Help - Benchmark Coord. Confusion

[+Ukulele Andy]

When I download Benchmark coordinates into Easy GPS on my computer the coordinate numbers are different when I open up the folder but when I send this information to my GPS unit the numbers match up with the coordinates on the website. Why would this be happening?

Also, are the coordinates for Benchmarks in a different format than what we use for Caches? I noticed that each Benchmark coord. has NAD 83 written next to it - do I need to change map datum info in my etrex before I start searching for Benchmarks?

[Sketch]

I read somewhere that up-to-date USGS maps use NAD-83 and that it is essentially the same as WGS-84 which is what GPS uses. I don't know how accurate that is and I'm sure others here will have a more informed answer.

[+Keithpilot]

I believe that the EasyGPS translates the coordinates into DDD.MMMMM, while most gpsr's are defaulted to, and Geocaching.com uses, DDD.MM.SSS

 

So, although they look different, they are most likely the same waypoint.

 

If you want to check, change the format in your gpsr to match what EasyGPS uses.

 

Keith

 

Keithpilot

The Yellow Dog Project

N43 10.00374

W070 54.87366

[jayhawk999]

See my comments on NAD 83 vs WGS 84 above.

 

One milli-arc minute corresponds to about 1.9 meters in latitude over the US, so that is about the lower limit you could even hope for in a hand held receiver. That is why these units don't display any more digits. Translation to other formats is simple arithmetic: divide arc seconds by 60 to get fractional minutes, divide arc minutes by 60 to get fractional degrees etc.

[+Slower Pace]

I located four benchmarks, geological survey triangulation disks the other day. The coordinates for those disks are right on, since they need to be for geological surveys. I found them by changing my map datum to NAD83, since that is what they were recorded in.

 

Then, out of curiosity, I walked away and went to WGS84. I found no difference whatever in my final readings. I don't feel there is enough difference, if any, to warrant changing our GPS units to NAD83 to search for benchmarks.

 

But when I use a topographic map recorded in NAD27, that is a different story. I do switch my GPS over to that map datum so I can pinpoint where I am on the map, as it is not as accurate as today's standards.

[jayhawk999]

Slower Pace, I was trying to imply just what you have observed -- there is no practical difference between NAD 83 and WGS 84. NAD 27, however, is different. There can be a considerable difference in coordinate values in this older datum. GPS receivers allow for NAD 27 use as some detailed maps use this datum and no other maps are yet available in NAD 83 or WGS 84.

If you want to explore in more detail issues and descriptions of various datums, both the NGS and NIMA web sites provide access to various documents.

WWW.NGS.NOAA.GOV and WWW.NIMA.MIL

 

The NIMA site has tr80-003 "Geodesy for the Layman" which helped me a lot.

[survey tech]

In a nutshell, the reason for the difference between NAD 27 & NAD 83 is that the earth is not a sphere. It is an oblate spheroid, which is to say that it is slightly flattened and elliptical in shape, something like an egg. NAD 27 uses an earlier model, or estimate, of the earth's size and shape. NAD 83 uses a more advanced, and therefore more precise, model. No model can ever be absolute, since the earth is dynamic, and its exact size and shape are in constant flux.