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cjf

Benchmark Numbers On Topo Maps

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I've been dabbling in benchmark hunting for a year or so (and yes, I've been lurking here in the forum.......). When one of the posts mentioned that benchmarks appear on topographical maps I had to check it out. I already owned TOPO! California which states it is USGS Topographic maps on CD-ROM. And, sure enough - there be benchmarks! But now I have some questions I haven't been able to find answers to on the USGS website. Perhaps someone can help?

 

1) Is there a simple explanation telling me just what the heck some of the "Control Data and Monuments" legend symbols mean? There are a couple that have BM , a number, then a triangle, X, or square. Then there's that one that just says "Neace" with a triangle under it. I'm just wondering what they mean. (Emphasis on simple. I'm having a good day if I can just figure out which way is north.)

 

2) I compared a benchmark I had already found HT1801 with a topographical map of the area. The topo map has "BM 21" where I found HT1801 and I'm assuming they are one and the same. What is the "BM 21" designation used for? Does it translate to any of the NGS information? (I've posted an extract of the map on the log for HT1801. I couldn't figure out how to post it here. Maybe that could be question number 3? :rolleyes: )

 

Thanks for any help you can offer,

 

cjf

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Would really need to see that part of the map in question. Can you show an image or close up photo?

 

Does this help or it this what you are talking about?

 

ctrldata.gif

 

More Map symbols info here

Edited by elcamino

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cjf -

 

As I understand it, the "BM" numbers on topographic maps are simply the elevation of the benchmark. It is not the mark's NGS Designation.

 

My limited experience tells me that a great many of the items listed on topographic maps are not in the NGS database (and therefore not in the Geocaching database).

 

On HT1801's page, click on "view original datasheet" and you get this. On there, look at the SUPERSEDED SURVEY CONTROL area and you will see "21.37 (f)". I believe that "BM 21" refers to this (superceded) elevation. Therefore, it is possible that HT1801 is actually the same station as "BM 21".

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cjf,

 

It appears that all of your questions should be answered by elcamino's USGS mapping information.

 

One thing that I would add from reading your description when you found HT1801; you had mentioned that you had a tough time finding the mark because the coordinates were off. As you will read that is not surprising that they are off due to the fact that they are scaled. If you plot the NGS coordinates on a topo map before you go out for all of the benchmarks that you intend to search for, you will see quite a few of these marks falling near these BM X marks on the quads. If you actually adjust the NGS marks to fit the quad positions, you will find that you are searching in a much closer area where these do appear on the quad.

 

Happy hunting,

CallawayMT

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One application of the topos that came in handy was for JV3388, reported not found in 1990. Gnbrotz figured it out, in part using the old Topo information IIRC. Road numbers had changed, but the old topo data showed the location making more sense of the original description. He put the info together and made the find.

Edited by embra

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Then there's that one that just says "Neace" with a triangle under it. I'm just wondering what they mean.

The marks designated with a triangle are, as the legend posted by el camino notes, horizontal marks, i.e. latitude/longitude. Technically "bench mark" - usually two words - is a position for which the elevation has been measured. There are also many marks, particularly newer, GPS-measured ones, that are used for both horizontal and vertical control.

 

In your example, NEACE is the Designation. The PID (that's the AA2222-style label) does not appear on topo maps. In my experience, most named marks on topo maps are in the NGS database, but I wouldn't be surprised if some are not. To check, visit this page on the NGS website.

 

-ArtMan-

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Thanks everyone! Things are starting to make more sense now.

 

Yes, elcamino, the legend you posted helped. The legend in the software I have abreviated the descriptions enough to make them confusing.

 

The legends use the term "Neace." I googled that term and all I learned is that Neance is a surname and a lot of Neances are interested in geneology. What exactly does "neance" refer to in the map legend? (I'm assuming the use of "cactus" is referring to a prickly plant growing in arid areas. Correct me if it's something else to map legend readers.)

 

I'm also curious about the use of "Third order or better" term. Is this referring to some quality of the monument or its location?

 

Finally, based on Black Dog Trackers reply, I checked the original data sheets for some other benchmarks I found that are also shown on the topo map. Yes! The BM # on the map corresponded to the elevation in feet of the BM as reported at the time the map was made. (That information could have come in handy on another BM I eventually found that day!)

 

cjf

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What exactly does "neance" refer to in the map legend?

 

Take a look at this topo map. The mark circled in red shows the name of the mark, or its designation with NGS. See the datasheet here.

 

The term 'neance' refers to the designation of the station that they used for the legend. Usually the designations used for the triangulation stations can be anything from a town name, to a landowner name, or from anything that the survey crew decided to call it. There are several strange names in use out there.

 

knoxville.jpg

Edited by 5Wishes

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5Wishes commented:

There are several strange names in use out there.

 

There's the understatement of the year! And it's only February! :huh:

 

By the way, a round of applause for all who responded to the original inquiry! There is an impressive body of knowledge in this forum....

 

-Paul-

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I'm also curious about the use of "Third order or better" term.  Is this referring to some quality of the monument or its location?

 

ORDER AND CLASS: HORIZONTAL

***************************

Horizontal station order and class for first-, second-, and

third-order stations are defined in the Federal Geodetic Control

Committee publication "Standards and Specifications for Geodetic

Control Networks". In addition:

 

Horizontal A-order stations have a relative accuracy of

5 mm +/- 1:10,000,000 relative to other A-order stations.

 

Horizontal B-order stations have a relative accuracy of

8 mm +/- 1: 1,000,000 relative to other A- and B-order stations.

 

Most concurrently published NAD 83 positions have consistent

coordinate accuracy, regardless of the date appended to the datum.

This means the relative accuracy of most stations will continue to

meet their published standard (A- and B-order, as well as first-,

second- and third-orders) even when the datum tags are different.

 

There is one important exception to the last paragraph. High

accuracy stations (A- and B-order) are routinely published prior

to the readjustment of the remaining horizontal control stations

in the region. These remaining stations that do not have a

corresponding adjustment date suffix will may not have consistent

horizontal coordinate values with the A- and B-order stations in

that region until the regional adjustment has been completed.

Until that time, the high accuracy stations in that region are

flagged as special-status positions. The following text

applies to them:

 

SPECIAL STATUS - The horizontal position of this high accuracy station is

hereby published prior to the readjustment of the remaining lower-order

(first-, second-, and third-order) stations in the region. The lower-

order non-suffixed stations in this region will not have consistant

horizontal coordinate values with this station until the regional

readjustment has been completed.

 

ORDER AND CLASS: ORTHOMETRIC VERTICAL

*************************************

Vertical station order and class for first-, second-, and

third-order stations are defined in the Federal Geodetic Control

Committee publication "Standards and Specifications for Geodetic

Control Networks". In addition:

 

Normal bench marks with unknown order will display a '?'.

Vertical control which were determined only for the purpose of

supplying a height for Horizontal Distance Reductions are

assigned an order of 'THIRD'. If these types of heights do

not have supporting observations then the Order is displayed

as 'THIRD ?'.

 

Class 0 is used for special cases of

orthometric vertical control as follows:

 

Vertical Order/Class Tolerance Factor

-------------------- ----------------

FIRST CLASS 0 2.0 mm or less

SECOND CLASS 0 8.4 mm or less

THIRD CLASS 0 12.0 mm or less

 

"Posted bench marks" are vertical control points in the NGS data

base which were excluded from the NAVD 88 general adjustment.

Some of the bench marks were excluded due to large adjustment

residuals, possibly caused by vertical movement of the bench marks

during the time interval between different leveling epochs.

Adjusted NAVD 88 are computed for posted bench marks by

supplemental adjustments.

 

A range of mean distribution rate corrections is listed for each

posted bench mark in the data portion of the publication.

A summary table of the mean distribution rates and their codes is

listed below. The mean distribution rate corrections which were

applied to the original leveling observations is a good

indication of the usefulness of the posted bench marks' adjusted

NAVD 88 heights.

 

Distribution Distribution

Rate Code Rate Correction

------------ ---------------

"a" 0.0 thru 1.0 mm/km

"b" 1.1 thru 2.0 "

"c" 2.1 thru 3.0 "

"d" 3.1 thru 4.0 "

"e" 4.1 thru 8.0 "

"f" greater than 8.0 mm/km

 

POSTED BENCH MARKS SHOULD BE USED WITH CAUTION. As is the case for

all leveling projects, the manditory FGCS check leveling two-mark or

three-mark tie procedure will usually detect any isolated movement

(or other problem) at an individual bench mark. Of course, regional

movement affecting all the marks equally is not detected by the two-

or three-mark tie procedure.

 

ORDER AND CLASS: ELLIPSOID VERTICAL

***********************************

The following ellipsoid height order and class relative accuracy.

 

Ellipsoid Height Maximum Height

Classification Difference Accuracy

------------------ -------------------

FIRST CLASS 1 0.5 (mm)/sqrt(km)

FIRST CLASS 2 0.7

SECOND CLASS 1 1.0

SECOND CLASS 2 1.3

THIRD CLASS 1 2.0

THIRD CLASS 2 3.0

FOURTH CLASS 1 6.0

FOURTH CLASS 2 15.0

FIFTH CLASS 1 30.0

FIFTH CLASS 2 60.0

 

The ellipsoid height difference accuracy (:huh: is computed from a

a minimally constrained correctly weighted least squares adjustment

by:

b = s / sqrt(d)

 

where

b = height difference accuracy

s = propagated standard deviation of ellipsoid height

difference in millimeters between control points

obtained from the least squares adjustment.

 

Source - NGS

Edited by elcamino

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