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Bill93

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Everything posted by Bill93

  1. 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.
  2. >I believe that the USGS became part of the NGS which is now part of NOAA, I don't think so. I've not heard that, and they still have separate web presence. http://www.usgs.gov/ http://www.ngs.noaa.gov/ >so USGS marks from the early 20th Century could/should be in the NGS database Even if the agencies were to be combined, the USGS data is in file cabinets and nobody is going to fund the effort to digitize millions of data sheets on marks that are often not as accurate as the NGS data, especially in these days of precision GPS. >I've heard through the grapevine that NOAA no longer cares much about intersection stations, but does still care very much about benchmark disks. That's official, and will be true at least until the planned new vertical datum is released in about a half-dozen years, based on GPS+gravity at any location instead of the older optically leveled data on the existing marks. >older disks were often stamped with their elevation as of when they were set. USGS has done a lot of that. CGS/NGS hasn't done that for a long time, if ever. >However, sea level has changed over the past 100 years Actually, irrelevant here. The older National Geodetic Vertical Datum of 1929 (NGVD29) was based on an average of sea level at several locations. Sea level is hard to measure precisely because of tide patterns that take years to repeat, differences in salinity, currents, etc., so locations will not agree with leveling measurements between them. After gathering much more data they released the North American Vertical Datum of 1988, which is still in use. It was based on picking one station and making all others in the US, Canada, and Mexico relative to that one. No adjustment is made in the vertical datum for sea level changes. The difference between datums can be quite significant but varies from region to region. The difference between datums has caused some people to think it is a measure of sea level change, but it is not.
  3. The designation is not E217 or E271. It is E 217 or E 271 with a space that is significant in searches.
  4. I've removed paint from disks. If the paint flakes off like Harry's picture, then it's no big deal. Just be careful not to scratch or abrade the metal - a plastic scraper is safer than metal. I'd have started by cutting around the edge of the disk with a knife instead of digging into the center, but whatever works. The paint in your picture looks too fresh to peel unless there are layers under it, but you could try. I've used a nasty organic (non-corrosive) solvent for some disks. Carburetor cleaner or non-corrosive paint stripper, cover for a few minutes to soak, rub off, repeat several times. Your environment may be different than mine, so don't exceed your comfort level. Around here, I wouldn't worry that it would be illegal. You're not going to harm the disk, and it shouldn't have been painted in the first place. Unless it turns into a big ugly mess with streaks running down the wall, it shouldn't be mistaken for vandalism. I think I could explain it to law enforcement if they happen by. This one looks to be out in open country, so you should attract little attention.
  5. Good article. I think all of us on this forum would resonate with this statement: "What’s the most enjoyable part of the job? Some of the places we find ourselves working are unique and beautiful, and are often closed for public access. It can be thrilling to travel to areas I wouldn’t normally go, nearby or further afield. We sometimes spend our field days “walking in the footsteps” of the original surveyor, inspecting untouched remote monuments from 100 years ago or more."
  6. I think the town was platted with lots, alleys, and streets that were never developed as originally expected. People on this end of town now own multiple lots and there is no visible evidence that the N-S streets were ever built. You guys are thorough investigators to dig that deeply, or else totally bored and needed a diversion ;-) Yes, there was a set of tracks when I took a quick look (certainly not thorough) for the elevation bench mark, with trees growing up between the rails. The north rail may not have moved much since the description was written. I'll see if I can get my brother's opinion on the age and location stability, because he used to do track maintenance and inspection. It'll take a very good estimate of position to find the buried disk, with 18 inches to probe and dig. I'm probably not up to probing much area that deep unless the ground is very soggy.
  7. I'm interested in LE0511 (GC link) (NGS link) because my sister is moving to that neighborhood and it would be a good puzzle to work on while visiting there. Reference marks 1 and 2 are reported as destroyed, with RM3 and 4 set in 1969. There is to-reach for the new RMs but no bearing or distance from the RMs to the buried triangulation station on the data sheet. It makes the buried station a little hard to find. I haven't looked yet, but have doubts about the old fences being there. Would somebody at NGS be able to find bearing and distance in records that didn't make it to the data sheet? Or maybe I shouldn't bother them - do they really care about tri stations these days?
  8. That's pretty strange and I can't explain it. I can only add confusion, as follows: Theory 1 would be an undocumented reset, but I'd have expected them to at least stamp it RESET. I think SOP would have been to leave the original stamping and add RESET 1960. Theory 2 would be that the monuments were set in 1935 by US C&GS but not leveled at that time. In 1960 the USGS was doing preliminary work for the topo maps and found this one disturbed. Either USGS or CGS reset it while working the 154 series. Since it had no data on it, they didn't stamp it reset. Then they did the level run through the 263 series some time before the 1965 recovery report. Some other monuments in the area are listed as MONUMENTED 1960 CGS, but I don't think the MONUMENTED tag is really a reliable indicator of who set the disk, but may have more to do with who took the data. But why would the 154's be monumented in 1960 and the much higher number 263 have been done in 1935 by CGS?
  9. I have no idea what happened to bend the line in G.E. I had Missouri DOT precise coordinates for the cast iron post, which was set in 1851 or 52, and found it within my Garmin's accuracy. You'll note that fencelines to the east and west are nearer the latitude of the post.
  10. I've painted around a couple marks because I thought it might help preserve them during nearby construction or clearing. One of them was a (non-geodetic) state line marker in a field (spanning the line) where the renter told me the owner was planning to clear the old fence and brush. I painted on trees and ground around the post, tied flagging on the post and overhanging tree, and painted a fat 6 ft long arrow on the ground pointing to the post. I see on Google Earth that there is a spot in that field being farmed around, but I have no idea whether or not it was due to my efforts. 40°34'47.82"N 93°27'34.65"W I avoid marking when there is no apparent threat, on the theory that it may attract unwelcome attention from vandals.
  11. That looks like a very effective tool for those marks that have been covered by an accumulation over the years. For deep ones like some buried tri-stations, a commercial tile probe or the one mloser had made are needed. But whatever you are using, always remember to look around for utility route signs, telephone pedestals, or fiber optic equipment pits. Puncturing any utility would be a very bad experience.
  12. Could be DaveD at work. He has harvested a lot of coordinates for scaled marks off this site, and could have noticed the disk type discrepancy from your photos.
  13. I'm surprised that the data sheet archived on Geocaching.com shows it as LF1239_MARKER: DG = GRAVITY STATION DISK but the current NGS data sheet has LF1239_MARKER: DS = TRIANGULATION STATION DISK while the latest recovery is the same 1977 on both. Somehow the discrepancy came to their attention. Did you submit a correction to them?
  14. It is rare to find a cadastral (land corner) monument in the geodetic data bases. They were set for very different purposes and mostly without regard for each others' coordinate systems. The stamping on that disk is shorthand for: in Township (6-mile nominal squares) number 26 south of the regional base line, on the line between Range (of townships) 6 east and Range 7 east of the regional principal meridian, at the southwest corner of section 7 in range 7 east, which is also the northwest corner of section 18 in that range, and adjacent to to section 13 in range 6 east. I'm not sure but guess the C means it is a closing corner as the survey coming from the east hit the previously surveyed range (north-south running)line, as opposed to a standard corner as marked when that range line was run to define section 13. The instructions for different states/regions were somewhat varied in how the accumulated error for each township was to be dealt with, but in many cases the last sections along the edge of a township were not made to match up with those in the next township. For more information about the overall system, search for Public Land Survey System.
  15. There are two main reasons you might not find a data sheet for a mark. 1) There are dozens of federal agencies and thousands of state, county, and city agencies, plus untold private companies who all set survey marks for various purposes. The vast majority of them are not measured to the exacting standards or processed through the "Bluebook" procedure to get into the National Geodetic Survey (former Coast and Geodetic Survey) data base. You'll find most of the marks are not in the data base for the National Park Service, the US Geological Survey, Army Corps of Engineers, most of the State Survey marks (of the 1930's), on down to the local level, and almost any cadastral (land corner) monuments. 2) The Geocaching data for bench marks is a snapshot of the NGS data base as of about 15 years ago and has not/will not be updated, so anything added at NGS after that won't show up here.
  16. The stamped elevation was probably within a fraction of a foot on the old NGVD29 datum, if the monument hasn't been moved. Since it's Geological Survey and not Coast & Geodetic Survey (predecessor to NGS) it might be a little looser than those on NGS data sheets. The stamped elevation could be different by several feet on the current NAVD88 datum - I haven't checked for the difference in that area. GPS of any grade does not measure elevations on either of those datums. It measures height above the mathematical ellipsoid model of the earth. That is typically several to many meters different from either of the elevations. Your handheld receiver probably has an approximate model of the difference (the geoid height) to give you a reading somewhat closer to the datums than the raw height measurement. Elevation isn't GPS's strong suit anyway, as you may notice by watching your reading wander around. Even surveyor grade GPS and the best geoid model isn't quite the same as NAVD88, but NGS is working on an improved gravity model that will let them replace NAVD88 within a few years. NAD83 is NOT exactly the same as WGS84, although the difference less than the typical repeatability of a recreational GPS so they treat them as the same. When WGS84 was defined, it and NAD83 were intended to be the same, and were to the accuracy that could be attained at the time. Now we know they are 3 or 4 feet different in the US, and growing further apart by 1.5 to 2 cm per year. Once you get to measuring things this accurately, it gets to be a headache to keep track of what you're measuring.
  17. A recent blog discusses the early 1800's layout of Manhattan. Ok, they're cadastral and not geodetic, but are a lot older than the bronze disks we usually see. Richard "Papa-Bear-NYC" may have mentioned these when he was active here (I don't remember for sure) as he was very active around there. Another article on New York by the guy who later became the head surveyor of the new World Trade Center. http://www.pobonline.com/articles/89732-web-exclusive-the-xs-and-ys-of-the-big-apple An interesting quote from that article: In the greater New York City vicinity, there are, by my count, at least 13 coordinate systems and seven elevation datums all pre-dating the state plane coordinate systems and the national vertical datums.
  18. I would agree that any time there is a significant change, I'd report again. This includes damage, reference objects destroyed so needing update, roads being renamed if they were important for finding the mark, or maybe even buildings being renamed if the location wasn't totally obvious. I probably would not report an unchanged situation for most of a decade, although we were once given a guideline of only a few years (I forget details). It's been long enough that I could start going through my list again. I'd do it just for updating, never mind the low challenge. I, too, recognize most of my finds as I drive past. That seems to stick with me better than remembering what I did yesterday.
  19. http://www.penryfamily.com/surveying/usgsdw5020.html
  20. I tried several variations on the designation. I figured BM was not part of the designation itself. The rest looked like either a 4-digit number or a letter and 3 digits, so I tried each. The standard format for a series of marks has a space between letter and number. The NGS search seems to have a wild card at the end of the characters you enter, but not at the beginning. You have to get the start correct.
  21. That would seem a good deduction from the stamping, except that it doesn't take into account the nearby marks in the same series, such as JV6913 B 197 01/01/1991 by MDSHA (MONUMENTED) A MARYLAND STATE HIGHWAY ADMINISTRATION SURVEY DISK and JV6914 C 197 A MARYLAND STATE HIGHWAY ADMINISTRATION SURVEY DISK which would indicate that the designation A 197 and the date 1991 were probably stamped by the same crew. So I don't think the quality of the stamping is evidence here.
  22. Some thoughts: It was DESCRIBED BY, not MONUMENTED BY the Maryland Highway Administration, so ArtMan is undoubtedly right that the MDSHA were the ones who measured it, by running level lines from other elevation marks. Note that they had a B 197, a C 197, and probably others in the series the same year. They worked in NAVD88, not the Refinery Datum. ESSO must have set some of these much earlier but didn't tie into the national datum then. It is worth a POOR report to NGS with the details. It seems unusual for the disk to come off the stem. I can't imagine it getting scraped off. Possibly, it wasn't in tight contact with the concrete and being driven over rocked it back and forth until the stem broke. What does the break look like? The only good way to keep the disk at the site would be to bury it alongside the post and note that in your report, but it may not stay there even if you do that. You certainly shouldn't try to glue it back in place, as that changes the elevation without making it obvious to a user. If you decide you want a souvenir, nobody will give you permission to keep it, but nobody will come after you for it. (You may get some unfriendly mail if you try to sell it on eBay, as that encourages people to steal good disks, although unfortunately no one will do anything about that either.) There are quite a few bench marks within a mile and a half, some with finds on them, so I doubt a surveyor would bother trying to figure out how much height was lost when this disk went away. If they did try to use this mark, having the disk or the exact vertical dimension of your souvenir would be important, i.e. "broken off stem flush with disk","broken off stem, leaving 0.1 inch depression in back of disk," or "broken off, taking 1/8" stub of stem with disk."
  23. On the pro forum there is a discussion that started with a historical marker sign about a C&GS baseline. It then went into gory detail as several people tried to relate the old measurements to the current data sheets. Not everyone's cup of tea to do the calculations, but scanning the discussion could help people appreciate the level of effort that went into the old measurements. https://surveyorconnect.com/index.php?mode=thread&id=318897
  24. >The things aren't where they say they are. Many bench marks have SCALED coordinates, meaning they were read off a map and not very precise. When you get close to one of those, pay more attention to the words than the coordinate numbers. Other marks (like triangulation stations) have ADJUSTED coordinates that are more accurate than your handheld GPS. The other problem is that many have fallen victim to road widening and other construction.
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