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Everything posted by trmcconn

  1. trmcconn


    Thanks, I should have noticed that. I knew that OPUS files record phase information for both L1 and L2. I wonder why tracking the L2 phase in addition to L1 makes units so much more expensive. Maybe the hard part is the logic needed to compute the ionospheric delay from the difference in phase lags, but surely there's just a formula for that. Oh well, back to the drawing board.
  2. trmcconn


    Does anyone have experience with the Juniper systems Geode sub-meter receiver? Will it produce files that can be uploaded to OPUS? A bit out of my price range (well, quite a bit) but if a used one came on the market for under 1K I'd give it serious thought.
  3. It's hard for a non-specialist to get understandable answers to basic questions in a very technical field like Geodesy. The NGS website, useful as it is, bristles with an alphabet soup of confusing acronyms and, when it comes to actual details, usually refers you somewhere else. (Some arcane article in the Journal of Geodesy, for example.) One should of course start with a basic undergraduate text, but basic (101 level) undergraduate texts have their own problems. Students at that level can't handle the unvarnished truth because they lack sufficient mathematical training, so such texts routinely lie to them or fall back on vague language in the interest of simplicity. It is hard for the reader to identify the BS without already being an expert. Here's my question: our current beloved datum (NAD83) consists of two main ingredients: (i) an ellipsoid (GRS80) that represents a crude approximation to the actual figure of the earth; and (ii) an origin and set of 3-dimensional cartesian axes. The origin coincides with the center of symmetry of the ellipsoid (anything else would be crazy) and I presume the z axis coincides with the semi-minor axis. This doesn't determine where the x-axis pokes through the "equator" of the ellipsoid, but it doesn't matter because the ellipsoid is symmetric under rotations around the z-axis. The real question is how is this whole geometric rig actually attached to the physical Earth? My understanding is that the origin was supposed to coincide with the center of mass of the Earth, but it's a meter or so off, and that's one of the reasons there's a new datum coming in 2022. Presumably the z-axis should coincide with the mean axis of rotation of the Earth. (One has to say "mean" here because the actual physical axis wanders around due to nutation and other more random effects.) What about the x-axis? Where does it poke through the (mean) equator of the earth? Is there a monument there or something? (For the older NAD27 datum, the Clarke ellipsoid was pinned down by declaring a particular point - Mead's Ranch - to have a specific latitude and longitude, and a nearby survey mark, station Waldo, to have a specified azimuth.)
  4. If the CORS network is going to define the datum, then doesn't that mean that the satellite positions are ultimately what defines the datum? How do we know where the satellites are? If the whole purpose of the NSRS is to define and determine positions on the surface of the earth it makes more sense to me to have everything ultimately depend on things that are actually stuck in the earth. I feel unmoored, and all at sea ...
  5. Ah, yes, nothing new under the sun. I should have known. I do have have a couple of related questions, though. I've been assuming that with sufficiently many independent GPS observations averaged, you can get as close as desired to the true location. This seems not to be true, though. According to GPS.gov the limiting accuracy of N averaged observations as N goes to infinity is about 2 feet. (Not nearly as accurate as adjusted horizontal control.) But I seem to remember hearing that professional grade gps recorders can get accuracy close to the survey mark accuracy by observing for 4 hours or so. If so, how do they do it? Do they use more satellites? (US + GLONAST + Chinese?) Note: the GPS accuracy of +/- 10 meters that people throw around is a different thing. It includes errors introduced by the receiver, rather than the accuracy of the source. Second, if an amateur, having mounted his/her own disk, can establish that their method is sufficiently accurate, could they apply for and get a PID? Has anybody ever done it?
  6. I was kind of bummed on my walk today thinking about how NGS support for the database of survey marks is going away soon. I love these objects, love searching for them, love the whole concept. What to do? It dawned on me: why not make my own horizontal control mark? Right in my own backyard! So here's my plan. I wonder if anybody else has done this, and if so, can they offer any advice? First step would be to purchase an appropriate brass disk from Berndtsen. Then with a number of bags of redi-mix and archived instructions from CGS I should be able to pour a "genuine" monument and mount a disk pretty easily. The hard part, of course, would be to measure the position of the disk with some kind of reasonable accuracy. As an amateur without surveying equipment (or experience!) I would have to rely on GPS. As everybody knows, a single GPS observation ( obtained, say, from an iphone) is not very accurate. The CEP (circular error probable) is about 10 meters, while bona fide horizontal controls have a CEP of a couple of centimeters. If you took multiple independent GPS observations, however, and averaged the results, statistics comes into play. With N independent observations the CEP gets divided by the square root of N. The square root of a million is a thousand, so with million observations averaged I'd get about 4 inch accuracy. I could live with that. How to get a million observations? I own a Garmin Montana GPS that can record gpx files. I'd lay that on top of my brass disk set to record a point every 5 seconds. If I let it run for maybe two hours every day I'd have enough data in a couple of years. (Of course, there are mathematical issues to deal with. E.g., observations taken 5 seconds apart would not be independent, but with so much data it should be possible to measure the correlation coefficient and adjust for it appropriately. More observations needed, probably.)
  7. That's weird, because when I download data sheets by PID in DSWorld (PID=PG0858) I only get some boilerplate definitions followed by: ----------------------------------------------------------------------------- Pid Name Lat Lon Elev O o Hv ------ ----------------------------- ---------- ----------- --------- - - -- >PG0858 MERIDIAN MARK 44 28 44. /073 11 50. XX >PG0858 MERIDIAN MARK 44 28 44. /073 11 50. NN "Mount Philo" is a different (and older) mark than "Philo". There is no tool shed there now, but I didn't know about Mount Philo when I was there and didn't look for the bolt. Another thing to do on my return visit!
  8. Before a recent trip to Burlington I scouted out benchmarks in the area for possible recovery and noticed one named "Meridian Mark" on the UVT campus. It has a pid (PG0858) and shows on the NGS data explorer as a red dot next to a sidewalk crossing one of the many quadrangles, but the data sheet is essentially empty. It only lists a pair of scaled coordinates and no history or recovery reports. I had decided not to bother with it but got desperate after failing to find two "easy" vertical controls mounted in retaining walls that no longer exist. (Something there is that does not love a retaining wall!) So on the way back to my daughter's apartment I diverted down that diagonal sidewalk just in case. There was nothing obvious, but there was a small rectangular patch of concrete flush with the ground next to the sidewalk in exactly the place shown by the red dot on Google Earth. There were no markings of any kind on it, and I decided it was most likely just the remains of an earlier sidewalk or part of the foundation for a vanished lamppost. Still, I wonder... The only thing I could find about Meridian marks is that they are temporary marks used by surveyors for calibration. Anybody have any further information about them? The trip had a happier ending. The next day we decided to do an afternoon hike to the top of "Mt. Philo", an 800 foot drumlin just south of Burlington. It is a nice mellow little hike and the view of Lake Champlain and the high peaks of the Adirondacks from the top is spectacular. I got to thinking "Hmm... Isolated peak in an area with a large amount of historical survey activity. I wonder..." I moseyed up to the exact summit and practically tripped over the monument of triangulation station Philo (PG1937). Sometimes you win a round with the benchmarking gods. (With no cell service I was unable to pull up the data sheet and didn't try to find the reference marks with a random search. But someday, I'll be back!)
  9. trmcconn


    Thank you, Bill93! As always your answer is helpful and informative. A good example of the type of mark I had in mind is station RED STONE (PG1953). It is located at a high point, but it is the high point of the University of Vermont campus and levelling surveys were also run nearby.
  10. trmcconn


    The data sheets for most triangulation stations list latitude and longitude as "adjusted" and elevation as "scaled", meaning that the latter is estimated (usually from aerial photos.) I noticed in one datasheet that scaled is replaced by VERTCON. What does this designation mean? More generally, I wonder how common it is to have stations for which both coordinates and elevation are adjusted? The location of such a station in space relative to the datum would be precisely known, and maybe such a station would be useful even in the age of GPS. (The endpoints of the original line on Long Island would qualify as examples, although they no longer exist, since they were at sea level. )
  11. I’ve often dug them up in NYS forests. I can rationalize this behavior by reasoning that (a) camping is allowed anywhere in a state forest that is at least 150 feet from a stream, body of water, or trail; and (b) making small temporary excavations is a normal part of camping. OTOH using metal detectors is illegal in state forests, probably to discourage the inevitable digging that would ensue, so the legality of this activity is murky at best. The key point is that I always re-bury the marks and try to leave the area exactly as I found it.
  12. I carry a Garrett Ace 250 with me but seldom use it. It did come in really handy on one find. RM1 was destroyed and RM2 was not found. The station monument was supposed to project 4 inches but was nowhere to be seen. I did know the exact distance from the center-line of a nearby road. That+gps got me close enough to find it with the Garrett. It was buried about 3 inches deep. Bear in mind that using medal detectors is often illegal in state and national parks and forests.
  13. Have surveys been done near the earth's poles, or to ask it another way, what is the distance to a pole from the nearest survey mark? I have a very indirect reason for asking. AZ marks are usually listed on datasheets with an exact azimuth and approximate distance. My understanding is that azimuth is measured from the surveyor's meridian to a curve on the surface of the earth connecting the surveyor and the AZ mark. Which curve, exactly? Is it a great circle, or a rhumb line (line of constant heading)? In temperate latitudes it makes essentially no difference at the typical distances of AZ marks, and probably surveyors in these regions just measure azimuth from meridian to a line of sight to the AZ mark. But near the poles it could make a difference, so if surveys have been done in polar regions the profession may have some standards that address the issue (I'm not a surveyor.) The deeper reason for my question is that I'm working on a program to generate polygonal gpx tracks connecting given points. The sides of the polygon can be either great circles or rhumbs. The program has an option to list vertices or heading/dist from a given point and I'd like to get things "right".
  14. Thanks, Bill93. Yes, put the wrong units on the size estimate. It was also odd that the RM measurements were off. Maybe the original crew measured slope distance rather than horizontal. Aren't the RM measurements usually horizontal? Or do I have it backwards?
  15. Last week on a business trip to Long Island I had a free afternoon so (what else?) I went benchmark hunting. Turns out there is a little plot of undeveloped land smack in the middle of densely populated Nassau county, and that this little plot of undeveloped land (90 acres or so) just so happens to have a triangulation station smack in the middle of it: BETH-KU2634. The land is owned by Nassau county and rented by a nice couple who operate a small truck farm there. They kindly gave me permission to visit the site. The data sheet says the station monument projects 2 inches but I had to dig down through about 2 inches of accumulated leaf litter to find it. One unusual feature of the site is worth mentioning: The station monument sits in the center of a roughly 10 meter square whose corners are marked by embedded chunks of metal that stick up about 6 inches out of the leaf litter. They resemble enlarged versions of the kind of angle irons you find in hardware stores for bracing corners of windows. They are very firmly embedded in the ground, sharp edged, and could inflict a nasty cut on an unwary passerby. The data sheet describes these as' "anchor stubs" and lists precise distances from each oneto the station mark as a kind of supplement to the reference marks. (I didn't find those, alas, but they're undoubtedly still there, buried under the same leaf litter that blankets the whole hilltop.) My guess is that these are remnants of the light platforms built when Beth was monumented. If so, I wonder why these were left in place, or, conversely, why you don't find them more frequently at other sites - these were the first I've seen. (Tried to upload a jpg under 2 meg but it keeps refusing to upload - no idea why )
  16. Yes, a database search on the AZ mark pid yields a fatal error of "No Marks Found" but coordinates are nevertheless given at the bottom of the page with a code of DD meaning "No descriptive text available". I hadn't realized that AZ marks are assigned scaled coordinates like vertical controls. Could be useful in future searches.
  17. Dsworld has a feature (don’t know if it’s new or old) that lets you pick a state and county and it will launch google earth and fly in above the selected county with every survey mark in that county filled in. I noticed it even includes azimuth marks, which prompts a question: since azimuth mark positions are very roughly known at best, what information does it use to compute their position? Today I managed to squeeze in a little benchmarking after visiting a high school in western New York. The goal was to recover station Boyce (Cattaraugus county NY) PID NC1013. The evening before I prepared by entering the coordinates for Boyce and its RMs in my handheld GPS. I also tried to predict the most likely location for the AZ mark by drawing a line at the proper azimuth in Google Earth and noting where it crosses a road at approximately the right distance. (In my experience, AZ marks are usually next to roads. If you get to pick where to put one, wouldn’t you put it next to a road?) Boyce was set by CGS in 1935 and recovered by a CGS party in 1964. The recovery report noted “the direction to the azimuth mark was found to be 20 degrees higher than the original direction and after double checking we feel that the original direction was an error.” The azimuth listed on the datasheet is 91 degrees, but there is no indication whether this is the original figure or the corrected one. Accordingly, I entered two waypoints for the AZ mark: one based on an azimuth of 91 degrees, and one based on 111 degrees. I considered the first possibility the more likely one since the original party had described the AZ mark as Northeast of the station, which would make sense only if the original reported azimuth had been 71 degrees. I did indeed find the AZ along a town road at 91 degrees azimuth from the station. The curious thing is that DSworld plots the Boyce AZ mark at 71 degrees azimuth and a distance of 1.8 miles from the station near a different town road. The 71 degrees makes sense, but the 1.8 miles is way off from both the original survey and follow up estimate of about ½ mile. (I found the AZ at a distance of about .78 mi.) The question remains: what data is DSworld using to compute the position of this AZ mark? I can only conjecture that it is using some very old database containing the 71 degree figure before it was corrected to 91 degrees based on the 1964 report. But databases didn’t exist in 1964. Did it take 40 years for the error to get corrected? Strange. After the 1964 recovery the only report is one from 2002 by our good friends in the Power Squadron: All marks recovered in good condition.
  18. What's the meaning of first order, second order, etc in a datasheet? Presumably first order marks are more important somehow. On a possibly related note, somebody recently posted a "triangulation diagram" which was quite fascinating. Some marks connect to only 3 others, while some may connect to 10 or more. Graph theorists call the number of connections the "degree" of the vertex. Is the degree recorded somewhere in the datasheet? Are marks of high degree "more important" than low ones? (I could argue that one either way.)
  19. Light travels slower in denser air than in less dense air, an effect known as refraction. If a beam of light is directed parallel to the surface of the earth, the parts of the wavefront in the denser air closer to the surface of the earth will be slowed relative those further away, causing the beam to bend slightly in the direction of the curvature of the earth. One effect of this is that distant objects appear slightly higher in the sky than they really are. Faraway ships at sea, for example, can be spotted by observers when they are still below the horizon. Since the deflection is vertical, this effect should not be a problem for triangulation surveys. Indeed, there could be a slight benefit in that lights over station marks could be mounted a bit lower than if the survey were done on, say, the Moon. On the other hand, observations made in the presence of horizontal pressure gradients should experience horizontal deflections. For example, if you're looking north with a high pressure system to your West, then light will tend to curve around the high, deflecting the apparent position of objects slightly to the East. I wonder if this effect is significant at the level of accuracy of our historical surveys, and whether the surveyors corrected for it? Could this be the reason relatively nearby azimuth marks were established, rather than relying on quite distant objects - church steeples and such?
  20. I use BenchMap 2.0.09 for android to "shop" for marks, dsworld to find coordinates of RMs, and Google Earth as the killer app for finding Azimuth marks: put a thumbtack at the station mark coordinates, extend a ruler line at the proper heading, and see where it crosses a road at the approximate distance reported on the datasheet. (AZ marks, in my experience, are usually next to roads.)
  21. I've found the Rolla, MO USGS office to be very helpful. I asked them for a copy of a datasheet in my area and they sent me copies of datasheets for all their marks in the same county. One thing to be aware of with USGS data sheets is that the datum of reference is not automatically updated as with NGS data sheets. Coordinates for the mark I was looking were relative to NAD27, and that made a difference of about 100 yards.
  22. If MH0026 is a vertical control, do you have to measure down to it from the flat plate and include that in your submission? Or does machine/software compensate? Boy, it sure looks flat around there!
  23. Probably you won't find that mark in the NGS database since it's not an NGS control. NGS is under the Department of Commerce and your mark was placed by USGS (Department of the Interior). I've found USGS to be very helpful. Try writing to their Rolla, MO office at mcmesic@usgs.gov.
  24. My understanding (as a mathematician, not a geodesist!) is that the ellipsoid is a simple surface chosen to approximate a complicated one (the geoid) over a limited area. A datum consists of a choice of shape and size for an ellipsoid as well as the location of its center and orientation of its principal axes relative to the celestial coordinate system. In the case of NAD27, the shape and size was that of the Clarke ellipsoid. The location and orientation was determined in part by requiring the two surfaces to cross at Meade's Ranch and also requiring them to osculate there, i.e. to have the same direction normal (=vertical) to both surfaces at the crossing point. That still leaves a degree of freedom for the ellipsoid to be rotated around the normal direction vector, and this was chosen to minimize the discrepancy (in least squares sense) between resulting geodetic and astronomic measurements at an existing network of stations.
  25. Now that I look at it, my submissions have clearly been edited, so even the automated dsworld submissions are vetted. Probably, this is a good thing. In several cases I submitted GPS coordinates for an AZ mark and this got dropped from the update. The very first thing I do when I find an AZ mark or a scaled mark is make a GPS waypoint. I figure this it at least some infinitesimal positive contribution to human knowledge. A GPS reading is better than a scaled guesstimate, even on a bad day. I wonder why they seem not to want them?
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