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Help with Network RTK in NAD27  

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(@jimscheideler)
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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.

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Posts: 9
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(@jimscheideler)
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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. 

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Mark Mayer
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(@mark-mayer)
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There are quite a number of rocks to look at under here....  

  1. 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.
  2. In 2019 it is rarely appropriate to use NAD27 for anything, except in some very specific circumstances.  Not sure why the surveyor went that way. So much so that I have to wonder if the note is in error. 
  3. 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.
  4. 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. 
  5. 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.  
  6. 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? 
  7. 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.

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Bill93
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(@bill93)
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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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(@jimscheideler)
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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. 

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