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Oxmmdox

Wgps84 Vs. Nad83

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Ok, I am pretty new to both Caching and benchmarking. Now I have my GPS set to WGPS84 and it works great for Caching, but when I go to find the Benchmarks I quickly found that I cannot depend on the Coords. posted. Today I found out that this might have been because I was looking at Scaled coords. I understand that they are two differnt Datums, Now my real question: Should I change my GPS over to NAD83 for benchmarking?

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Oxmmdox -

 

As a beginning benchmark hunter you should be sure to read the entire Benchmark Hunting FAQ.

 

You could change your GPS to NAD83 for benchmarking, or not. The difference between NAD83 and WGS84 is somewhat less than the precision of your GPS receiver so it really won't make any difference in your finding.

 

As you have found out already, possibly from the FAQ, the difference between a location Adjusted mark and a location Scaled mark is hundreds of times more important than the difference between NAD83 and WGS84.

 

Have fun benchmark hunting!

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Scaled coordinates can be off by hundreds of feet, even if your GPSr is set to the same datum as that used by NGS. As we understand them (not without dimness), the differences between the two datum systems you mention are small, and are rarely likely to cause much trouble during a search. However, if you submit handheld coordinates as part of a log, they should be NAD83 coordinates, as that is the datum used at NGS.

 

Our experience has been that the To Reach descriptions at the end of the data sheet are more useful than the GPSr in maybe 90% of our searches, but we sometimes use the GPSr to refine scaled coordinates and submit handhelds as part of the log.

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There was a big discussion of this that you can find by searching the history on this forum.

 

As I recall the summary, we recommended you set to the NAD83 datum for both geocaching and benchmark hunting.

 

NAD83 is the correct datum for the NGS data, and is plenty close for geocaching. In that protracted discussion no one posted an example of where their handheld unit gave a different reading for WGS84 and NAD83. The differences are variable around the country but small enough that most manufacturers do not try to model them. Future improvements in accuracy may make it possible to see the difference in handheld equipment, so if you get in the habit of using NAD83 you will be correct.

 

The only time I would switch to another datum is for working with maps based on a different one. For example, the topographic maps are usually on the older NAD27 CONUS datum and that makes a significant difference in most areas of the US. You should be able to enter coordinates in one datum, switch your handheld to the other datum, and see the same physical point converted to the newly selected datum.

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From a surveying standpoint, the differences between NAD83 and WGS84 can be significant in certain areas, however, for benchmark hunting the datums are virtually identical. Stick with the WGS84 datum, particularly if you do a lot of caching. For hunting benchmarks, it'll get you close enough, even to the un-adjusted coordinates. Then, as suggested by M&H, use the 'to reach' descriptions to find the mark.

 

Good luck on your hunts.

 

- Kewaneh

Edited by Kewaneh & Shark

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Try HTDP for all your NAD83 and WGS84 conversion needs.

Since most, if not all, consumer GPSrs assume NAD83 is exactly the same as WGS84 you are probably better off just leaving it on WGS84 and doing the conversion with HTDP if you actually feel the need to get that extra meter or so of accuracy.

Edited by Rotareneg

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You can't squeeze any more accuracy out of the current handheld GPS receivers by using mathematical conversions between WGS84 and NGS83.

 

Perhaps when the Gallileo system is finished in 2010.....

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