Benchmarks

[Guest gonzobiker]

If you read a topo map carefully, you may have noticed "BM" with a triangle symbol and number next to it. This is a benchmark, a brass marker placed by the US Geological Survey (USGS) at strategic geographic locations, such as at the highest point in a mountain range, etc. Some benchmarks are placed by other gov't agencies. Benchmarks may be identified on either a paper or electronic map (like Nat'l Geographic's Topo! software). Then the lat/long coordinates may be computed and recorded. Feed the numbers into your GPS and search for benchmarks. Some are easy to find (on a large rock), others aren't as easy (hidden in tall grass). I'm going to start bagging (photos) benchmarks!

 

[This message has been edited by gonzobiker (edited 18 July 2001).]

[Guest rebobbitt]

You can search for benchmarks here:

 

http://www.ngs.noaa.gov/datasheet.html

 

You can do a radial search from a lat/lon and specify the distance up to 20 miles.

 

Many benchmarks do NOT have good horizontal positions. You will want to search for horizontal control points (or GPS points too). The NGS data base includes many USGS monuments but not all.

 

They even have pictures for some of them!

 

These points are great for showing people how accurate (inaccurate) their GPS's really are.

 

Rick

[Guest walkerk]

If your're successful, make your discovery useful and fill out the "Mark Recovery Entry Form" and let the NGS know the condition of the benchmark.

 

Here's a page with some in my local area: http://www.crocuta.com/NGS/index.html

 

Remember to set your GPS to NAD83.

[Guest walkerk]

If your're successful, make your discovery useful and fill out the "Mark Recovery Entry Form" and let the NGS know the condition of the benchmark.

 

Here's a page with some in my local area: http://www.crocuta.com/NGS/index.html

 

Remember to set your GPS to NAD83.

[Guest rebobbitt]

The errors from WGS84 to NAD83 should be less than a meter so probably only WAAS enabled receivers have to worry about it, see:

 

http://www.mentorsoftwareinc.com/RESOURCE/Nad83.htm

 

Rick

[Guest BigFig]

Many of the old ones were set with NAD27, but the USGS has stopped "supporting" NAD27. I don't think my Garmin IIIs have NAD83 as an option - I'll have to look, but I think my only close option is WGS84. I thought WGS84 was just NAD83 internationalized?? In my part of the world (kentucky), if you are using USGS maps, you'll need to use NAD27 when looking for benchmarks - even though the USGS site list in NAD83. What do professional surveryors use??

[Guest stayfloopy]

quote:

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Originally posted by rebobbitt:

You can search for benchmarks here:

 

http://www.ngs.noaa.gov/datasheet.html

 

Rick

---

 

Very interesting. I came across one of the benchmarks while I was at High Point State Park (northwest NJ) on Saturday. I did not know what it was at that time. It was a brass disc set into the large rock just a short distance from the High Point Monument.

 

PIDs for those benchmarks are: LY2605, LY2606, LY2607.

 

And oh yeah... I placed a cache somewhere there too.

 

http://www.geocaching.com/seek/cache_details.asp?ID=4574

[Guest stayfloopy]

quote:

---

Originally posted by rebobbitt:

You can search for benchmarks here:

 

http://www.ngs.noaa.gov/datasheet.html

 

Rick

---

 

Very interesting. I came across one of the benchmarks while I was at High Point State Park (northwest NJ) on Saturday. I did not know what it was at that time. It was a brass disc set into the large rock just a short distance from the High Point Monument.

 

PIDs for those benchmarks are: LY2605, LY2606, LY2607.

 

And oh yeah... I placed a cache somewhere there too.

 

http://www.geocaching.com/seek/cache_details.asp?ID=4574