RS2+ (Base) & RX (Rover) using Emlid Entrip

I am using a Emlid RX2+ as a BASE (setup on a known Government monument with XYZ) and then using a Emlid RX as a Rover and using the Emlid Entrip Service to send correction, I have access to 3 other Government Minor Controls that I am using to test the accuracy of the RTK corrections but I am getting an average of 5 cm diff on Northings & 50cm on Eastings, I am using manual base setup to input the XY value of the BASE (WGS value of Control), I have also followed the setup recommended on the Emlid site for both Base and Rover. any help??? or is this differences in Easting what I should except using the above mentioned setup.

5 cm may happen sometimes but it is fairly high. 50 cm is not normal.

  • Do you have a FIX status on the RX ? What is the baseline length ?
  • What is the accuracy of these control points ? For example here in France, some minor controls have an accuracy of 50 cm, so in that case, the problem is you compare with a less accurate reference than what the RX is capable to achieve
  • What is your local geodetic datum ? What country are you in ?
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Hi Rob,

I second Florian’s questions. And one more from me: is the shift the same on all the measured points?

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Florian, thanks for your reply,

  • Yes we have a FIX status.
    -Accuracy of Control Point being used as Base is around 7 cm, its what they refer as a Major Control Point, the other points I am observing to test accuracy are Minor Control Points that have been set by Total Station between 2 Major Control Stations at a minimal accuracy of 1unit per 18,000 units
    -Datum is NAD27 Spheroid / Clarke 1866 (country of Belize)
    -Base Line Distance is 400 meters and 900 meters

and to answer Julia’s question- Yes the shift is the same on all Minor Control Points.

we do a transformation from NAd27 to WGS84 to get the values that are used on the Base

Based on what you are describing 5cm horizontal seems to be in line with the existing network but the vertical definitely has something else going on. Are all of your antenna heights double-checked and corrected. Are you using the same vertical datum as the original survey? In the US we see this sometimes because the ground-leveled benchmark is in conflict with the network.

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its 50 CM of the Easting values and 5 cm on the Northings

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I understand now. I misread as that being the z delta. :wink:

When were the passive control marks established ?

How were the passive control marks established ?

Who established the passive control marks ?

Are datasheets published for these marks ?

What is the datasheet accuracy statements ?

These are just a few of the questions for your marks. Conversion of NAD27 to NAD83 datum is problematic due to the different ellipsoidal models used in both horizontal and vertical datums.

Adequate vertical benchmarks would be required to convert from NGVD29 to NAVD88 vertical datums in order to have a precise model for conversion. Simply converting from NAD27 to NAD83 for a single point will not adequately give you a reliable conversion.



Thanks for your answers.

I agree with Bryan about the datum conversion. And I suspect this is where the mismatch comes from. Are all the benchmarks in NAD27? I mean the one you set your base on and the ones you measure with the rover.

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