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KS&k

Newbie Question - Nad 83 Vs. Wgs 84

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Recently got a GPSr to start geocaching and I want to find some benchmarks to see how close I can expect it to get me to the listed coordinates. I d'loaded some benchmark coordinates, but I notice they've got "NAD 83" next to them. I'm pretty sure the geocaching coordinates are listed in WGS 84. How can I enter the NAD 83 benchmark coordinates and find them w/out messing up the WGS 84 geocache coordinates I've already entered into the GPSr?

 

I've looked in the forum & FAQ's, and I can't find anyone else w/ this question. That means I'm either the first person to run into this problem or there's a very simple answer that I just can't seem to see...

 

TIA

 

KS&k

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Hi KS&k: Welcome to Benchmark hunting!

You are by no means the first to ask that question. I'm surprised you didn't find anything in your search. The experts here will rightly say that you should use NAD83 as the datum for benchmarks. However, the difference between NAD83 and WGS84 is very small. It is much less than the accuracy of a handheld consumer grade GPS receiver. So - for practical purposes, you can stick with WGS84. It is good to know the difference, and that maybe in the future it might be good to use NAD83, if we get better GPS systems.

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KS&k,

 

Just use WGS 84 datum. They are so close, as to be for all practical purposes, identical.

 

More important is to check the benchmark page and see if the coordinates are "Scaled" from a map or "Adjusted".

 

Scaled coordinates can be off quite a bit (the farthest off we have found is about 4/10 of a mile.) while adjusted will get you within 10 feet usually and we have had days where the GPSr read less than 3 feet off.

 

John

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There is a difference between the NAD83 and WGS84 datums, but for the purposes of geocaching and benchmark hunting, the systems are essentially the same. Both systems are a spherical grid system comprised of latitudes and longitudes, but the difference in the systems is the point of origin. That difference in origins is less than a meter and a recreational GPSr does not have the accuracy to show that difference. (The difference between the datums, however, is important to someone using a survey-grade GPS system.)

 

For recreational purposes, including geocaching and benchmark hunting, there should be no need to convert from one datum to another, and you won't 'mess up' a WGS84 coordinate by entering a NAD83 coordinate. There should be an option on your GPSr to select which datum is your primary datum. Most units come out of the box with WGS84 as the primary datum. Just make sure you have the correct datum selected for the coordinate you're entering, and you'll be fine. Your GPSr knows the difference between the datums and will act accordingly. (By entering a coordinate in one datum and then selecting a different datum as primary, your GPSr will convert the entered coordinate to the selected datum.) Many units also have the option of a secondary datum. Mine is set to WGS84 as primary, and NAD27 as secondary. Many (older) maps are printed using the NAD27 datum and there is a substantial difference between NAD27 and WGS84.

 

- Kewaneh

Edited by Kewaneh & Shark

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I remembered reading an excellent post on this subject from last year and found it.

 

Post by DaveD

 

Nad 83-Wgs 84

Edited by mogle1

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OH NO, THAT Thread!!!!

 

If you are REALLY interested in the subject, then slog through it. I followed it as it "evolved", and it was fascinatiing! The "interpersonal dynamics" aside, there is lots of great info in it, including an opinion from one of the professionals at NGS. We even learned a meter is not always a meter!

 

Again, for our "handheld consumer grade GPS purposes", you will very very likley never see any difference at all between NAD83 and WGS84 datums. There is "learned speculation" that in fact when you change your datum setting on your GPS to NAD83, it really doesn't do anything except change the display. The corrections are so small and so difficult to get right (there IS no "formula" per se), that "our" GPS manufacurers just give us the same WGS84 coordinates anyway. So - change to NAB83 if you really want to be technically correct when hunting benchmarks, but you don't really need to do so.

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Thanks Kewaneh for adding the correct link, apparently I can't edit my post after someone has posted after me.....or perhaps I'm just blind and can't see the edit button anymore.....now where did I put my bifocals.....

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Thanks to all for your replies regarding this issue. I'll quit worrying about it and just go try and find some markers!

 

Thanks again!

 

KS&k

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