Hello dear people, I am here because I need your help. I am part of a university students project in Germany. Our task is to build a UAS/UAV, which flies a survey mission above a certain area and map the location of targets on the groud (with 1 m precision), which have to be identified with an onboard camera.
For this purpose we have decided to use RTK for the UAS's navigation, and have purchased two u-blox ZED-F9P GNSS modules to be set up as base and rover. Now here comes the problem:
To set up our base station (i.e. find out it's precise location), we want to use the F9P's RTK ability and use the german state-run NTRIP caster service called SAPOS, which is available for free. The problem is, SAPOS sends it's correction data in the ETRS89 reference frame, while we have to present the target locations in WGS84. The F9P's manual states, that the RTK solutions the GNSS-chip puts out, automatically switch to the reference frame used in the correction data and that, if another reference frame is required, a datum transformation has to be performed. The u-blox Chip offers configurations to input several transformation paramters, but I don't have a clue on where to find those parameters.
While searching the Internet for Information, I have found out, that between WGS84 and ETRS84 (or the most recent realization thereof) there is a difference of about half a metre, which is of no concern for most appications, therefore both CRSs are sometimes treated as equal. In our case however, 0.5 m could mean the difference between staying within the 1 m precision-limit or not, because we also expect some inaccuracy due to the whole being-on-a-plane-thing. While trying to look up transformation-parameters, the closest thing to what I THINK I am looking for is this, but this again seems to tell me, that there is no difference between ERTS89 and WGS84 at all. On top of that, I did read something about different types of coordinate transfomations having different accuracies, which could be another error source to consider.
Right now I am utterly confused about the whole CRS/datum topic and don't know where else to look for information regarding my problem. I am uncertain about, whether I should dive deeper into the topic, or if I maybe just get away with ignoring the whole thing. At the moment our GNSS is totally able to get RTK-fixed solutions using the abovementioned SAPOS-Service, I just don't have any way to tell the "absolute" precision of these fixes, or in which datum format they actually are.
Maybe there is someone on this forum, who has experiance whith the topic, who can provide me with some explanation or a push to the right direction. Also any links to Information, which answeres my question or provides the necessary background would be very much appreciated.
Thank you for reading!
When referring to data as WGS84 requires a qualifier indicating the version. WGS84 coordinates have undergone periodic revisions. These revisions are in the form e.g. WGS83(G1762) where the G1762 refers to the GPS week associated with the revision. WGS84(G1762) was implemented 16 October 2013 and used 2005.0 as the reference epoch.?ÿ
A source for more information on this is here:
The reason for the various versions of WGS84 is the decision by the US NGA to keep WGS84 consistent with the current version of the ITRF. This means that ITRF and WGS84 can be considered consistent at the few centimeter level as long as they cover the same date range.?ÿ
ETRS## are developed from ITRF##?ÿ and WGS84(G####) is consistent with ITRF (where ETRS and ITRF use year references and WGS84 uses a GPS week reference.?ÿ
For more information and links to tools see:
https://sigeo.cerege.fr/?p=467
Why do you wish to report coordinates in WGS84 at all??ÿ
I don't have more time to address these issues. I may revisit them later.
Good luck,
DMM
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Other references on the issue:
This document is from the International Civil Aviation Organization. It includes some discussion of ITRF and WGS.?ÿ
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https://earth-info.nga.mil/index.php?action=home
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Unfortunately the transformation parameters link on this page does not work. I reported it to the site operator.
https://www.epncb.oma.be/_productsservices/coord_trans/
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Some screen captures:
@geeoddmike Thanks a lot for your input, I will try to work through the links you provided in the next couple of days.
Regarding your question on why to use WGS84: Actually this is an assumption I made, becaue our task is unclear in its requirements. We have to provide the target's coordinates in the format of X/Y (no alt) in decimal degrees. Because GNSS-systems (to my understandig) usually use WGS84 I assumed, that this is the reference frame we have to use. I probably should have stated this more clearly. It may as well be, that even the people who created the task do not exactly know about this whole topic.
Thanks again for the links!
I thought that ETRS89 was the EU-recommended frame of reference for geodata for Europe??ÿ
I do not see this explicitly specified but note that the datasets seem to all be in ETRS89.
Remember that what SAPOS provides is corrections with respect to ETRS89. WGS84 is not involved.
You mention that you intend to provide only two-dimensional (latitude and longitude) values in decimal degrees. While I expect you will do so to the appropriate significant digits, do you intend to not use ellipsoid height information at all?
I provide yet more links below.?ÿ
Yet another source of information about WGS84 realizations and the its relationships to ITRF and ETRS89 is: https://confluence.qps.nl/qinsy/latest/en/world-geodetic-system-1984-wgs84-29855173.html#WorldGeodeticSystem1984(WGS84)-WGS84,ITRFandETRS89
BTW, there is a link from the Wikipedia article on ETRS89 to a memo by C. Boucher and Z. Altamimi where they describe the transformation of ETRS89 to ITRF in great detail. See: https://en.wikipedia.org/wiki/European_Terrestrial_Reference_System_1989
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