GPS Datum, NAD 27 vs. WGS 84 Help!

[eyewitness]

OK, now I'm totally confused. I looked up NAD 27 and discovered two things. First, apparently most maps for navigation are and have been made under the NAD 27 format. At least I'm pretty sure.....Anyway, I pulled up another report which states that NAD 27 does not work for using GPS without quite a bit of error in distance. It goes into a lot of statistical mumbo jumbo that I'm not going to take the time to repeat here. Second, my understanding is that ALL GPS systems are based around the WGS 84 format which is not the same as NAD 27. So I go into my Lowrance Global Map 100 and pull up the datum options in the menu system. There are several different options for the North American Datum 1927 (NAD 27)and who knows what's what right? So I go to the Lowrance Manual and in the list of Datum options it states that the North American Conus East 1927 covers Minnesota. Ok, so now what I want to know is this. What the he!! do I use for the reelbottom and Hot Spots maps for my GPS? Should I be using the WGS 84 or the NAD East Conus? Boy have times changed! My Dad can hardly program the clock on his VCR and I have to be a ##### rocket scientist to use my GPS correctly. Anybody out there that can set this straight for me before I blow a gasket? LOL

[Mike Michalak]

In the Navionics HotMaps, the datum is changed to the WGS84 standard for electronic use, while the paper maps themselves have the NAD 27 configuration, which work just fine. You're right about the mathematics involved to make these conversions, it's complex.

Now, Fishing Hot Spots cannot speak for any changes being made to the data once it leaves here, and that's what's happening. Each map is properly georeferenced for GPS use. The issue, I'm afraid, is only answered by Navionics and Lowrance. Through our experience however, the guys at Navionics have been responsive to queries from customers on issues such as this.

I'll see what I can do to make things any clearer from this end.

[eyewitness]

Thanks Mike, I guess what I'm really confused about right now is whether I should be setting my datum on my GPS to one or the other. If I'm interpolating waypoint coordinates off of a hard map and entering them into my GPS should my datum be set to NAD 27 East Conus BEFORE I input the coordinates, or only when I am going to try and actually use the unit to find a given waypoint while on the water?
Thanks,

[Boatnut]

Assuming you're using a NOAA chart, it should tell you what datum the chart is based on. My lake erie chart says -
" The horizontal reference datum of this chart is North American Datum of 1983(Nad83 and for charting purposes is considered equifvalent to the world Geodetic Stystem 1984 (WGS84). Geographic positions referred to the North American Datum of 1927 must be corrected on average of .173" northward and .321" eastward to agree with this chart".

I always set my LMS 350 to WGS84. works fine.
hope this helps

[Tired]

Mike,

In a previous post you said to make sure the NAD # was the same as the GPS. So, we all have been trying to determine where to find that majic number to match the map with the gps unit. Also, you say to use WGS-84 as the electronic conversion requires it, but also tell us to use the NAD # in the other post......... Very confusing.

http://www.walleyecentral.com/cgi-bin/dcf/dcboard.cgi?az=read_count&om=17967&forum=generaldiscussion

Also,

http://www.walleyecentral.com/dcf/boatsandmotors/12240.html

and,

http://www.walleyecentral.com/dcf/boatsandmotors/12090.html

Please help this clarify this mass confusion. Navionics and Lowrance are not and it does not help anyones business with the inaccuracies being questioned.

[Mark Lassig]

I talked with Navionics on this topic. As Mike stated above Navionics has changed the datum to WGS84 for the Hotmaps and WGS84 is what the datum should be set to to get the most accuracy out of the electronic map. Now what I cannot answer is if you are entering a waypoint from a paper map with a datum of NAD27 and the GPS is set at WGS84 I do not understand what if any error will exist.

[Mike]

More info. for you to consider.
Peter Dana may answer some of your questions.
http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html
You may have a very accurate map, but if your datum is not set correctly in your GPS, you could be hundreds of meters off.

[wow]

If I read that data correctly, the error from the conversion of NAD 27 to WGS-84 is at least 30 meters or larger?? If so, the problem lies in the conversion of the Maps for GPS use. Then why bother? If the accuracy is not there, they are nearly worthless as you will only get close enough to a fishing spot to have to hunt for it with the sonar anyway. You can get right on it with a regular Navionics chart.

Too bad they didn't take the time to produce an accurate product with all the bells and whistles for the inland fishermen.

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[HUH?]

I've been trying to follow these posts on both the Boats and motors forum as well the general discussion as I'm in the market to purchase a sonar/gps or sonar and a gps?

With all the discussion regarding the new tech Lowrance has produced not matching the formats of hard maps, what would be your recommendations as far as what to purchase?

a combo unit-
seperate units-
Garmin / Lowrance or competition-
mapsource-

I'm just learning the GPS side of things and may not even know what I'm asking but if someone could steer me in the right direction, it'd be appreciated.

Thanks

[Crazy Boys]

Most beleive that the GPS units are fine, but that the FHS maps are not accurate in the Navionics conversion for GPS. However, most agree that Navionics maps (not their FHS conversions) are just fine. BlueChart maps and Garmin GPS are said to be good as well.

The real question is if you really need the FHS info on your GPS anyway? If you can see the location on a paper map, you should get real close with a BlueChart or Navionic's chart on a GPS, locate the exact spot with sonar, and simply note/mark the spot with the GPS for future reference.

But, you might not want to use the Navionics-FHS for Navigation due to perceived inaccuracies mentioned on this board.

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[Aaron Burke]

As Mike from FHS pointed out, Navionics HotMaps are converted to WGS84 datum for electronic use.

Each map is properly georeferenced for GPS use, but if you find inconsistencies within the accuracy of the coverage, please contact Navionics sales at 800-848-5896 and give them the information about it, so we can check into it and fix them.

Aaron Burke
Marketing Manager
Navionics, Inc.

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[Sunshine]

Aaron,
I did as you suggested and called your company. The woman that I talked to took all the info and said someone from your company would be calling me back. I'm still waiting, if you would like to correspond by email that would be fine.

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