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Discussion Starter #1
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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dang dusty, seems like you would be the one to answer that question. i bet the new stuff is nad83 or wgs84 datum, but you rarely find the old stuff right where it is supposed to be or even some of the new stuff. i always put the public numbers in then drive around in circles till i find it, then remark it with the same name but with a + sign at the end.
 

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Discussion Starter #3
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.
 

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hell yell

when you are out there you can swap your datumsback and forth on your gps. if you want to come by the house we can look though the numbers i have and see what we can find. also mbt has quite a list on their website and if you stop by their shop i think they have them plotted out on a map.
 

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i think you are worrying about it too much. use the default setting on your gps and im sure you will be fine for any of the new deployments.
 

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I really don't know and can't answer that. I just use the default setting. If you are going to be out in the kayak anyway, go to a known public number, maybe even the boat ramp or a close in public number that you know. Anchor on it and start going through the datums. Use whichever is right on.
 

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WGS 84 is the standard spheroid for us navigational charts and navigational instruments. However, a user can select a different spheroid for most GPS units. Using a different spheroid can result in significant (miles) of error, depending on the location. If a person publishes numbers or a company publishes a chart in other than WGS 84, they should note that (since it is non-standard in the US)

Probably more info than you need, but a spheroid is the grid a particular system lays over the earth. Different spheroids have different starting points and orientations, so they don't match up, so you get errors if you use one when you should be using another.
 

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In the 1980s, the United States Department of Defense used the Geodetic Reference System, 1980 (GRS 80) and Doppler satellite images to create a new, more accurate world geodetic system. This became what is known today as WGS 84. In terms of reference, WGS 84 uses what is called the "zero meridian" but because of the new measurements, it shifted 100 meters (0.062 miles) from the previously used Prime Meridian.

Similar to WGS 84 is the North American Datum 1983 (NAD 83). This is the official horizontal datum for use in the North and Central American geodetic networks. Like WGS 84, it is based on the GRS 80 ellipsoid so the two have very similar measurements. NAD 83 was also developed using satellite and remote sensing imagery and is the default datum on most GPS units today.
 

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Here's the answer to converting your different GPS numbers.

http://boulter.com/gps/

Just put the numbers into the converter and pick the one that it converts it to. No it wont convert LORAN numbers.
 

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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?</A>
 

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Won't corpscon do that conversion too? I only use it for state plane to WGS84, but I think it will do the others.
 

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corpscon may do it if you want to download the software

but the point of my last post was with only a sub-millimeter difference between the two and the fact that the gps on your boat is only accuratewithin 6' +/- at best, whats the point?
 

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Did anyone try the converter that I posted the link to?
 

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sealark (1/29/2010)Did anyone try the converter that I posted the link to?
i did and its cool

but it does not convert datum to datum. it converts one datum to degrees minutes and seconds, decimal degrees and degrees and decimal minutes and plots it on a map.

but it is still cool
 

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Discussion Starter #18
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?

</A>
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!

Dustin
 
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