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Guest Jeff Renner

Check GPS vs. survey marker

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Guest Jeff Renner

Interested in checking your GPS receiver against a precisely located survey marker? I was, and found out how to locate these. This is elementary to a surveyor, but new and useful to me.

 

Go to http://www.ngs.noaa.gov/datasheet.html then go down to Retrieve NGS Data Sheet(s). If you know the name of the station go to Station Name. If you have an area, go to Radial Search and put in lat/long and the radius distance you want to search.

 

I did this with a five mile radius from my house and got a few dozen. Some were not high quality ? i.e., not precisely located in decimals of seconds lat/long or decimals of meters of elevation. But one was 1.5 miles away and had six decimals of seconds of precision lat/long and three decimals of meters elevation. Good enough for me!

 

NAD 83(1994)- 42 17 27.08086(N) 083 47 48.63919(W) ADJUSTED NAVD 88 - 289.526 (meters) 949.89 (feet)

 

So I keyed these into my eTrex and set off. It was a survey marker disk in concrete beside a bridge. I placed my GPS receiver on the disk and waited for it to settle in. It immediately said it was within 9 feet, then kept going down to 4.5 feet and swinging around. However, it also said it was 50 feet lower tha it was actually. Then the elevation began to come up closer but the distance began to increase. It finally settled down to about 9 feet horizontally and two feet high. Hey, I can live with this.

 

Of course, this is with a clear view of the sky, but I was pleased. Earlier, I used a horizontal control marker (not precisely located for elevation) at one of my caches and took a photo of the GPS about 12.5 feet off horizontally and four feet in elevation. I?ve encouraged visitors to test their receivers there. BTW, I researched the area and incorporated it into my description ? much more informative and enticing, I hope, than a bare bones entry.

http://www.geocaching.com/seek/cache_details.asp?ID=1993

 

Jeff Renner

Ann Arbor, MI

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Guest Dan Bollinger

Jeff, I did this too last year when I first got my eTrex. It was at the airport. I got the same accuracy you did. Just as I was walking towards the benchmark a surveyor drove by. He said his company just started using a GPS. They use it to find survey stakes hidden in the woods. It makes their work faster. Of course, they know by memory where all the benchmarks are. Dan

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Guest Muzikman

maps. they are indicated with a + and have a 2 letter identifer next to them.

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Guest Jeff Renner

Thanks. No, I didn't know that. It appears not to be the case for all of them, or at least not for the location of the marker I found, see http://www.topozone.com/map.asp?z=17&n=4685638&e=269426&s=25 It does not have such a + mark that I can see. How would you find the coordinates from this two letter mark? They aren't on the marker itself.

 

Jeff Renner in Ann Arbor, MI

 

This

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Guest Rocky

Glad to see someone else is having some fun with USGS markers. I have had some great adventures finding markers in the middle of some large forest. Some locations have 3 markers, the main and 2 reference marks to form a Azimuth

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Guest JCutter

Interesting info on using the USGS markers. What do you do in the woods? My GPS tends to lose the satelites in the woods...any suggestions?

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Guest rocky

I am in a high altitude in Western NY, have only had my GPS since last Oct. We just now are getting full leaf out. I may be asking the same question

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