Hi I would like to start by saying I am NOT a surveyor. I have a top con hiper v base and rover that I use for my job sites (I am a construction supervisor). I usually localize to control given to me by the surveyor. I have also been getting into drone topography work. Most of the time the job already has control so the surveyor gives me the text file of control and I localize to it then shoot my own ground control points. However, a builder wanted me to give them a new drone topography of a property that they own where there is no control. They want me to give them a bid to do the dirt work on the job, and I told them since the job was designed well over 10 years ago I want to do a new topography to supplement and verify the existing contours on the plan I was given. To complicate matters further, the job is in "NJPCS (NAD27)" I take that to mean the horizontal coordinate system is NJ State Plane NAD 1927 and the vertical datum is NGVD 1929, however I didn't see anything that says NGVD1929. So I went out into the field and brought my topcon hiperv rover and connected to the topcon network. Under "Control Options" I selected "Known Projection/ Datum" and selected "Projection: SPC27-New Jersey" "Datum: NAD27" and ELL CLK66 automatically came up. So I started shooting ground control points and soon realized my elevation was 10 feet too high next to the existing road, I was shooting 50 and the plan said 40. I realized that maybe the height I was getting was the ellipsoid height and not the orthometric height, so I applied a geoid model (unfortunately the only one I had was 12a) and my elevation went to 40 so I figured everything was good. When I went back home and imported my points,(into google earth and pix4d and my takeoff program) they seem to be off by 20 feet horizontally. My question is: when I was reading a elevation 50 was that the ellipsoid height "h"? and when I applied GEOID12A did I make a irreversible mistake? I read that 12A is designed for NAD83 and should not be used for NAD27. How should I have gotten the orthometric height "H" while network surveying in NAD27? Is there a way to undo applying a geoid model? Is my data unuseable?" If I go back out and re do it, how should I shoot my ground control points? I am most likely going to tell the builder that I need to have control from the surveyor for this job, then re do it, but I would still like to understand what happened here. THANK YOU.
I guess my real question is: can I get my points back to WGS84? I use pocket 3d on a FC-5000. When I shoot points, is there some meta data that includes the original WGS84 reading, or does it only record the XYZ in your given projection? I opened TopCon 3D Office and tried to convert coordinates back to WGS84 but they still don't match up when I plot them on google earth to where they should be.?ÿ
There are quite a number of rocks to look at under here....?ÿ?ÿ
- This data may be retrievable, but it would take a few software tools you probably don't have access to, and, frankly, some knowledge you don't have. Probably your best bet is going to be to return to the site, get things set up right, and do it again.
- In 2019 it is rarely appropriate to use NAD27 for anything, except in some very specific circumstances.?ÿ Not sure why the surveyor went that way. So much so that I have to wonder if the note is in error.?ÿ
- Yes, Geoid12 is intended for use with NAD83(2011). Geoid12A is not the current version, that is Geoid12B. But there should be no significant difference between the 2 in NJ.
- I doubt that there is any Geoid model appropriate for use with any flavor of NAD27. Certainly none that the NGS maintains. Probably you would just use a standard geoid seperation for the site, determined with reference to a local benchmark, in lieu of a geoid model.?ÿ
- For an area as small as a building site you could just go with any geoid model , or without a geoid model at all, and add or subtract an offset to the elevations as needed to match local conditions and not be far enough wrong to matter.?ÿ?ÿ
- Some states define doing topographic mapping, with or without a drone, as practice of professional land surveying (or professional photogrammetry). I'm not familiar with how NJ feels about that. Probably as long as you operate on a construction site and restrict your self to layout and quantities you are in the clear, but when you are off collecting for design, you might get yourself in trouble. Is this worth it??ÿ
- Punctuation! That's one long paragraph. Hard to read.
This site has a lot more traffic on weekdays. You will probably get more responses on Monday morning.
Geoid12a vs Geoid12B explained by NGS here.
It says there is some differences from Geoid12 in the southern US, but 12a and 12b are identical except in Puerto Rico.
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Thank you for the response. I actually made some progress, I used Topcon 3d Office to convert my coordinates back to WGS84 using the same geoid file, so far I have done 3 of them and they look perfect. My plan is to now do the whole photogrammetry in wgs84 and then find a way to bring it into my takeoff. As I said I am not a surveyor, I would prefer to act as a subcontractor to a surveyor, meaning someone asks the surveyor for a topo, the surveyor sets control then I go fly it, give my info to surveyor he provides final product. I believe that in this case that is what should be done. However, I do progress topos for my sites and pile volumes which I should be able to do without a problem.?ÿ
Since you are not a surveyor, you are likely not familiar with the issues of referencing your positional information to "WGS 84."?ÿ Assuming your work is intended to have a relatively high level of positional accuracy (lets say anything better than 2m/6.7 ft) than you should cite the source of your "WGS 84" positions.?ÿ The U.S. Defense Department defines and maintais the system and they do not provide any form of foundation control, either passive or active with any level of positional accuracy to the civilian community.?ÿ Your data should be reference to the datum from which it was originally referenced - I would assume the Topcon network you referenced is actually providing NAD 83 (2011) epoch 2010.00.?ÿ If ultimately you do think you have "WGS 84" then the coordinates should be defined it their appropriate iteration of the system along with the epoch (year and decimal date) of the observation.?ÿ The current realizations is called WGS 84 (G1762), therefore an observed performed today would be WGS 84 (G1762) epoch 2019.03.?ÿ Additionally any good documentation should also provide the positional uncertainty of any coordinate value, hopefully at the 95% confidence (2 sigma level).?ÿ Without such information it makes providing any transformations to other systems such as NAD 27 basically a wild a## guess.
You can't compute a true projection from WGS84 to NAD27. There are two methods, a local and international. One will compute within a foot accuracy, the other method will be within 30ft. Vertical datum has nothing to do with NAD27. CORs in NJ are on NAD83(2011) and NGVD88(GEOID12b). You may be wanting to get to vertical datum NAVD29. These old datums can only be achieved by tieng into original monuments. Translations may get close, but do not work.
Also you are out of your league here. Providing topo for a subdivision is likely required to be done by licensed Land Surveyor in NJ. Advertising on this board to survey without a license may get you a Class E felony. You may want to check with state board.
I very much appreciate the help here, first of all the topo I am doing for this job is for my own purposes to provide a quantity takeoff to the builder. My sole purpose is to verify that the existing topo hasn't changed over the years and give myself a nice digital contour layer to work from so I don't have to trace every line on the plan. If I found that there was a major change to the existing ground then I'm sure a legitimate re-survey would have to take place by a licensed surveyor.?ÿ The data that I create is not going to be used by anyone but me for this specific purpose. The job has already been designed and engineered, it is just old. If this is illegal for me to do then I was unaware of this and won't do it.?ÿ
As far as my problem goes, I think that I need to stick to getting control from the surveyors and localizing to that before doing any jobs, since converting to and from different coordinate systems is by far the most convoluted, overly complex, head ache inducing endeavor I have ever attempted- and I would very much like to leave it to the professionals.?ÿ
This entire thread is an example of why a professional license is required. If you make an error in your topo, it could end up costing the developer $$. As far as you justifying this is for personal use, that would only apply if you owned the land you were surveying.?ÿ
Since mentioned flying a drone for commercial use, I see you understand the FAA requires a Part 107 sUAS remote pilot to perform this. Very big fines can be issued by FAA. This is more commonly known today, I see you already have a Part 107 and hope others understand this before advertising in a public forum.
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Yes I have a part 107 license, and like I said I am going to go back to working in tandem with a licensed surveyor so they can sign off on my work and give me control on the job.
There is another disturbing issue here. NAD27 assumes that the ellipsoid and the geoid are the same surface; ellipsoid height is everywhere zero.?ÿ
Geoid 12A should show something like -31?ÿmeters in New Jersey.
So, how did using Geoid 12A deduct 10 feet from the elevations? And why would the software permit using a geoid with NAD27?
As a reference to anyone who may have a similar problem, I have been messing around in Topcon 3D Office to try and figure out what happened for my own curiosity. I was able to convert my points to WGS84 Lattitude Longitude and Ellipsoid Height by selecting PROJECT-UTILITIES-COORDINATE CALCULATOR and selecting GRID-GEODETIC (INVERSE) option. I entered in my grid values and used the same geoid file and it gave me an output labeled WGS84 LATTITUDE LONGITUDE and ELLIPSOID HEIGHT. From there I am able to start over and replicate my results. So when I take the point I shot where I noticed the discrepancy and enter in the WGS84 values and use the GEODETIC-GRID conversion and select SPC27 NewJersey as the projection it gives me a Northing Easting Elevation and seperation from wgs84 ellipsoid. Under the elevation it came up with: 50.264. When I load the geoid file 12a it changes the elevation to 41.102. I used vertcon to see what the difference between NGVD29 and NAVD88 is for that point and NGVD29 should be 1.165' higher. So I am at a loss for what happened there.?ÿ
The generally accepted method to use existing NAD27 control SPC values is to calibrate to them. The more advanced way would be to create an LDP that closely mimics the NAD27 control. As long as the control is consistent (internal angles, scale factor) an LDP can be designed for it.
Geoid 12 can be applied also to the control elevations and if it was leveled correctly that will work. There seems to be some idea that only NAD83 (2011) ellipsoid heights can result in NAVD88 elevations using Geoid12. What also works is applying the Geoid height to the NAVD88 elevation to get the ellipsoid height. If you have NGVD29 elevations applying Geoid12 will result in an ellipsoid height that is smaller or greater than the NAD83 ellipsoid height and no one will care. The resulting file will carry NGVD29 elevations forward without any issues with the exception that Geoid models aren't all that accurate anyway.?ÿ
I do not know how anyone can survey using WGS84, it isn't available. Some programs call the geographic basis WGS84, but how do you get it? Look at an OPUS report, NAD83 or IGS08 are your options in the states.?ÿ
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I do not know how anyone can survey using WGS84, it isn't available. Some programs call the geographic basis WGS84, but how do you get it? Look at an OPUS report, NAD83 or IGS08 are your options in the states.?ÿ
Seems like a poor choice, but isn't WGS84(week#) essentially IGS at that epoch??ÿ So you should be able to use HTDP to shift.
It would be good if softwares were a little clearer on what they were using.
When I say WGS84 I am literally quoting from Topcon 3d Office. When they say WGS84 or NAD83 there is no mention of which version they are talking about which makes things very confusing. I also know that the phantom 4 rtk geotags images in wgs84, or at least that's what it says. The Phantom allows you to connect to the topnet (or any network rtk) to get rtk but then if the topcon network is giving corrections in nad83(2011) and the drone is reading wgs84, and if your saying that the topnet cannot correct wgs84 then that looks like an issue.?ÿ
@MightyMoe. Since the NGVD29 elevations are orthometric heights, using them with a geoid to approximate nearby NAD83 ellipsoid heights ought not cause great consternation. But I don't think that was what James was trying to do.
It's a strange software button that applies a 31-meter height adjustment to some sort of elevation and gets a 3-meter change.?ÿ
I wonder if the coordinates and elevations were interpolated in manner from an old USGS Topo map.
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@MightyMoe. Since the NGVD29 elevations are orthometric heights, using them with a geoid to approximate nearby NAD83 ellipsoid heights ought not cause great consternation. But I don't think that was what James was trying to do.
It's a strange software button that applies a 31-meter height adjustment to some sort of elevation and gets a 3-meter change.?ÿ
I wonder if the coordinates and elevations were interpolated in manner from an old USGS Topo map.
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NAVD88 and NGVD29 will be relatable using a shift (approximately -2.4' in my part of the world) and if the area is small enough (a few square miles) then the shift should be a constant number. The Geoid model can be mapped as a contour model that models the difference between the ellipsoid height and the orthometric height, over a few square miles that number can change quite a bit and the contour map using say a .1' interval will have a number of contour lines where the same .1' interval mapping the shift between 88 and 29 will show nothing.?ÿ
The point being is for a small area the difference between 88 and 29 should be a constant number. This allows the use of the latest Geoid model by either simply occupying the 29 bench mark and using the model, or applying a constant shift number to the 88 number, applying a constant shift number is probably the best way when using a network rover.?ÿ
There is another disturbing issue here. NAD27 assumes that the ellipsoid and the geoid are the same surface; ellipsoid height is everywhere zero.?ÿ
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Good point, almost. It is Geoid Separation?ÿ(a.k.a. geoid height) that is zero everywhere in NAD27, not ellipsoid height.?ÿ ?ÿ
The point being is for a small area the difference between 88 and 29 should be a constant number. This allows the use of the latest Geoid model by either simply occupying the 29 bench mark and using the model, or applying a constant shift number to the 88 number, applying a constant shift number is probably the best way when using a network rover.?ÿ
For a small area the geoid seperation is going to be virtually constant as well. So you could just go without a geoid model at all - especially for dirt work - and just use an offset.?ÿ The trick might be if a remote benchmark was visited.?ÿ ?ÿ