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I understand that if you collect GCPs using an RTK GPS they are in WGS84 / EPSG:4326 - and you could then convert them to another EPSG. But I'm wondering how GCPs are collected in other coordinate systems like for example EPSG:2274 or NAD83.

Are they being collected with another type of device that doesn't use GPS, or are they collected with a GPS and then converted.

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  • GPS has been in full operational capability only since 1995. Before that all data were collected by comparing an unknown location to some other places with well known coordinates. See for example icsm.gov.au/mapping/surveying2.html.
    – user30184
    Jun 27, 2017 at 19:49
  • RTK are using a control network with known coordinates to "tighten up" what the GPS is reporting. The control network will be reporting ITRFxx or NAD83 (2011 or CSRS), etc. The only way to get true, accurate WGS84 coordinates is to a US military-grade receiver and access to the WGS84 control network.
    – mkennedy
    Jun 27, 2017 at 20:01
  • @user30184 and were the well know coordinates of that origin arbitrary I guess? Jun 28, 2017 at 0:29
  • There was and are a network of triangulation stations and the most accurate of them are used for calibrating the GPS system. Lower level GCPs are not as accurare but they are not arbitrary. In the field work the surveyor could for example take GCPs from a map by putting the first pole near a corner of some building that appears on the map.
    – user30184
    Jun 28, 2017 at 4:30

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