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Bill93

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Everything posted by Bill93

  1. https://www.shapeways.com/shops/triangulation
  2. Phenomenal work! It must take a serious investment of time to get the detail right, and a not insignificant production cost. It reminded me of the equipment models a guy posted on the professional surveyors forum. He also makes practical things for his work. https://rplstoday.com/community/threads/more-printed-surveying-models.241503/ https://rplstoday.com/community/threads/got-my-1-4-scale-geodimeter-today.232161/ https://rplstoday.com/community/threads/i-received-my-christmas-ornaments.227217/
  3. How does anyone amass such a collection? That's a large percentage of the styles in George Leigh's writeup. Plus a couple of USGS to boot.
  4. As I think I understand it, you can define the horizontal datum origin and orientation independently of the vertical, but in order to add any other points to your data base using that datum, you have to have the scale factor to convert surface measurements to distances on the ellipsoid model. I.e., if you are at lat-lon A,B and go some measured surface distance and direction then you need to convert that to distance on the ellipsoid before finding a new lat-lon. The conversion factor depends on elevation. They didn't have a very good geoid model at the time so in order to define the elevations they assumed the geoid was at the ellipsoid at Meades Ranch. That slightly inaccurate conversion of elevations introduced a significant distance (hence lat-lon) error as they got further away from that initial point toward the coasts. One of the benefits of NAD83 was fixing that problem, using a better geoid model. Hope I got that right.
  5. I decided to post the question in the professional forum and got an authoritative answer there, that explained why that statement is true. Following the explanation really tests your understanding of geodesy.
  6. The current Wikipedia entry Geodetic Datum has a statement in a paragraph discussing NAD27: "The geoidal height at Meades Ranch was assumed to be zero." At first glance this makes no sense to me. I thought the horizontal datum was on the mathematical ellipsoid and would have no relationship to the geoid. Is there some small second order effect that causes an interaction of the vertical with NAD27 (or NAD83 either) that would need such an assumption at the time? I didn't find a data sheet for Meades Ranch, but look at the data sheet for KG0640 Meades Ranch Reset (NGS) (Geocaching, older data sheet) and note the NGVD29 elevation is within a meter of NAVD88, and the current geoid separation is more than 26 meters. Should that Wikipedia statement be deleted?
  7. All of these that weren't updated are bench marks or triangulation stations. Ironically, although NGS generally doesn't want intersection station reports, they did accept one in July that I submitted because I thought I had potentially useful information. See NJ0770 (NGS) (Geocaching) as discussed in this thread. I had data for the intersection station, a triangulation station, and a RM for a triangulation station that showed a fairly consistent pattern of offset between the NAD83(1996) and NAD83(2011) coordinates in this area that is not modeled by the GEOCON2 conversion software, due to the fact that it is a long distance to the nearest stations having both classical and official GPS coordinate measurements to allow such a determination. While my work wasn't NGS quality it seemed significant.
  8. I have reported via the NGS web site form on perhaps a dozen disks this summer. Of those, 5 have not been updated. Some from early July were and some weren't. Two of three that I submitted on the same day in September were updated a week ago and the third one still hasn't. Perhaps a couple of my reports included unconventional information that I thought was useful, or were pretty trivial (reporting wrong direction from witness post on mark set in 2014), so if they were editing closely someone might have rejected them. But some of these missing reports were pretty straightforward. Anyone else have this experience or insight into what's going on?
  9. Very few elevation marks have an underground backup disk. I've seen some MORC data sheets that had a rivet below the pipe the disk was on, but that was not common practice for either USGS or C&GS/NGS. Only triangulation stations typically had underground disk. If your disk had an elevation stamped it was very likely USGS as C&GS/NGS did not stamp elevations as a regular practice. And only a small fraction of USGS disks were measured to the standards to get in the NGS data base. So I am not optimistic for you but wish you luck. --------- Edit: the locations of elevation marks were usually SCALED off a topo map, which might be good within a few yards where there were features on the map to compare to, but the the values were truncated (not rounded) to whole seconds of lat-lon. Thus you can figure that even if the disk was perfectly located on the map, you have up to 100 ft north and 75 ft of west of the coordinates to search. The way to find bench marks from the data sheet is to go to the lat-lon coordinates and then put the GPS away. Get out the tape, and measure from the features listed in the to-reach instructions on the data sheet.
  10. They took the sightings at night because of more stable air, avoiding heat waves. They were sighting 10 miles or more at times, so visibility was a major concern. Another reason was that heat from the sun would cause some expansion of one side and move the instrument. There are some historical pictures of the work somewhere on the NGS web site. Also search this site for posts having to do with Bilby towers, which were double (inside and outside) towers designed by Jasper Bilby to to be easy for a crew to put up and take down. The crew was on the outer tower and the instrument on the inner tower so they didn't shake it. I went to the dedication (2014) of the Bilby tower installed in his home town of Osgood, Indiana by the Surveyors Historical Society with the help of others. This tower was either the last known to exist, or perhaps there is one other. It was located because of discussion on this forum, which I passed on to the SHS. Salvage operation pix Rebuilding pix Dedication ceremony pix including links to more info. A State of Indiana historical plaque in Osgood will be dedicated November 5, 2016. There were former members of tower crews in attendance at the dedication, who probably did that work as late as the 1970's. They reminisced about being like a gypsy tribe who moved from place to place, often with the families living in trailers and helping each other with child care and various problems that came up.
  11. Bill93

    Easements

    A lien means the land owner owes money, so that is not applicable here. Back when the agencies were setting triangulation stations, there might or might not have been some sort of written agreement with the land owner, but I've never heard of an easement being recorded in the public records for that. It would be interesting to know if anyone has found such an instance. Most of the elevation bench marks were placed along roads or railroads, so there was already a right of way easement if that land was privately owned, and the placement of a disk was usually not questioned even when that use was not originally included in the easement rights. For those disks placed on private property, I suspect the owners just cooperated, and if not the disk was placed somewhere else. One could make an argument that an easement by prescription now exists for them, but IANL so I don't know for sure. For us benchmark hunters, I don't think we can assume any right of access and need permission if it is on private land. I've even been chased off the road right of way in front of someone's house because measuring scares people. Interesting question. I'll be interested to see if someone knows more.
  12. https://rplstoday.com/community/threads/uscgs-historic-markers.327890/
  13. I found an interesting disk in Grand Canyon Village, near the area where you change between east loop and west loop shuttles. It appears to have no PID so I scabbed it onto the nearest mark in this data base. I had never seen one quite like it, nor with so many agency stampings.
  14. NJ0770 isn't anything special except for the history I have on it. Its position was probably within a foot or two of correct back in its day, and after roof work it probably still is. It is also mentioned in my recent post about finding the accuracy of local stations.
  15. Your enthusiasm is good to see, and I note that you have been finding bench marks for a while. Your reports to NGS can be valuable to them. I hope you have picked up the "flavor" of the official recovery reports. All business, no "found while geocaching with a friend on Saturday between rainstorms" comments. Use "Found as described" or add any helpful information you can, such as "School building is now a community center", "Road realigned and disk now xx feet from center", "Railroad dismantled and used as recreational trail", "[item they measured from] is gone. Disk is xx feet NW of new culvert". If you didn't find it, add helpful comments about how the location has changed. Give handheld GPS coordinates in degrees, minutes, and seconds format for marks that only had SCALED coordinates before. Please, no comments about how far off the old coordinates were, and never give your coordinates for a mark with ADJUSTED coordinates because they are more accurate than your device. Intersection stations (church spires, towers, water tanks) are no longer of interest to NGS, so do not report them. Good condition for professional purposes means that the POSITION hasn't been compromised. They don't care a lot about the cosmetics of the mark. A disk that has been hammered on by vandals can be GOOD if they can set their pole on the exact same location and height that was originally measured. The most pristine looking disk in a post that is seriously leaning is at best POOR and perhaps DESTROYED. You will be asked when using the NGS entry form whether the location is suitable for GPS. You can select "Don't Know" and that's fine. The criteria for professional GPS are generally clear sky view except for minor items like a pole or small tree. They are interested in visibility from 15 degrees above the horizon up. If you extend your arm, spread your hand, and set the little finger on the horizon, your thumb will be close enough to 15 degrees.
  16. I'm not sure where one would find a United States Geological Survey data sheet. You probably found a data sheet on the National Geodetic Survey (NGS) site for a US Coast and Geodetic Survey (US C&GS) mark (the predecessor name for NGS). There was a thread with pictures long ago on this forum about a recovery of Buttermilk. Buttermilk, of course, was a geodetic mark as opposed to the M&D cadastral (land boundary) stones.
  17. In doing the experiment described in a related post, I've tried to sort through the various datums, datum tags, and other mysterious designations on the data sheets. This is more detailed information than most forum posters get into, so I thought I'd summarize what I think I know about datum realizations. Correct me where I'm wrong. Some of this summary comes from: http://geodesyattamucc.pbworks.com/f/25Feb2010_NAD.pdf http://www.plso.org/Resources/Documents/Dave%20Doyle%20PLSO%202014%20HISTORY%20OF%20GEODETIC%20DATUMS.pdf You may also want to read articles such as: http://www.ngs.noaa.gov/PUBS_LIB/develop_NSRS.html http://www2.arnes.si/~gljsentvid10/datm_faq.html http://www.ngs.noaa.gov/INFO/OnePagers/NewDatumsOnePager.pdf http://www.ngs.noaa.gov/faq.shtml http://help.arcgis.com/En/Arcgisdesktop/10.0/Help/index.html#//003r00000009000000 Following regional triangulations begun by Hassler, the US Coast & Geodetic Survey worked through the 1800's and connected across the continent and formed the US Standard Datum. Adjustment of the data collected into the early 20th century resulted in the US Standard Datum in 1901 and then an update to the North American Datum of 1927 (NAD27). These used the Clarke ellipsoid model of the earth and were defined relative to Meades Ranch. NAD27 had a network accuracy of about 10 meters and local accuracy of 1:100,000. After several decades of use and the gathering of more data (including pre-GPS satellite doppler data and VLBI vectors), a new datum was defined. It used the internationally adopted GRS80 ellipsoid earth model that fits the whole world better than Clarke and (with minor differences) is essentially the earth model used for GPS. It was defined as the North American Datum of 1983 and became available in 1986, hence NAD83(86). This realization had a network accuracy of about 1 meter and local accuracy relative to nearby stations of 1:100,000. That would be 2 arc seconds or 1 foot in 19 miles at 95% confidence. The topographic maps of the country were prepared in NAD27 coordinates. You will find that even a recreational GPS can see the difference between that and NAD(83); in my area it is about 50 feet so you need to account for it when using topo map with GPS. GPS operates with a datum that is used internationally, IGS08 which is for most purposes the same as the International Terrestrial Reference Frame (ITRF2008). The US military calls their version WGS84 and update it to match the international datum every several years with a datum tag of the GPS week, like (G1674). NGS chose to fix NAD83 to the North American tectonic plate to minimize coordinate changes. The two datums drift apart by a couple centimeters per year. Additionally, it was learned that the initial realizations of those two datums were not as close as hoped, so the difference is more like a couple meters. Most recreational GPS units ignore the difference. For precise work the NGS horizontal time-dependent positioning program (HTDP) provides good estimates of the conversion at a given time epoch. Since then additional GPS data has been used to improve the NAD83 realization. State-by-state updates were done in the 1990's under the name High Precision GPS Network (HPGN), renamed High Accuracy Reference Network (HARN) which you see on signs as you recover some stations. Some states had two versions, so the latest in any state is called the Federal Base Network (FBN), a label you see on data sheets. You also see Cooperative Base Network (CBN), which is a densification of the high accuracy stations. The 199x FBN adjustments updated all horizontal control stations in the data base using additional VLBI, CORS, and other GPS measurements. Successive adjustments held the highest precision stations (VLBI and CORS) and adjusted other GPS stations to them, and then adjusted stations with no GPS data. Only a minority of the old tri-stations had HARN GPS data. Additional HARN stations resulted from GPS measurements on elevation bench marks that had no prior accurate lat-lon. The FBN stations improved accuracy by a factor of at least 10 relative to NAD83(86) but the tri-stations that were not occupied by GPS improved less. HARN/FBN was still NAD83 because it used the same definitions, but the realization was different because it used additional measurements that shifted things a bit, hence names like NAD83(1995). I think of realizations like a fish net. After you tie down the lat-lon of some points on the net, there is still some sag or wandering by the rest of the net. Each realization gets its points better aligned, but not all points have equally good data. NGS created NAD83(CORS96) which included only CORS stations and had an accuracy of about a centimeter. The next realization (update) was NAD83(2007) with the goal to remove inconsistencies between the state adjustments and with CORS. It held fixed the NAD83(CORS96) positions. This time only GPS data was used, so triangulation stations that didn't have GPS data were omitted, and you still see their NAD83(199x) coordinates at the top of their data sheets. Those old coordinates typically have a fraction of a foot to even a few feet of local warpage relative to the stations with GPS data, which is not modeled because it is random error in the old measurements. Intersection stations are going to be less accurate than triangulation stations. The latest is NAD83(2011, epoch 2010.0), which you see on data sheets having good GPS data. If a project design is based on an older realization, then measurements tied to the adjusted monuments can be converted accurately enough to 2007 and FBN, but stations that were not occupied with GPS will usually not match well. (corrections welcome)
  18. You may want to read the related post about datum realizations to help this make more sense. I have a project that I've worked with occasionally over several years to play with measurements, and get a better understanding of the C&GS triangulations. I observe angles to tall objects such as the red lights on radio towers, serving as intersection stations. I have a few points with known coordinates as a basis and do least squares fitting of the measurements to estimate coordinates for everything. A church spire NJ0770 didn't seem to fit very well when I used the latest data sheet NAD83(96) coordinates and converting to NAD83(2011) with GEOCON didn't help much. Then last fall I acquired an "antique" professional GPS, a Trimble 4000sst. Anybody who used one when they came out in the early 1990's would consider them junk today because they only process L1 and sort of process L2, but no P code. Newer receivers process all of those signals. I figured out how to manipulate the data files so the NGS OPUS service would process them and return NAD83(2011) coordinates. I got a couple OPUS sessions on the ground near the church and did triangulation to tie in the spire, and find it to be about 1.6 foot east of where the data sheet plus GEOCON would put it, and much closer to my older measurements. So I went to first-order triangulation station NJ0775 a few miles away, dug down to the disk, and got a 4-hour GPS session that came up 1.0 ft mostly east from the 96+GEOCON coordinates. The nearest tri-station NJ0769 is deep in someone's yard and doesn't have enough sky anyway. Its RM3 is also somewhat obscured and gave me a file that was so weak in L2 OPUS wouldn't give useful results. I got someone to run the L1-only portion of the data and it came back 1.0 ft mostly east of prediction. My conclusion is that the old triangulation network in this area was very good in relation between nearby tri-stations, and good to about a foot with respect to stations that are tens of miles away, but the intersection stations (church spire) can be off more than that even with respect to nearer triangulation stations. The triangulation network had some warpage in this area for stations without GPS data, relative to the stations used in the '96 adjustmen,t because the nearest stations with both triangulation and GPS data are tens of miles away. That is well within the goals of the old triangulations. This has been an interesting and educational exercise. I'd like to eventually check more tri-stations around here to better characterize the shift. (Corrections welcome)
  19. The topic comes up occasionally about careless logging by people who don't check the stamping to see they have the right disk. One of my first few NGS logs turned out to be WRONG despite having the right agency and stamping. I guess the lesson is that you need to understand civil engineering terminology to be sure something hasn't been rebuilt. I thought the road had just been raised over the old bridge but it seems this is a new culvert, with a new disk that somebody in their questionable wisdom decided should have the same stamping. NJ0584 STATION RECOVERY (2014) NJ0584 NJ0584'RECOVERY NOTE BY NATIONAL GEODETIC SURVEY 2014 NJ0584'THIS REPORT WAS SUBMITTED BY STEVEN MILLIGAN. THERE IS STILL A NJ0584'MONUMENT WITH A DISK STAMPED LINN CO ENG DEPT 1402, BUT IT IS NOT NJ0584'IN THE SAME STRUCTURE AS DESCRIBED IN THE 1934. IN 1934, THE DISK NJ0584'WAS IN THE TOP OF THE CENTER OF THE SOUTH CONCRETE GUARD RAIL OF A NJ0584'TWIN 5 BY 12 FOOT HIGHWAY BRIDGE, AND NOW IT IS IN THE CENTER OF NJ0584'THE TOP OF THE SOUTH HEADWALL OF A TWIN 6 BY 12 FOOT RCB. A +2.75 NJ0584'HOUR OPUS SESSION SHOWS A DIFFERENCE FROM THE PUBLISHED NJ0584'ORTHOMETRIC HEIGHT OF THE DISK OF -1.013 METER. I CONTACTED LINN NJ0584'COUNTY AND FOUND THAT THE ROAD WAS RELOCATED IN 1968, AND THE NJ0584'BRIDGE WAS REPLACED BY A CULVERT. ALSO, LINN COUNTY SHOWS TWO BM'S NJ0584'WITH A DISK STAMPED 1402 WITH A ORTHOMETRIC DIFFERENCE OF -3.15 NJ0584'FEET.THE 2004 GEOCACHER DID NOT FIND THE 1934 DISK, BUT INSTEAD NJ0584'FOUND THE POST 1968 DISK.
  20. The disk you saw was probably never entered in the NGS data base. Only a small fraction of the disks out there in the country were ever measured to the standards needed and submitted for inclusion in the NGS list, even though those disks serve a useful purpose for someone. The Geocaching list of bench marks is a snapshot of the NGS list from about 2001, which is long enough after 1988 that it should have been there if the data was submitted at the time the disk was placed. We don't expect the Geocaching list of marks to ever change. Nothing is deleted, and nothing is added to keep up with NGS. Once a disk is entered at NGS, it stays in their list, although if it is reported destroyed you have to use a different search to bring it up there.
  21. I don't see any relationship to the old thread, so would have started a new one, myself. I can convince myself I see part of GENERAL but agree is isn't the same style of disk as the other picture. Have you looked at the topo map for this location to see what it shows?
  22. Never a set of several identical repeats. I've found some weird things in the data base, though, and gotten them documented and/or corrected. I did find a mark that had two different PIDs and different to-reach that ended up in the same location. One PID had the wrong longitude by exactly 1 degree (if I remember correctly) so that led to the duplicate entry. Then there was the classical triangulation station PID with newer Iowa DOT data by GPS, and also a PID with Minnesota GPS data and the same designation. The coordinates were only a couple feet apart and there was only one disk there. It turned out that the Minnesota data had a 1.0000 meter error in one of the GPS XYZ coordinates. That got straightened out, too, by NGS combining the corrected data on one data sheet and doing away with the incorrect duplicate. Corrected, except that a PID really isn't a Permanent identification. NGS re-used the PID from the deleted data sheet for something a couple hundred miles away, and my recovery note pointing out the original discrepancy on the data sheet they kept looks pretty foolish now. Lesson: never put anything in the recovery notes that could be wrong if a problem is fixed. I found a set of three marks that shouldn't be in the data base. Somebody carefully reset disks with designations 5, 8, and 12 in some series to be outside the construction area for a widened highway, and the resets got new PIDs. The problem was that the original disks did not have PIDs and the office calculated the reset elevations based on the data for designations 5, 8, and 12 in another county. Since the elevations are on the order of 100 ft wrong, I doubt any surveyor will be misled by them. And I had one with ADJUSTED coordinates that I couldn't find until I re-studied the to-reach and discovered that to be 0.6 mile away at a different road intersection, where I found the disk in good condition. It turned out that somehow they had used the data (including coordinates) for an unpublished PID of a USGS series temporary mark with the proper description and to-reach of the disk that they intended to publish. NGS got that straightened out. My recovery report looked so foolish and confusing after they fixed it that I got them to delete part of the report from the data sheet.
  23. It appears the precise measured value changed slightly and now they round up to a whole meter instead of rounding down. I haven't done the research. Does anyone know what their definition of height is? When you get precise about it, elevation is a very complicated topic and not entirely intuitive.
  24. I've happened across several similar ones. Many counties have contracted out the placement of such marks to base their GIS systems on.
  25. It appears I answered the wrong one of your duplicate posts, so will copy my response over here. You've already gotten some good advice. You are definitely looking for professional equipment, as any recreational or car GPS unit will be a couple orders of magnitude poorer than your requirement. If your company needs only a moderate amount of data, it will be advantageous to contract a professional surveyor to perform the service for you. If you have an ongoing need for a lot of data, then you or someone in the company may want to get educated on it, but it will be a big commitment. Depending on what the data is used for (e.g., land boundaries and perhaps some kinds of construction), there may also be regulations governing who is licensed to perform the work. You might try registering (free) on www.surveyorconnect.com and asking for advice from the professionals there. The 5 cm specification is not quite complete, since any measurement has statistical error. Are you looking for a standard deviation of 5 cm, or 95% confidence (i.e. 19 times out of 20) within 5 cm, or what? Is the requirement to find positions relative to a national or world coordinate system, or is the primary need to have accurate relative positions among certain points? How long are you willing to have the equipment on a point to get a reading? With a professional receiver operating by itself, "static autonomous" measurements of 15 minutes to a few hours may be needed to get that accuracy, depending on satellite positions at the time, and how clear the view of the sky is. Some people use a base and rover setup with a radio link. If the base is placed on a known point (Ordinance Survey trig point?) the rover can get positions relative to the base with very short occupation times. Of course, it is advisable to make the rounds of your unknown points twice to confirm that there was no anomalous reading. There are Real Time Kinetic networks (RTK or RTN) in some areas that you can subscribe to, which eliminate the need for you to have a base unit. I don't know anything about RTN coverage in England. Leica certainly sells equipment that will meet your needs, as do other manufacturers as well (Trimble, Javad, etc.) Besides equipment capability and price, pay a lot of attention to software compatibility with your CAD system and how easy it is to get training and service support from the company. Professional equipment will require some training to get good results. And there is a lot to understand about what you are really measuring at the cm level. Are you familiar with horizontal datums and vertical datums? Map projections? Map grid versus ground scaling? If you use the native GPS coordinate system, do you know how to account for continental drift that will be significant over a few years? And many other issues.
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