Ground Datum Use

Generally speaking network are adjusted to certain datum, but they all broadcast WGS84 (Lat, Lon, Ellipsoidal Height.In other words, the network receivers track the same information from the satellites than your Emlid Reach does. It is the software that takes care of transformations. In RTKLib (and Reachview on the RTK side) the options are limited to WGS84 and a couple of vertical datums (ERMS96 and something else I can’t seem to remember(maybe ellipsoidal height?). So, after you get your coordinates processed in RTKlib, then you will need one of the methods others in this thread have highlighted to be able to fit the coordinate system you are intended to use or your final deliverable coordinate system is.

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NAD 83 is based off of locations only on the North American tectonic plate. WGS is based off the entire planet. They differ by a few cm per year.
From a surveyor’s perspective, it doesn’t matter so much what datum you’re on because GPS standalone is somewhat meaningless on the ground. 1 arc second east-west is not the same distance as 1 arc second north-south. From a survey perspective, physical distances in feet or meters is what matters.
Also, if you’re dealing with property lines, you need to work from monuments in the ground. If you’re working with buildings, streets and sewers, you need to work off vertical control benchmarks. If you’re working on highways and large projects, you need to tie to horizontal control for the local state plane coordinates.
A surveyor will never define a location by WGS lat/long.
CORS is helpful, but only in that you have a receiver that is perfectly fixed over long time periods so you can reference to it again and again.
I wouldn’t worry about transformations between the two datums because the one that matters is the transformation from a datum to physical coordinates.


Well said!

Right on. This is a huge gap in the Survey/GPS/GIS and now drone worlds. I have had a really hard time getting people to understand the middle-stance of drones, GPS equipment and GIS and how work on the ground uses ground datum. WGS84 is used to define the boundary of the project then the project is surveyed or layed-out inside of that on a YXZ coordinate system. The localization translates the two on the fly. There has been a hard transition over the last 15 years of mainstream GPS use and surveyors and engineers coexisting in these two realms at the same time. You will very rarely, if ever hear a surveyor or engineer speak in LLH. Now they are because of localizations. That is the transformation.


they are not the same and are over 1.5 meters apart where i am. they were the same originally. if you just need relative accuracy as in the case of construction site work then i guess it doesnt matter you just stick with one. but if your expecting your work to relate to other GIS layers then you might want to dig deeper. i really like the online materials at penn state

as RTK_hunter has stated, HTDP is a informative tool to experiment with. where i live everything is moving 2 to 4 cm per year so i capture in wgs84/igs08 and then store in nad83 2011 which in my case is like freezing in time.

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Yes, but say you survey an XYZ point/monument on the ground. Nathan then goes out with his Emlid and also surveys the point LLH. So at this point XYZ=LLH. But Nathan then corrects his location in PPK (but NAD83 base coordinate from CORS) than passes off his point to me to go stakeout at a later date. I setup my unit add my file to stakeout and go out to the site. I use the stakeout function to go to Nathan’s “point”. I arrive to his “point” but can clearly see the monument and I am at least 30cm away. Why?

They are all generally apples to apples describing the same point. True, what a surveyor is describing in XYZ can also be described in LLH. But when Nathan doesn’t transform the coordinate correctly his point now becomes an orange because my LLH would no longer match your XYZ.

Between your XYZ and Nathan’s LLH a transformation wouldn’t necessarily matter if it was just the two of you. But between my LLH and Nathan’s LLH a transformation matters entirely so I can match your XYZ. Else, something like this would occur: UNAVCO NTRIP epoch alignment problem


Wow, this guys gets it!

Localization solves all of this. You have a primary network set from your perfect world whatever LLH for person A. Person B then enters YXZ coordinates which represent those points from the CAD file or plat and localize by occupying point B and using the measurement from point A. Everybody gets what they want. You can worry about whatever transformations you want after that, but this is how the real world is built.

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Ok, so just to make sure I understand localization and compatibility with RTKPOST, etc, please correct me if I am wrong.

My CORS reference stations are published in NAD83, ellipsoid elevations.
Benchmarks are published in NAD83, ellipsoid and ortho heights.
RTKPOST only has a selection for the use of “WGS84” for coordinate system processing.

Process 1:

  1. Convert Base coordinates to WGS84 to use in RTKPOST (making sure to do use the correct date for conversion)
  2. Convert RTKPOST results back to NAD83 (for comparison to benchmark for accuracy study).


Process 2:

  1. Feed RTKPOST NAD83 coordinates and ignore “WGS84” as the coordinate system.

NOTES: The RINEX header positions for our network are in XYZ (NAD83). When I choose “Use RINEX Header Position,” the coordinates will not be ECEF. This is synonymous with Process 2, and I have experienced adequate <2cm error results this way. XYZ can be NAD83 or WGS84 (if ECEF).

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The localization I speak of has more to do with marrying the coordinate values from WGS84 or other system using LLH to a YXZ format that is almost always used on the ground. The other thing localization helps out with is the fact that the grid coordinates we end up using and that our CAD/Design data is on is not always a direct transformation from LLH to that state plane. Shifts happen in CAD (they shouldn’t) and scale factors are inconsistently used. A direct transformation from WGS84 to state plane will not come in at the same place as a CAD file with a scale factor so I need the localized transformation to adjust my drone data if I want to be in the same place and have the ability to integrate my drone data with all the other VDC/BIM data. This is where I put enter the manual base coordinates and also when I am post-processing in RTKPOST.

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Ok, I think we are getting confused chatting here because XYZ(ECEF) is earth-centered XYZ. Your XYZ you are referencing is really a form of ENU of some local cartesian system like a state-plane system where the Z is the elevation. XYZ when referencing emlid documentation does NOT have the Z-direction changing elevation.

I don’t think we are confused, but it may confuse others. You are exactly right on the difference between our XYZ’s. I write YXZ because it is Northing, Easting and Elevation and not related to LLH. Hence the localization and transformed ground datum not using an ellipsoid or geoid.

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XYZ is NOT Northing Easting and Elevation when referencing RTKLIB. It’s also NOT Northing, Easting, and Elevation to the NGS NCAT tool. Z=0 is at the equator.


We’re not talking about either of those. Not the point of the thread. Please stay on topic.

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I apologize if I was too blunt. I am trying to help people understand what may be a whole new use-case to people using these products for surveying and drones in particular. Could you please start another thread with what you are trying to convey? I think it is very important as well and would like to understand it better myself.:grin:


Sorry to distract from this thread with the technical points. I’ll try to bring it back around to the main point about using these units for surveying.

I think surveying is a profession that requires a license in many places. Because of the accuracy required, it justifies more expensive and robust surveying equipment. I don’t L1-only units meet the robustness and accuracy needed for most surveyors, and the rest of us cheap non-surveyors just need a little positioning here and there and aren’t too concerned with repeatability.

Those people who do need the repeatability have a lot of learning to do that a website or forum cannot teach. That’s why there’s trade schools, apprenticeships, etc.

I work for a company that does Aerial Surveying, but it takes a LOT of experience and knowledge to do it well. Surveying organizations and boards are cracking down on non-licensed individuals who are providing geospatial products. I can somewhat understand that when they are providing bad data.

In short, I think many people know that these products are needed for the use case of surveying. Many people also know that they aren’t qualified to provide the accuracy required for surveying, so they just call out the surveyor to provide control points and then they align with those third-party surveyed points. We do this in the Aerial Surveying industry all the time.

I want to learn more about this also as I am not quite up to par when it comes to the VERTICAL in the USA.

Seems most settle with the WGS84 ellipsoid height? But how do you bring it to the “real world” to the actual GROUND elevation either if BELOW or ABOVE the WGS94 ellipsoid height? Does one use a GEOID in addition to a WELL KNOWN POINT set by a registered surveyor? I guess the key is to use a known point which you set your base on and go from that? Or even localize to?

A GOOD TUTORIAL from start to finish would be awesome explaining all of this one day. Point to point using a BASE and a ROVER. Then a more complex one using drones etc?

btw Nathan, nice website etc!

I have been in surveying for almost 18 years, 5 of those under an RPLS and 10 more under a Civil Engineer. While I no longer do certified surveys I treat the construction surveying and layout as if I were. I agree and have had trouble getting the L1 devices to perform well enough for our normal needs, but I am finding uses for it in general small layout and the drone. The Topcon Hiper L1/L2 and like brands are pretty much untouchable as far as performance goes, but they are also unreachable as far as cost to most companies.

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I think new drone mappers assume and don’t know any better. They also don’t realize why we need it or much about construction. Ellipsoid and Geoid are all but irrelevant in land surveying. GIS and Geospatial think in global terms, but there is a reason that they are conceptual and to date not in relation to the ground. I think that is changing with drones and the ability to capture much more accurate data in the type that we both can use.

Land Surveying

  1. Registered Surveyor studies available existing data including registered surveys and existing datum.
  2. RPLS send a crew out to find the datum that is actually on the ground.
  3. Techs prepare the collected data and put it in a format for the RPLS to further analyze.
  4. RPLS provides the plan of action and the field crews.
  5. Field crews update the ground condition of the data and return with an asbuilt.

Construction (At least in our company)

  1. CAD Techs coordinate retreiving data from the Engineer of Record.
  2. CAD Techs and VDC Manager analyze the received data and make sure it matches the Red Stamped construction plans.
  3. CAD files are optimized and standardized for company use.
  4. Points are made for known benchmarks and property corners and transmitted to the field crews.
  5. The field crews setup the base station in an arbitrary position and measure each found point for localization of WGS84 HORIZONTAL to ground STATE PLANE.
  6. Level-loop is run from known vertical benchmarks through the control as localized and final GROUND elevations are established.
  7. Primary control network is preserved and a logistically positioned secondary network is created for field layout.

Because the secondary network is logistically planned it is also normally suitable for the GCP network. With the introduction of PPK and RTK drone capability this will be greatly reduced, but in our company will never disappear. The workflow for the drone is the same as surveying. Difference being the Standard Operating Procedure and what must be done at the beginning of the project. Very generic example.

  1. Google Earth and Street View review for physical complications.
  2. AirMap or similar review for airspace restrictions.
  3. File for waivers if necessary.
  4. Site visit for physical measurement of aerial obstructions.
  5. Log Book completion and existing conditions survey performed.
  6. Flying
  7. Log out
  8. Processing
  9. Exporting
  10. Analyzing
  11. Transmitting
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Lately there have been some great announcements of at least L1 / L2 tech for lower cost. I think a lot of this is getting ready to change for the better pretty rapidly here. Before you know it, smart phones will be more mainstream with at least L1/L2 capability. There is 1 at least already.

L1 will soon be OLD news.

Ok, back on topic! ; )