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There are several systems for finding lat/long coordinates. This includes GPS satellites, DoD TRANSIT, Russian GLONASS, Galileo, differential GPS, LORAN, land-based systems, and more. Maybe even using laser interferometry directly from Greenwich.

I want to know, using all of this technology, what is the actual maximum accuracy which a location on Earth is measurable?

This is not a question about technology or measurement. The theoretical limit is nanometers because of lasers. But the actual limit will be worse because of the physical characteristics of Earth, it is constantly moving.

So what is is actual best measurement we can make?

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    I'm not sure what the best possible accuracy is, but survey-grade GPS units that utilize a rover and a base station can achieve sub-centimeter accuracy as long as the base is set up over a known, high-accuracy NGS benchmark or one you've created yourself by postprocessing a static occupation (e.g., with OPUS). – lambertj Nov 29 '18 at 16:39
  • I should mention-- the unit we use is a Topconn GR5 which utilizes GPS and GLONASS. – lambertj Nov 29 '18 at 16:44
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    With GPS survey, if you remain static for several hours or days and collect enough data from satellites and ground stations, it is possible to obtain an relative accuracy of a few millimeters. Tectonic studies typically monitor changes of a few millimeters in tectonic motions. However, most international and national reference frames that are typically used to express positions (and lat/lon with the use of a reference ellipsoid) will have a practical accuracy around one centimeter because of small inconsistencies in the reference frames themselves. – FSimardGIS Nov 29 '18 at 17:36
  • See also gis.stackexchange.com/a/2761/59 – Kirk Kuykendall Nov 29 '18 at 17:43
  • Thank you this is all very helpful and these would definitely qualify as "answers" here – William Entriken Nov 30 '18 at 16:56
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It is currently possible to achieve sub-centimeter accuracy when surveying a point.

To express a latitude and a longitude, it needs to be reference to something. We need a datum: a reference frame and a reference ellipsoid. The reference ellipsoid is simply the mathematical shape that we use to perform the conversion to a geographic coordinate system (lat/long/height). The reference frame is the set of physical points (stations) that serve to locate and orient a coordinate system. For example, the ITRF2014 (International Terrestrial Reference Frame) is defined with the coordinates of hundreds of stations around the world, as well as their velocities (because of crustal movements). The ITRF2014 is globally non-rotating relative to the crust, is centered on the geocenter, and has no translation. It is an ECEF (Earth-centered, Earth-fixed) frame of reference.

This most recent realization of the ITRS (ITRF2014), with modern geodetic techniques (mainly VLBI, DORIS, SLR and GNSS) has a very good overall accuracy and consistency, and the errors are typically only a few millimeters. Countries often use other local or national datums to express positions, using reference frames that locally follow the crust and reference ellipsoids that better approximate the shape of the Earth in that country. Accuracy and consistency of these datums can vary a bit depending on which techniques were used to realize their reference frames.

Now for the practical measurement part, if you wish to determine the lat/long of a point, RTK GPS, and even traditional surveying techniques can be used to achieve a reasonable accuracy of a few centimeters pretty quickly. These techniques require a nearby known reference point or broadcasting station for GPS, usually from a densification network, a local geodetic network or some other previously measured point. With more advanced GPS survey, if you remain static for several hours or days and collect enough data from satellites and post-process your data using nearby ground station corrections, it is generally possible to get down to a few millimeters in accuracy. Sub-centimeter accuracy surveys can help analyzing crustal deformation, tectonic motion, isostatic rebound, sea level rise, etc.

  • Excellent, thank you for sharing this state of the art for surveying – William Entriken Dec 4 '18 at 16:21

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