On Sat, Jul 17, 2021 at 02:38:55PM -0400, Christian Hopps wrote: > > Benjamin Kaduk <[email protected]> writes: > > > Hi Christian, > > > > Sorry for the very delayed reply (and thanks to Rob for the nudge). > > > > On Wed, May 26, 2021 at 06:04:58PM -0400, Christian Hopps wrote: > >> > >> Benjamin Kaduk via Datatracker <[email protected]> writes: > >> > >> > Benjamin Kaduk has entered the following ballot position for > >> > draft-ietf-netmod-geo-location-08: Discuss > >> > > >> > When responding, please keep the subject line intact and reply to all > >> > email addresses included in the To and CC lines. (Feel free to cut this > >> > introductory paragraph, however.) > >> > > >> > > >> > Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html > >> > for more information about DISCUSS and COMMENT positions. > >> > > >> > > >> > The document, along with other ballot positions, can be found here: > >> > https://datatracker.ietf.org/doc/draft-ietf-netmod-geo-location/ > >> > > >> > > >> > > >> > ---------------------------------------------------------------------- > >> > DISCUSS: > >> > ---------------------------------------------------------------------- > >> > > >> > I think we lack sufficient precision (forgive the pun) in how we talk > >> > about "accuracy" and "precision". Are the leafs that claim to specify > >> > "accuracy" specifying a precision? If so, the precision of a specific > >> > measurement, the precision of the measurements that led to the creation > >> > of the coordinate frame, or something else? Are they doing so in > >> > relative terms (e.g., percentage) or absolute terms (e.g., degrees and > >> > meters)? (There are "units" directives only for "height-accuracy" and > >> > not the others.) How can we we say that we'll have 16 fraction-digits of > >> > precision for lat/long when the maximum accuracy we can say that a > >> > geodetic-system has only gives us 6 fraction-digits for coord-accuracy? > >> > When we say that the "precision of this measurement is indicated by the > >> > reference-frame" is that the same thing as the relevant "-accuracy" > >> > nodes, or something else? > >> > >> Yes, the geodesic-datum is what defines the values and their accuracy. For > >> the > >> precision in the value we choose the fractional digits based on what might > >> be > >> needed, but not to prescribe anything. For decimal degrees e.g., we only > >> need > >> 100s values the rest can be left to the fractional portion. > > > > Unfortunately, even your description here still doesn't help me understand > > what the intended semantics of these values are. > > > > To help illustrate my confusion, here are a few possible things that could > > be what is intended to be conveyed: > > > > - the geodetic-datum description of the object has been measured to be > > within a known delta of the actual object being described, at all points > > on the object that the coordinate system can describe > > > > - the geodetic-datum description of the object is capable of determining > > relative differences between points on the object to within a particular > > delta of precision, but those individual coordinate values may be farther > > than that delta from the actual point on the object that was referred to > > > > - the values that are reported in this YANG module reflect measurements > > that were made and are known to be within some delta of the coordinate > > system's value that they are reported as > > > > - the values that are reported in this YANG module reflect measurements > > thare are known to be distinguishable from other measurements to within > > some delta of other measurements relative to that coordinate system, even > > though the actual position being indicated may diverge from the reported > > value by more than that delta > > > > - the values that are reported in this YANG module reflect measurements > > that were made and are known to be within some delta of the actual point > > on the object that the coordinates refer to > > > > - the values that are reported in this YANG module reflect measurements > > that were and are known to be distinguishable from other measurements of > > points on that object within some delta, but the actual distance from the > > measured point to the point on the object indicated by the reported > > coordinates may be larger than that delta > > > > In short, there are at least three classes of things at play here: the > > actual object itself, the coordinate system used to model the object, and > > values reported in the YANG module (which are assumed to ultimately derive > > from some form of measurement). To talk about accuracy or precision > > implies a relationship between elements of two of those classes, and I > > don't even know which of those classes you're trying to talk about. > > Let's start with a simple baseline, if you want to dig any deeper than the > well understood Lat+Long; do you know what a geodetic datum is? This is > required knowledge if you want to get into anything more than the obvious > Lat+Long use of this grouping. It defines the coordinates and also the > accuracy of measurements.
Well, I thought I did, but the fact that you are asking me makes me less sure that I actually do. Limiting just to the Earth for simplicity of discussion, it is "well known" that the earth is not a perfect sphere; it's not even a regular ellipsoid. Even discounting local topography on the surface, "mean sea level" varies due to the differing internal density, angular momentum, and myriad other factors. So if we want to talk about coordinates of a point on the earth, we have to build a model of the earth in which we define what our coordinates mean. We'll have to anchor our coordinate system to actual points on the earth in some way, whether by defining an arbitrary origin at a physical object, using the center of mass (which can at least in theory be measured to very high precision), or some combination thereof. But the coordinate system remains a model of the actual earth, and there will be some skew between them for points that in an ideal coordinate system would match up exactly with the physical object. The coordinate system will have inherent accuracy limits based on how much skew there is between the idealized points in the coordinate system and the actual points on earth they're supposed to represent. When one makes a measurement with respect to a given coordinate system there may also be inherent limits to the precision of measurement that can be made with respect to the coordinate system, e.g., if the coordinate system is defined with respect to GPS points, the limits of GPS resolution are a bound on how precisely one can make a measurement. That's all well and good, but what I describe above is a property of the coordinate system itself, not a property of individual measurements made with reference to that coordinate system. The YANG grouping we define here allows overriding the coord-accuracy and height-accuracy on a per-grouping-instantiation basis, i.e., for a single list of coordinates. But those coordinates in the instantiation are certainly sometimes going to be derived from measurements, and I expect that measurements will be the overwhelming majority of usage. Measurements, however, *also* have accuracy and precision, but this time with respect to the coordinate system they are being measured in. Instrumental error and other factors can introduce a systemtic bias in the measured values, leading to bad accuracy, even if the precision of the group of measurements remains quite good, so that relative comparisons within the dataset are reliable even if the absolute numers are not reliable with respect to the coordinate system. So, when we refine the coord-accuracy and height-accuracy for an instantiation of the grouping, what does that mean? > It is out way out of scope for this YANG grouping to try and explain the huge > field of geographic locations and geodetic datum and systems. Of course. But we should have enough of a reference so that people can have a way to read up and understand what the fields we are defining actually mean. -Ben _______________________________________________ netmod mailing list [email protected] https://www.ietf.org/mailman/listinfo/netmod
