Difference between averaged Base and Rover position

Hello all. I have been reading quite a bit on the forum regarding RTK with Emlid Reach RS+, but I am encountering a discrepancy for which I cannot find a proper explanation. Maybe someone can help.

I used 2 Reach RS+ in a base-rover configuration to collect RTK points. Before starting the survey, I calculated the base position with a 10-minutes average, noting down coordinates.
The position recorded in the RTCM3 file matches the one I noted down within 0.04mm in X,Y,Z.

The base position obtained from the 10-minutes average is:
Lat: -46.558107459851804
Lon: -67.43280756912664
BaseHeight: 34.053
Base Antenna Height: 1.305

After averaging the base coordinates, I started the RTK survey. The first point I collected is the base position itself (just in case, I thought).
The position I have out of the Rover is:
Lat=-46.5581079411258
Lon=-67.4328064124377
RoverHeight=33.850
Rover Antenna Height: 1.775

By making some calculation, it turns out that the XY distance between these two points is around 10 cm, which is OK as I did not put exactly my rover above the base (that is in my field notes).

What leaves me a bit more puzzled is the height. If I understood correctly how data is stored by EMLID, then I should calculate the height of the point on the ground with the following procedure:

PtgroundBase=BaseHeight-Base Antenna Height=34.053-1.305=32.748m
PtgroundRover=RoverHeight-Rover Antenna Height=33.850-1.775=32.075m

So, while the two altitudes should be virtually the same, I have a difference of 0.67m between them. The pole heights are OK, I re-checked them.

Any hints you can share?

Hi Alessio,

Can you post here the logs from the survey (raw data from both base and rover) and the Simple System reports so that we can look into them and check what may go wrong?

Hello, thank you for the prompt reply. The data files are at this link:

Hmm, how did you measure the base position with the rover, when using the base for RTK corrections to the rover?

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Maybe another point was shot and traversed to? Or was another base point averaged in?

I just put the rover antenna close to the base and shot a point while the two were connected over radio. Maybe I am being dense, but I expect to see almost exactly the same point recorded by the base and the rover.

I am pretty sure I did not average another base point.

Ok, missed that part. 10cm is probably right, but the elevation difference may be an antenna offset issue. Maybe double-check the measurements and entries in your data collector software?

I did, and I also re-checked the rover/base antenna heights from field photos. Nothing out of the ordinary. What I’ll do now is see if the error is repeated in other survey days.

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Also verifying you had a good strong RTK fix? I would shoot in a new base point with the rover. Document the coordinates then set the base there, manually enter the coordinates and stake back to the original base.

This is a standard survey check that we do every time.

That’s a good suggestion. Yes, there was a strong RTK fix. I’ll process other similar datasets to see if there are the same problems. But this one is bugging me. I have some points taken with Trimble Omnistar at the same location, so I will cross check.

Yes, I’ll double check that as soon as I am back in the office and I can access my two units. But the antennas I use are always the same and I did not find anything unusual.

Maybe the problem is in how I subtract the antenna heights.
Are the antenna heights already included in the CSV and Base elevations as exported?

Hi Alessio,

Sorry it took so long to get back to you!

Unfortunately, the logs don’t contain time when you re-collected the base position. So I can’t check if the data quality affected the solution.

However, could you clarify one more time whether you added the antenna height offset value (0.134m) to the base and the rover tripod height?

Have you had a chance to conduct new tests?