2 ms·

There are large networks of GPS monitoring sites that regularly report their positions, including movement, down well below centimeters.

jandrewrogers 7y ago
The point is about repeatability, not precision. Nominally fixed points on the Earth's surface are constantly in motion relative to GPS and relative to other fixed points. Many of the effects that cause this lack of repeatability can induce centimeters of measured deflection from the mean position over relatively short periods of time. There are models that can be employed to correct some of these effects (e.g. fluctuations in the gravity field, which is measured by satellites) but not all of them. Consequently, the measurement may be precise but there is never a reference frame in which the points are actually "fixed" over time, which is indistinguishable from reduced precision. Local changes in surface geometry are also easily detected with LIDAR. For surveys that require maximum repeatable precision, they will often use the mean position as measured over e.g. 12 hours. The magnitude of the variance varies quite a bit depending on where you are on Earth.
- jhayward 7y ago
  I've done a fair amount of work with GPS and precise positioning, and I'm aware of all the dynamics of earth, orbit, and electron path that are involved. I've never before seen a claim of 1cm being some kind of physical limit on position. There are both short and long-term motions of the earth. All are sufficiently measured and modeled such that repeatable measurements are very much achievable well below 1 cm. In fact, it is GPS that is used to develop the models of earth movement. So I don't quite get what you are saying.
  - jandrewrogers 7y ago
    The centimeter limit is an engineering heuristic used by people combining high-precision GPS, LIDAR, remote sensing, and sometimes other sensors to build high-precision models of the world e.g. maps for autonomous systems. LIDAR alone can routinely detect changes in relative local surface geometry in excess of a centimeter across a day in practice. Autonomous driving typically uses an error bound of around 10 cm on absolute positioning even though the sensors used to determine that positioning often have sub-centimeter precision, for similar reasons. Real-time localization is an interesting problem because measurement with orthogonal sensor modalities can produce conflicting results with differences greater than the nominal precision of those positioning algorithms. Getting the last few centimeters of functional precision out of contradictory measurements is one of the things you need AI for. I agree that parts of the world have repeatable measurements below 1cm if you can account for the myriad effects that cause displacement, but others do not. The models are also approximate, since some effects require real-time measurement of things we do not have real-time measurements for, or which are not practically available in contexts where local position needs to be determined.

jandrewrogers 7y ago

The point is about repeatability, not precision. Nominally fixed points on the Earth's surface are constantly in motion relative to GPS and relative to other fixed points. Many of the effects that cause this lack of repeatability can induce centimeters of measured deflection from the mean position over relatively short periods of time. There are models that can be employed to correct some of these effects (e.g. fluctuations in the gravity field, which is measured by satellites) but not all of them. Consequently, the measurement may be precise but there is never a reference frame in which the points are actually "fixed" over time, which is indistinguishable from reduced precision. Local changes in surface geometry are also easily detected with LIDAR. For surveys that require maximum repeatable precision, they will often use the mean position as measured over e.g. 12 hours. The magnitude of the variance varies quite a bit depending on where you are on Earth.

jhayward 7y ago
I've done a fair amount of work with GPS and precise positioning, and I'm aware of all the dynamics of earth, orbit, and electron path that are involved. I've never before seen a claim of 1cm being some kind of physical limit on position. There are both short and long-term motions of the earth. All are sufficiently measured and modeled such that repeatable measurements are very much achievable well below 1 cm. In fact, it is GPS that is used to develop the models of earth movement. So I don't quite get what you are saying.
- jandrewrogers 7y ago
  The centimeter limit is an engineering heuristic used by people combining high-precision GPS, LIDAR, remote sensing, and sometimes other sensors to build high-precision models of the world e.g. maps for autonomous systems. LIDAR alone can routinely detect changes in relative local surface geometry in excess of a centimeter across a day in practice. Autonomous driving typically uses an error bound of around 10 cm on absolute positioning even though the sensors used to determine that positioning often have sub-centimeter precision, for similar reasons. Real-time localization is an interesting problem because measurement with orthogonal sensor modalities can produce conflicting results with differences greater than the nominal precision of those positioning algorithms. Getting the last few centimeters of functional precision out of contradictory measurements is one of the things you need AI for. I agree that parts of the world have repeatable measurements below 1cm if you can account for the myriad effects that cause displacement, but others do not. The models are also approximate, since some effects require real-time measurement of things we do not have real-time measurements for, or which are not practically available in contexts where local position needs to be determined.