My other thread on a USGS benchmark (as a possible component of a future trial "SPC Grid to Ground" traverse opened up a can of worms (for me at least). I'm finding that control is not necessarily control is not necessarily control.
I've found a number of candidates, about 1.3 miles apart from the farthest. They include an "NGS GPS", a "Vtrans GPS" (Vtrans is Vermont's Agency of Transportation), and a USGS benchmark (which I'm pretty sure by now is useless for my purposes).
But, putting aside for the moment the question of whether two marks at almost the same elevation, and perhaps 3/4 mile apart in longitude are far enough apart to be a proper test of skills, how do I interpret the horizontal accuracy of these marks? The data sheets (even on the NGS mark) don't have the "FGDC Geospatial accuracy and Distance" section on them. Are there simply assumed standards for each type of mark?
Not to barge in here, but this seems like a classic use for OPUS Projects if you have enough equipment. If not, call your county surveyor or maybe some peers in your area to assemble adequate gear. NGS is your friend and they do it all, once you attend the 1 1/2 day class. Well worth it.
Good luck
> Not to barge in here, but this seems like a classic use for OPUS Projects if you have enough equipment. If not, call your county surveyor or maybe some peers in your area to assemble adequate gear. NGS is your friend and they do it all, once you attend the 1 1/2 day class. Well worth it.
>
> Good luck
Hey Wayne...You might not be up to speed on my "project", but it's way old school still...not studying GPS yet. I thought it would be a good exercise to traverse (with a total station) from one "known" point to another, then apply scale factors etc. to the traverse, adjust it etc. to see how close I could come to "reality". Using OPUS (or fellow surveyors with $50k worth of gear), kind of defeats my purpose right now.:-)
Single point to single point requires azimuth shots at each end, or near each end. Polaris and some other commonly visible points if possible. NGHS data sheets should indicate any Church spires or other azimuth points. For your purposes you could use new cell towers that appear on geo-referenced ortho photos as a last resort.
email me if you need help on the latter.
Paul in PA
You could traverse between known points and see how the distance compares, even if you didn't know true azimuth.
If you use a tower for reference, you need to know its position to some moderate accuracy. Tower hazard lights may work, especially if you can see a distant one from multiple places on your traverse, but I think some towers move by tenths of a foot when the sun is warming up one side.
Cell tower antennas are problematic because it is hard to be sure you are using the same one of the three, and if you try to take an average of the three azimuths you aren't finding the center nor the same point as an average from another setup.
If you are lucky, maybe you could call Vtrans and see if thay have an adjustment report or some metadata for their points from which you could derive a network accuracy classification.
> You could traverse between known points and see how the distance compares, even if you didn't know true azimuth.
>
> If you use a tower for reference, you need to know its position to some moderate accuracy. Tower hazard lights may work, especially if you can see a distant one from multiple places on your traverse, but I think some towers move by tenths of a foot when the sun is warming up one side.
>
> Cell tower antennas are problematic because it is hard to be sure you are using the same one of the three, and if you try to take an average of the three azimuths you aren't finding the center nor the same point as an average from another setup.
Yes, I think that was my original idea: distance only. The idea was to do the traverse, then apply SPC scale factors to the distances, and see how close I can come. I didn't think I even needed the azimuth between the points to do that.
If it is the case that I need azimuths, or that it would make the traverse more accurate, at the north end, up the road a bit is the first mark I found (and mentioned in another thread on State Plane Coordinates...Its an NGS mark with Uber accuracy ( I think), and it's paired with an azimuth mark down the road in the direction of the proposed traverse. I could start on the azimuth mark, back sight to the primary one, then turn and go on my way.
On the other end, there is another ngs mark nearby but I think it'd be visible from the end point. so I could get an azimuth there too.
Not really sure I understand how it factors in though.
You could just test it out for yourself by running a careful traverse between the two stations. Using ground distances, (a scale factor of 1) and a bogus back bearing to get going, traverse from one station and close into the other, then rotate your survey onto the computed bearing (inverse) between the two stations. Inverse your measurement (ground) between the two stations and compare your measured distance against the inverse distance between two given sets of coordinates for the stations (grid). If your spot on with your traverse and given coordinates for the stations are tight, you should find the difference (misclosure) in distances correlates very closely with the scale factor to go from ground to grid in your area and your survey is now oriented to the grid azimuth between the two stations. Kapish?
Just because I'm paranoid, doesn't mean they aren't out to get me.
> You could just test it out for yourself by running a careful traverse between the two stations. Using ground distances, (a scale factor of 1) and a bogus back bearing to get going, traverse from one station and close into the other, then rotate your survey onto the record bearing (inverse) between the two stations. Inverse your measurement (ground) between the two stations and compare your measured distance against the inverse between two given sets of coordinates for the stations (grid). If your spot on with your traverse and given coordinates for the stations are tight, you should find the difference (misclosure) in distances correlates very closely with the scale factor to go from ground to grid in your area and your survey is now oriented to the grid azimuth between the two stations. Kapish?
Wow. Uh. I think I need to stay in a Holiday Inn Express, then read it again.:-D
But I think what you're suggesting is exactly what I'm going to try to do.
Do I even need a bogus back bearing if I have a bona fide back bearing. The first mark of the traverse is the Azimuth pair of another nearby NGS station.
> I could start on the azimuth mark, back sight to the primary one, then turn and go on my way.
If you're really interested in refining the accuracy of your survey, you'd be better off learning how to do astro observations for azimuth. As inexpensive as optical theodolites have become, you should be able to find a decent one-second theodolite for under $500. All you'd need in addition to that would be a sun filter for the eyepiece, a digital stopwatch, and a shortwave radio for time signal (or possibly an iPhone), and a recreational-grade GPS receiver. It should be possible to get astronomic azimuths with standard errors in the range of 1.5" to 3.0" by solar observations that can be converted to near-geodetic azimuths and then to grid azimuths of the SPCS.
> If you're really interested in refining the accuracy of your survey, you'd be better off learning how to do astro observations for azimuth. As inexpensive as optical theodolites have become, you should be able to find a decent one-second theodolite for under $500.
Kent:
Thanks for the suggestion. I had a nice Topcon optical Theodolite last summer but sold it to get my current instrument; probably should have held on to it; it was cheap.
I've done one polaris shot and a couple of lunars, with less than stellar results 😉
Knowing how to do something though is not the same as competency. I've got a solar filter for my TS on my list of "To Gets". I'd need one of those no matter what instrument I use. But the problem is that given so much to learn, I've got to focus and get a bit more methodical.
What started as a desire to learn simple coordinate surveying in the context of a local topo survey morphed into using SPC coordinates, which morphed into possibly traversing from a NGS horizontal control 2.5 miles away, which led to learning how to locate control information, which led to learning how to read the station data sheets, which morphed into designing a traverse from two different NGS stations, etc. As with the suggestion to use/learn GPS and OPUS, celestial azimuth determination might be a good one to practice this winter (heck, I can probably do it from my deck). I'm just not sure it makes sense to try to combine it with the "station to station"/SPC traverse I'm planning.
Depending on the progress of that, if I get a filter, and get the routine down, I might be able to throw it in there at one end (or both).
I realize it's a journey, not a destination and I'm really enjoying all of the input from you and the folks here.
As mentioned in another post, I find Polaris much easier, particularly due to the fractional-second time accuracy required for good solars.
> As mentioned in another post, I find Polaris much easier, particularly due to the fractional-second time accuracy required for good solars.
The practical advantage of the Sun, though, is that it can be observed during normal daylight hours at an altitude significantly lower than Polaris.
Performing a distance only tie between two monuments and rotating to the inverse can be flawed as there can be a significant angle error in the traverse that may not be apparent. I have found Trailing Edge solar observations to fall within a 6" tolerance. The key is to go to a monument with a published reference mark and practice the proceedure until you have confidence. In about three sessions, you will have it. Of course, I am going from memory since I started using GSP about twenty years ago.
> The practical advantage of the Sun, though, is that it can be observed during normal daylight hours at an altitude significantly lower than Polaris.
In this case, Polaris has the advantage of being observable with a total station instrument without additional fittings. Granted the zenith angle will be high, but still do-able.
> In this case, Polaris has the advantage of being observable with a total station instrument without additional fittings. Granted the zenith angle will be high, but still do-able.
Would he even be able to observe Polaris in Vermont using his instrument without a right-angle eyepiece?
> Would he even be able to observe Polaris in Vermont using his instrument without a right-angle eyepiece?
I've done it in British Columbia. It's not easy or comfortable. But it can be done. Have a chiropractor standing by.
you need two points on each end (preferably all on the same datum) or as others have said a solar/polaris tie. Your traverse will have to be adjusted for both angles and distances to have any meaning at all. Using OPUS or (maybe RTK) to setup the four points, is about the best/easiest way to get the control you need, other than that, I would find a more suitable location that has more monuments to get started from
> how do I interpret the horizontal accuracy of these marks? The data sheets (even on the NGS mark) don't have the "FGDC Geospatial accuracy and Distance" section on them. Are there simply assumed standards for each type of mark?
>
If I may hijack my own thread back 😀 ....
Would someone please confirm for me that I'm reading this correctly:
(p.s. I've read all 6 pages of the "FGDC accuracy standards" document and it's of some, but not a lot, of help)
1. I should be reading the "Median Local Accuracy" line...i.e. .95 cm horizontal, 1.41 cm elliptical, because I'm really working locally between the next proximate station.
2. That if I do the traverse perfectly, I would expect to be within the combined uncertainties of the starting station (this one), and the end one (which, so far, I don't have the same kind of data; trying to get it from VTrans...it's a GPS site.) This, of course, after application of the grid factors, convergence, adjustment of the traverse etc.
> Would he even be able to observe Polaris in Vermont using his instrument without a right-angle eyepiece?
Done it. No Problem. My Rod Girl (my wife), gives me a neck massage afterwards.:-D