Help with Network RTK in NAD27
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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