+cjf Posted February 27, 2005 Share Posted February 27, 2005 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? ) Thanks for any help you can offer, cjf Quote Link to comment
Z15 Posted February 27, 2005 Share Posted February 27, 2005 (edited) 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? More Map symbols info here Edited February 27, 2005 by elcamino Quote Link to comment
+Black Dog Trackers Posted February 27, 2005 Share Posted February 27, 2005 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 On there, look at the SUPERSEDED SURVEY CONTROL area and you will see "21.37 (f)". I believe that "BM 21" refers to this (superceded) elevation.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". Quote Link to comment
CallawayMT Posted February 27, 2005 Share Posted February 27, 2005 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 Quote Link to comment
+embra Posted February 27, 2005 Share Posted February 27, 2005 (edited) 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 February 27, 2005 by embra Quote Link to comment
ArtMan Posted February 28, 2005 Share Posted February 28, 2005 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- Quote Link to comment
+cjf Posted February 28, 2005 Author Share Posted February 28, 2005 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 Quote Link to comment
+5Wishes Posted February 28, 2005 Share Posted February 28, 2005 (edited) 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. Edited February 28, 2005 by 5Wishes Quote Link to comment
+PFF Posted February 28, 2005 Share Posted February 28, 2005 5Wishes commented: There are several strange names in use out there. There's the understatement of the year! And it's only February! 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- Quote Link to comment
Z15 Posted February 28, 2005 Share Posted February 28, 2005 (edited) 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 ( 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 February 28, 2005 by elcamino Quote Link to comment
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