I have a sample project that I want to calculate ground point locations by hand (well, using my hands on a keyboard with CAD, too).
The property is almost rectangular, ~ 420'x130'.
A recent survey, stamped by a PLS, shows the POB as a N/E coordinate from the nearest "triangulation station" or published NGS PID.
I set up a static GPS on site over an unknown point on the site and got a solution on that from OPUS.?ÿ The 2 points (PID and Static point) are about 17,000 feet apart.?ÿ Same with the PID vs the POB. So I have 3 fairly accurate locations in lat-long?ÿ and SPC.
If I place my property in CAD at the SPC coordinates (derived from a trig offset POB) and then place my static point at the published OPUS SPC coordinates, it starts out looking like my static point on the map is about 40' west of where I believe it to actually be.?ÿ Sump'm aint right.
I took an average combine scale factor and applied it to the 17k x 17k x 350' triangle created by the 3 points and get a ground adjustment that "moves" the static point about 0.5'.?ÿ Map convergence factors at the distant points are 0-9-58 to 0-9-10 or so?ÿ a difference of about 48" which I haven't taken into any account.?ÿ I don't know if I need to consider laplace, but it doesn't feel like any of these factors would get my static point anywhere near where I thought it should be (based on standing on the property and looking at where the unknown point was with regard to the surveyed
building to the north).
I goofed with a couple NGS tools from their tool kits and didn't see anything that would really help me take any steps I needed.?ÿ Might be wrong about that though.
Question is: given 3 points like that in SPC, what's the procedure to get your unknown point placed correctly on the property in ground coordinates?
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Where did you get the convergence angle??ÿ Isn't it this mark KAILUA?
TU2968;?ÿ SPC HI 2 - 57,366.056 533,654.158 MT 0.99998066 +0 06 54.4
or this one KAILUA RESET
TU1091;SPC HI 3 - 24,077.370 523,340.259 MT 0.99999673 +0 04 55.4
Man, there are so many similarly named ones it's confusing.
I have been surveying in Hawaii for almost 20 years, the local coordinate shown on the map/deed does not always correspond to the real northing and easting distance from the trig station, it would be nice if it did. Your best bet is to localize on the found corners and go from there.
Where did you get the convergence angle??ÿ Isn't it this mark KAILUA?
TU2968;?ÿ SPC HI 2 - 57,366.056 533,654.158 MT 0.99998066 +0 06 54.4
or this one KAILUA RESET
TU1091;SPC HI 3 - 24,077.370 523,340.259 MT 0.99999673 +0 04 55.4
Man, there are so many similarly named ones it's confusing.
It can be confusing.?ÿ KAILUA trig TU2968 is on Maui while mines on big island (Hawaii), Kona side. This one (by my limited experience) references was probably casually called by the surveyor and is really the KAILUA RESET, which was obliterated pre '72 but is still referenced by PID TU2594 and by many local maps here. I got the convergence angles and CSFs for KAILUA RESET from the PID and for the unknown static point, from OPUS.
What I was wanting to do, just as an exercise, is assume I have access to the internet and CAD, but no fancy pants magical datum transformation apps like TBC or StarNET.?ÿ I want to place a benchmark point on this map from my static solution.?ÿ Using SPC to set up a projected map of the 3 points, and assuming (probably incorrectly) that the N/E coordinate POB from the stamped map is SPC as well - because he's referencing a PID with calculated HI zone 1 coordinates.?ÿ With that info I should be able to place the BM in the boundary and show all 3 as projected points.
If I want to show the 2 on the map as "ground points" - I know the difference would be slight - what calcs/CAD/reference tools could I use to do that precisely?
I imagine I would use the site CSF (.99997006 from OPUS) and just scale the distance between POB to Static BM, "moving" the BM slightly south, but how about the OPUS convergence, -0 9 33.7 ??ÿ Would I also theoretically apply that to the relative Az between the 2?
I know this is an inefficient and practically useless exercise but I just wanted to do it and feel it for myself.
Tangentially, where does Laplace come in??ÿ Is that used to get geodetic to SPC (ie, already calced by NGS processing)?
Mahalo!
@antcrook Thanks.?ÿ I suspected as much, but it's good to hear from your experience on it.?ÿ Is that just lazy surveying or is it of not enough significance to make any difference??ÿ With GNSS, which this firm uses, it seems like an easy reference to make accurately with a note on the map describing your tools and data sources, or, if he's referencing a deed or prior record that calls that POB coordinate, seems like that should be noted as well.?ÿ I don't know processes and statute well enough to know what's a responsible standard of practice.
I believe the Laplace correction is only used to relate astronomical observations to geodetic ones. It is dependent on the tilt of the geoid at your location, which is not part of a GNSS solution.
I'm pretty sure the convergence angle for one SPC zone is not usable in another SPC zone, so TU2594 would be relevant and the others I looked at that not for work on the big island.
The data sheet for TU2594 says it was reported destroyed 50 years ago, and apparently its old position has been transformed to the 1993 datum realization.
Convergence is a simple calculation. In TM it's sin lat times delta long. In Lambert it's sin of lat sub o times delta long.
There are more cimplex ways to do it but I've never seen an appreciable difference.
It's been a long time since I've applied Laplace Corrections, or second term corrections to a survey traverse. Depending where you are will have an impact if you'll even consider doing the Laplace Correction. The Laplace Corrections can be found by asking the NGS for them for your location. Lat, Long, Height will be what they want,,,,,,I think. I haven't gotten one since before working with Heights.?ÿ
However, since you're on the big Island and some consider Mauna Kea to be the worlds largest mountain if the underwater portion is included then Laplace corrections should be interesting. I would like to hear what you find out if you obtain some for your location. I know from experience that they can be helpful, but you need some distance along your traverse to make much of a difference. The last time I applied them I was working along the eastern slopes of a mountain range and when applied the traverses closed better.?ÿ
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My post above was misleading about where Laplace correction is useful. It isn't used with GNSS data. But I tend to forget it may be used in other situations, because around here it is pretty negligible unless you have a large angle above the horizon, such as astro shots, and even there it's only a few arc seconds.
But as Moe points out, when your total station is pointed up or down a large angle from the horizon, and the deflection of the vertical is large such as near a mountain, then it may indeed be significant.
I see that it is indeed large for the station TU2594 mentioned above. I don't know if TS measurements at the site would use large vertical angles, but this does need to be kept in mind.
?ÿ?ÿ?ÿ?ÿ Xi?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ Eta?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ Hor_Lap?ÿ?ÿ?ÿ Deflec99
-11.67 ?ÿ -67.66?ÿ?ÿ?ÿ 24.15 arc sec