Geographical Coordinate Systems

[+larryc43230]

Please forgive what might be a real newbie question. I've done some forum searching and haven't been able to find any specific information on this topic.

 

My GPS unit is set to display map data in the WGS-84 system (probably like most units). The benchmarks I've looked up on Geocaching.com list coordinates using the NAD-83 system. My GPS unit (a GPSMAP 60CSx) lists both systems in the list of available "Map Datum" settings, as if they actually are different systems, even though other reading I've done indicates they're equivalent.

 

Are these two coordinate systems functionally equivalent, or do I need to convert NAD-83 coordinates to WGS-84 when I encounter them? And is there a straightforward way to do this conversion?

 

--Larry

[+geognerd]

First, NAD83 and WGS84 aren't coordinate systems. They are datums, or models of Earth's shape.

 

Second, there isn't much difference between coordinates that are using NAD83 or WGS84. I think the difference is less than a meter, someone correct me if I'm wrong. That makes it a wash, considering that on a good day your GPSr is accurate to about 3m.

 

Third, you can set your GPSr to use NAD83 and enter the coordinates. When you switch it to WGS84, the GPSr will convert accordingly.

 

Take a look at this topic about NAD83 vs WGS84 from the Benchmark section of the forum. Here's another topic discussing the same thing.

[+GPSlug]

Yeah, don't worry about it for the accuracy of your GPSr.

 

WGS84 was originally pretty much the same as NAD83, but it's moved since then. But it will still be within 1-2 meters. Don't bother with the conversion.

[+Kewaneh & Shark]

...Take a look at this topic about NAD83 vs WGS84 from the Benchmark section of the forum. Here's another topic discussing the same thing.

 

In geognerd's above reply, the Other Topic Discussing the Same Thing he links to was started in the Benchmark Forum by one of the forum's frequent visitor's, DaveD. The topic of datums is frequently discussed in that forum as benchmarks are listed in the NAD83 and NAD27 datums, not WGS84, and he began that thread as an explanation to the datum differences. (The ensuing discussion can get a bit longwinded, but there is much information to be found in it too.) It should be noted here that DaveD is neither a cacher or a benchmark hunter (in the GC.com sense). He is a Geodesist for the National Geodetic Survey (a branch of the NOAA), with an interest in the benchmark hunting side of geocaching. His office is responsible for the maintenence of one of the Nation's largest survey network databases. It should also be said that he is the nation's formost expert on survey datums. He quite literally helped to define both the NAD83 and WGS84 datums we all use today.

 

- Kewaneh

[+Sand-Man]

I guess we better go see what the man has to say!

[+larryc43230]

I've obviously asked a pretty popular question! I apparently didn't do a very good job of searching the forums before posting the question, since there's already a lot of information available.

 

Thanks, everyone, for taking the time to answer my question. The bottom line seems to be that, though the two coordinate systems datums are technically not the same, they can be treated as equivalent within the accuracy of my GPSr. The longer threads you've suggested, when I get the chance to go through them, should answer any more detailed questions I might have on the subject.

 

Thanks again for clearing this up!

 

--Larry

 

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