GCP measurement

Hi everyone

I am really sorry maybe to be repetitive but still I don’t understand at all the result of RS+.
I need to take GCP for drone survey.
My RS+ ( just one ) is correctly connected to NTRIP service and get the corrections.
Most of the time in my area the ratio of solution is 999.9 and simply I register the point needed for 5 seconds.
Later on I try to post process the same point and all the time I have a slight different value ( even if the one from Rs+ reach view was a point with 999.9 ratio).
So which one is correct?
Another question mark in my mind is the RMS…with reach view the error on 3 axis in 0.001 max 0.003 which I don’t believe.
Can someone be nice to explain me the correct procedure to have a GCP point with 3 to 5 cm accuracy and if I have to post process or not a fix point.

Thanks a lot

In order to get repeatable 3-5cm accuracy your base station should be on a known point and I would recommend that your GCP observations be no less than 30 seconds. RMS values mean very little as they are just live indicators of the quality without relativity to other shots or a different surveying session.

Hi Michael

Thanks for reply
Sorry but I don’t understand.
The known position is retrieved from the gps.
Let’s say, I put on tripod the rs+ connected to a NTRIP service and I wait until the Ra is higher as possible and I star the observation.
I observe a fix point for 30 sec and I save it.
How can I know the precision of that point?

Are you averaging/observing the base point each time or manually inputting the first observation you made? You can’t really tell the absolute accuracy of the point you derived without observing the point for hours or verification from a surveyor.

I just save the point needed from ReachView… do i have only one rs+…My base is fix base doing part of NTRIP service

You have one receiver? How are you shooting the GCPs? Using that receiver as a rover on NTRIP?

That’s correct

If that’s the case then I don’t understand how you are shooting GCPs that are relative. A 30-minute observation and a 30 second observation can easily be different, but what was the point of setting a base if the GCPs were going to be set with a rover/NTRIP configuration?

The workflow you describe, as I understand it, would normally have two receivers. The “base” or rover would be used to derive the base point. Then the coordinate derived would be manually entered into the base and the local base would send corrections to a rover. The NTRIP/rover setup should be standalone, but if it is trying to be related to another survey or some design files then there is usually some type of shift in my experience. Very rarely does native GNSS data transform directly to a local grid in design files. We sometimes have a problem with scale factors causing issues.


Yes correct.
It’s a stand-alone configuration.
One EMLID rs+ connected to NTRIP service.
The setup for rtk correction is set to have correction via internet from a near base doing part of national fix Gps stations.
The base is 4 km away.
I am correcting position just with NTRIP.
When I don’t have fix solution I use postprocessing.
My point is: how can I know the precision on my coordinates?

Ok I now get what I wasn’t understanding. You are not setting a base point, but referring the CORS as a base. It still remains that either you need to occupy for longer, compare and see the residuals or have another surveyor verify the coordinates until you are comfortable with it.

Buy 2 M2’s as static receivers approx 300m apart and PP each GCP located with rover. I would occupy minimum 5 minutes per GCP.

You can PP in commercial software, most also gives you loop closures for each computed point.

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If you have two receivers why not just use the workflow and RTK as I mentioned above? Of course one nice thing with the Emlid is that you can easily log at the same time if want to verify with PPK later.

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Sorry maybe i can explain well my question.

1- the configuration is standalone ( 1 receiver connected to NTRIP)
2- I want to take a position for drone GCPs
3- in EMLID reachview I use survey and I collect the point I need and download the .csv file

My question are:
1- how can I know the precision in cm of the points collected
2- do I need to use rtklib to post process a fix position?

  1. At some point you just have to be comfortable with the equipment and process. When mapping with GCPs you should always check-in with a known local or site benchmark. It’s always a good practice to collect at least two existing things in case your points need to be rotated or shifted later to coordinate with other information. This is something that disturbs me about people trying to map without knowing anything about or checking in to the local datum. In the same manner you should always set you base on one of your collected points and check into another collected point, not back to the base. It’s just a process that is standard in the way I was taught to survey.
  2. I would log for PPK while collecting RTK points (1) Just in case something happens and (2) In case you are ever asked for verification or want an RPLS to sign-off on your work. If you are comfortable with the performance and the report on the points from RTK then move on.
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Hi Davide,

I just want to add a few words to what Michael said.

First, there is indeed no information about accuracy by axes in CSV files. However, you can check RMS values in the Survey project, if you open points list.

Also, if you have a significant discrepancy between PPK and RTK results, please reach out to us so that we can investigate this.

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