> On Jul 17, 2021, at 6:14 PM, Benjamin Kaduk <[email protected]> wrote: > > 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’s supposed to mean the accuracy of the measurement that is recorded in the grouping. So if the coord-accuracy is .1 and the measurement is lat/long then the accuracy is within 1/10 of a decimal degree. if the measurement is in cart coordinates the accuracy would be 100cm. I don’t think we need to make this anymore complex than that. Is there some text you would like to see to make that clearer? Thanks, Chris. > > >> 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
