Topo Quad Bench Mark question

Take a look at this page showing USGS map symbology: http://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf

Looks like it should have been a "tablet."(corrected after I read the linked document again..:'(

> Take a look at this page showing USGS map symbology: http://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf
>
> Looks like it should have been a "tablet."(corrected after I read the linked document again..:'(

Thanks. I assume "recoverable" means something might be there. But out of curiosity....What's a "Tablet"?

I think it's likely that it is a disk (a.k.a. tablet) set in a rock outcrop or a concrete post. If the Coast & Geodetic Survey, now NGS, didn't use the disk then the data would only be on paper in a file cabinet somewhere - USGS didn't digitize their information.

The USGS data wouldn't be of as high accuracy as NGS standards since they were interested in efficiently getting mapping accuracy.

> I think it's likely that it is a disk (a.k.a. tablet) set in a rock outcrop or a concrete post.

I may go poke around some then. Of course the published accuracy standard is only Class III, which is way below my standards.:-D

Here in The West we use to contact USGS Rocky Mountain Mapping Center in Denver, CO with such questions. I can think of a couple instances where they were able to provide unpublished BM info. I haven't dealt with them in a number of years though, and I don't know what the equivalent office is for your area. Good luck.

As otherwise noted, you can always try to contact the USGS for more info from their non-digitized files. I used to call their center in Rolla, MO. The NGS Advisor to Vermont http://geodesy.noaa.gov/ADVISORS/AllStates.shtml#VT might have a current contact number.

I expect the USGS point is for elevation only (a bench mark). No idea when it was established but monuments along the roadways in areas that are plowed are frequently destroyed. Worse still is when they are disturbed.

I looked at the USGS MidContinent Center web page and did not find a contact for control information. As indicated above, I expect Dan Martin from NGS to have current contact info. The USGS site URL is: http://mcmcweb.er.usgs.gov

Good luck,

DMM

I know we have books (and yes they are blue) that list the bm's and horizontal stations shown on the quads we produced at TVA. We get calls from time to time asking for information on them. I would imagine some agency would have that. There is a book for each quad. I know I had them down in Florida and those were USGS Quads. I can't remember who we got them from or they may of come out of the defense mapping agency.

Also, be aware that the datum for an elevation stamped on a disk is most likely NGVD29 and could differ significantly from NAVD88 elevation. Compare superseded data on an NGS data sheet in your area to see how different they may be. The USGS topo map is probably in NGVD29.

It has been my experience that there is a lot of control out there that was established by USGS probably to support the National Mapping Program. All of it precedes NAD83 etc.

The data WAS published, just not by NGS and was obtained from your local USGS mapping centers and held as paper records filed by quad location identifiers.

I suspect this is the data several posters have given references to help find them, at the USGS Mapping Centers such as Rolla, Denver, Menlo Park and Reston. Almost all of it is Third Order. It was very useful when I started it was most of what we had. The marks could still be useful if updated with opus for example.

- jlw

Any County Surveyor's office worth a lick of salt, should have a hard copy of the USGS level loop runs (for their county) sitting in a book shelf gathering dust.

Although they may not be as accurate as newer control, they still have served as a reference for many years.

I would contact Dale Benson at USGS at 303-202-4394.

Found it!

> I looked at the USGS MidContinent Center web page and did not find a contact for control information. As indicated above, I expect Dan Martin from NGS to have current contact info. The USGS site URL is: http://mcmcweb.er.usgs.gov
>
> Good luck,
>
> DMM

I emailed Dan Martin. Good find!
He showed me the link that maintains information about all control in VT:

http://vtransmaps.vermont.gov/VOLGIS/map.htm

Here's the data sheet on that benchmark:

This would not be suitable for horizontal control, and I'm not sure the vertical is good enough either. Still trying to interpret that part.

But at least there's some information that will help me find it.
Thanks for the advice.

Found it!

Looks like you posted a different datasheet after I wrote the info below...

The datasheet you posted is NOT an official NGS product. Note that it is indicated to be a product of the NGS site description program Windesc. I see it's horizontal position was determined by GPS and its height by applying the geoid-ellipsoid separation to the ellipsoid height.

The network and local accuracies on an actual NGS datasheet for a horizontal control point are derived by the adjustment of the data for the station. That has not taken place yet.

I assume Mr Martin can provide additional details.

In any event and as others have noted, th USGS data for the point was undoubtedly NAD27 and NGVD 29. You want to use these "new" values.

First-order horizontal control is now an anachronism and refers to length-relative precision. First-order was one part in 100,000 plus a base error whose value eludes me.

Dale Benson retired over a year ago. I don't know if anyone is left there who know how to research these data.