How to get the surveyed points recorded and exported with certainty in a local ETRS89 coordinate system

Q&A forumCategory: QuestionsHow to get the surveyed points recorded and exported with certainty in a local ETRS89 coordinate system
mihails.cugunovs asked 5 months ago
We are accurately surveying points in Finland, Europe, with an RTK Calibrated Surveyor Kit, using SW Maps and a national RTK service. The exported point shapefile seems to be in a WGS84 coordinate reference system, as shown by QGIS on examination. However, we need to have a good accuracy, and an extra ~1 m error between the two coordinate reference systems is unacceptable to us. How do we record and export points as intended in Europe and by our local RTK service, i.e. in an ETRS89 system? Or is it so that there is no error in any case? I.e. is it possible that while the layer/shapefile itself is in WGS84, the points themselves are keeping to ETRS89, as supplied by the RTK service? At this point, I reprojected the shapefile in QGIS to ETRS89, but it’s hard to judge about the accuracy as compared to the proper European coordinate system.
Thank you! 

1 Answers
Ardusimple Staff answered 4 months ago
The RTK Calibrated Surveyor kit doesn’t do any transformation. If your correction service is in ETRS89, the positions will be output in ETRS89. Usually only global RTK-SSR corrections services use WGS84, it’s a bit strange that the Finnish service is using this system. Let’s do some checks to understand what can be happening:

  1. Are you in RTK FIXED mode when taking the measurements?
  2. Are you using the ETRS89 mountpoint?
  3. Are your other maps in ETRS89 or in local Finnish coordinate system?
replied 4 months ago

Hello! And thank you for further questions. Here are the answers:

1. Sometimes SW Map GNSS status shows RTK Fixed, and sometimes RTK Float.

2.Not sure, but here is the description by the Finnish National Land Survey about their RTK service:
“In the VRS method, the user’s device must send the FINPOS service an approximate position as an NMEA message, based on which the service creates a virtual base station (VRS) for the user. The user’s receiver receives the precise coordinates of the virtual reference station and the observations by which it determines its location in relation to the virtual reference station. From the mountpoints, VRS-FKP and VRS-FKP-OLD transmit data of a virtual reference station.

In addition to the VRS method, the user may receive data from a single reference station. The SINGLE mountpoint automatically connects the user to receive data from the reference station which is closest to the position the user has sent.

The coordinates of the base station and the virtual base station to be transmitted from the RTK service are in the EUREF-FIN coordinate system.”

And some more about the Mountpoint: ”
The service utilises error modelling of the entire reference station network and sends individualized virtual data (RTCM3.2, MSM4) to the user’s location. Data for GPS, GLONASS, Galileo and BeiDou”

3. Our other maps are supposed to be in a Finnish coordinate system “ETRS-TM35FIN”, which, to my understanding, is still tied to the general ETRS89 system and is a subvariant of it.

And a further question: if we disregard the coordinate system discrepancies and treat them as interchangeable and exactly same (as QGIS apparently does), will we (and our successors) run into problems with our measured position data in 50, 100, 200 years? Or do the dynamic updates of WGS84 keep the discrepancy with regard to ETRS89 at about the same level (0.5 m)?

Thank you and hoping that we can gain some better understanding on this issue!

Staff replied 4 months ago

Thanks for the extra info. This explains the offset between the measurements (EUREF-FIN) and the maps (TM35FIN).

About your other question I have bad news, there’s an additional problem, and is that a same coordinate system gets updates. This means there’s for example ITRF year 2000 and ITRF year 2014, and there’s also an offset between them. Unfortunately the earth is moving every year in centimeter-level and therefore the coordinate systems need to move as well. It’s a complex topic, we share with you a good article that explains with examples:

replied 4 months ago

All satellites uses WGS84 and you have to transform to local coordinates in your data collector app/mapping software using up to date transformations. As mentioned WGS84 is not good for long term documentation as the tectonic plates moves in various directions. (2,5 cm/y in Europe) Having WGS84 pinned to local points would create a very crooked grid after a few years.

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