84 might blow you a raspberry now and then. As well might I, if this goes on any longer.

Yrs. humbly,

John Vigor
Greaser's peggy
Aux. SV "Jabula"



jvigor@qwest.net

[Michael Heintz]

Geez, And I thought getting to the rendezvous was going to be easy.

Guess I'll just have to throw out all of that datum stuff... and go for it

Michael Heintz
Captain ( and I guess Commanding )
Macht Nichts CD 30 MK II



mzenith@aol.com

[Jay Hubbard]

I too wondered why the positions shown on those Maptech charts didn’t’ agree with my GPS but then I discovered they didn’t always agree with the other sources either. An excellent web site to learn about GPS errors and projections is http://www.cnde.iastate.edu/staff/sworm ... rials.html

In addition Copies of Complete Maps prepared for NOAA by Maptech are available at the following site. http://mfproducts.nos.noaa.gov/ Like the ones on Maptechs site they provide good overview but are not recomended for navigation.

Jay Hubbard
Fiddler's Green
CD 28

Hi Dave,
I have never seen this web site so I appreciate you leading me to it, if unintentionally. I do have a question for you. In general I thought maps off the internet were not accurate enough to use in actual navigation. In your post did you mean to imply that these GPS coordinates could be used or are these maps just useful to get an idea of where you are going?
Thanks
rich
s/v Inerarity CD36

mailbox1@worldnet.att.net

[D. Stump, Hanalei]

Captain Vigor,

Honored Sir, and have read your last. I have my Garmin GPS 45XL here on the chart table in front of me. It is in fact set up for WGS 84, but the same question still applies. If the chart is the same datam, won't the GPS Lat/Long's and the chart Lat/Long's be the same? Also, if I ask the GPS to find Napatree, RI, wouldn't it show the proper Lat/Long no matter what datum on the GPS was used? I mean, the Lat/Long is a point on the face of the earth, main reference being the Greenwich meridian.

This lubber of a navigator still doesn't understand....Good thing we beat old Boney when we did....I remain....Your most humble servant...

Dave Stump
Captain Commanding
s/v Hanalei CD-30

[John Vigor]

Captn. Stump, sir--You honor me in error. I am not a captain, and never have been. I am obliged to admit that the highest rank I have achieved in Her Majesty's Navy is that of greaser's peggy. (Forgive me if I dissemble. It was not precisely one of her Her Royal Majesty's
vessels I served upon, but a 20,000-tonner engaged in the Africa trade. Nevertheless, she did wear the Blue Ensign.)

It would be true that I am the commander of a fine Cape Dory were it not for the fact that I sail with my wife ... I presume, sir, that you will understand.

As for the matter at hand, I have this to convey: You ask, nay, you state, that lat/long is a point on the face of the earth. In this, my dear sir, you are quite mistaken, despite the efforts of myself nd others more qualified than I to impress your susceptibilities. Latitude and longtide of any one spot on the earth differ according to the datum base used.

For example, in the United States, the venerable administration known as NOAA has been changing from the North American Datum 1927 (NAD 27) to the newer World geodetic System 1984 (WSG 84). Now, if you have a given latitude and longitude on a chart drawn to the earlier datum, it may differ from EXACTLY THE SAME latitude and longitude drawn on the later chart. (I apologize for having to shout, but I find this often brings results when communicating with foreigners and those whose brains are not yet in gear.) Differences between the two may range from 40 meters in Florida to 150 meters in Maine--plenty enough to put you on the rocks. And you may take it from me, sir, as a respectable source, that many, many new charts drawn to the old datum are still being issued, while GPS is giving positions in the new datum.

It is regettable that a man of your fine sensibility should find this distinction so difficult to grasp. Forgive me if I suggest that your success against the little Corsican has addled your brain. Perhaps, if your thoughts had strayed less toward Josephine and more toward datum bases, this conversation would have been rendered unnecessary

I urge you to consult the Navionics website that one of your esteemed correspondents was good enough to suggest. Take your time, sir. Read the words slowly and give them time to sink in. Move your lips while you read, if you must, but pray concentrate, for this is important to understand if you are about to lead a flotilla into an anchorage beset with hazards.

Yrs. humbly,

John Vigor

Captain Vigor,

Honored Sir, and have read your last. I have my Garmin GPS 45XL here on the chart table in front of me. It is in fact set up for WGS 84, but the same question still applies. If the chart is the same datam, won't the GPS Lat/Long's and the chart Lat/Long's be the same? Also, if I ask the GPS to find Napatree, RI, wouldn't it show the proper Lat/Long no matter what datum on the GPS was used? I mean, the Lat/Long is a point on the face of the earth, main reference being the Greenwich meridian.

This lubber of a navigator still doesn't understand....Good thing we beat old Boney when we did....I remain....Your most humble servant...

Dave Stump
Captain Commanding
s/v Hanalei CD-30

jvigor@qwest.net