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GPS question

1754 Views 17 Replies 6 Participants Last post by  Team Bloody Waters
This may be a stupid question but I've had some charts that use different datums but are most numbers in WGS 84? More specificly the new nearshore reefs that were set last july?

thanks for the help in advance.:letsdrink
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thanks Mark I just got a gps and bottom machine for my kayak and there are some nearshore reefs I wanna try. These are new numbers so your probably right, I just wish they would tell you when they publish the numbers because there can be a huge difference between different datums:banghead.
can someone please answer this question?
I've only seen numbers in NAD 83 and WGS 84, does any one know the difference between the two?

Thanks for all the info!
nextstep (1/29/2010)again you are worrying too much, let me try this again

Not much difference!

Ref: North American Datum of 1983
NOAA Professional Paper NOS 2
Charles R. Schwarz, Editor
U.S. Department of Commerce

by:Sam Wormley [email protected] 3/7/97
<HR><CENTER><A name=1.4>1.4 How do I convert between NAD83 and WGS 84?</CENTER>

I think the proposition of doing a "datum shift" between NAD83 and WGS84 is a waste of time. The difference between the two is the philosophy of how an inertial datum should be defined, and is ignored by all but a handful of geodesists that are working at extraordinary levels of accuracy.

The ellipsoid of the World Geodetic System of 1984 is defined by four constants, one being the "bar C sub 20" Normalized Second Degree Zonal Harmonic Coefficient of the Gravitational Potential - the "J2" term is computed. The ellipsoid of the Geodetic Reference System of 1980 is defined by four constants, one being the "J2" Dynamical Form Factor - the "bar C sub 20" term is computed.

Geoid models (EGM96 versus GEOID96) will yield meaningful differences in ellipsoid/geoid separations (vertical) for North America, horizontal positions will not reflect anything beyond micro-fractions of a millimeter at best.

The differences between the semi-minor axes are computational (secondary DEPENDENT) variables based on different definitions of the gravity field. They are called "Derived Geometric Constants." The semi-major axis of the WGS84 ellipsoid is known to an accuracy of plus or minus two meters.

Transformations based on a derived difference of a tenth of a millimeter with an constant's accuracy of two meters is, IMHO, silly.

"Therefore, as long as the preceding is recognized, it can be stated that WGS84 and NAD83 are based on the same ellipsoid."

whats wrong you dont trust your old fishing/surveyor buddy?

Thanks Mark:letsdrink!

Yeah I probably am worrying too much. But I'm going to be paddling about 2 miles to get to the spots and I just wanna make sure they are gonna be there. Circling around for a while looking in a boat is a little diff than having to paddle more than I need too:hotsun. I can't wait til it warms up and when snapper season opens!

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