[Crm-sig] Issue 388 measuring position

Øyvind Eide lister at oeide.no
Fri Mar 5 17:11:25 EET 2021


Dear Martin,

I am considering a possible practical counter-example from my own manual measurement experience. I am not sure if it is relevant but maybe the first sentence (”Any position measurement is based on triangulation with multiples distances to reference points and angle measurements”) should be modified.

I am mapping the location of a boulder. Using a compass and my steps, fairly well calibrated to one meter, I measure the distance and direction from my fix point, in this case the spot where a creek is crossing a path.

The fix point itself is established through stereo aerial photography and thus, based on a sort of triangulation. But my measurement from the fix point and the boulder is based on the distance and direction from the single fix point.

Maybe it makes sense to still call this a triangulation as the compass shows the direction to the magnetic north. 

Whatever can be argued about this, maybe it makes sense to add to the GPS descriptions a bit more on analogue measurement practices? They were the basis for a massive amount of museum and cultural heritage documentation. 

All the best,

Øyvind

> Am 01.03.2021 um 21:19 schrieb Martin Doerr <martin at ics.forth.gr>:
> 
> Dear All,
> 
> I revise my previous proposal for measuring positions:
> 
> 
> Any position measurement is based on triangulation with multiples distances to reference points and angle measurements. GPS measures distances to satellites. Distances are Dimensions. If directed distances use georeferenced directions, i.e. angle to the rotation axis of earth, etc. angles are again dimensions. Hence, a position measurement is an evaluation of a combination of multiple associated distance and angle measurements from a particular spot to certain reference points of known position in the same reference space. If stars are used, they constitute (extremely) distant reference points. Gravity and Earth Magnetic Field also provide reference directions for angle measurements that do not need a second reference point. Classical longitude measurements use temporal simultaneity of a common event with a reference location, which evaluates to an angle. All methods are fairly complex, but the details are a standard routine or even hidden in a modern GPS module. 
> 
> Therefore we argue that position measurement is a specific (composite) observation which results in a position expression, but the constituent dimensions may or may not be documented.
> Hence, P40 observed dimension (was observed in): E54 Dimension may not be instantiated.
>  
> All position measurements are approximations of other places. Therefore, they result in a declarative place defined by an E94 Space Primitive. Since in general we talk about moving reference spaces, moving things and evolving processes, the time of measurement is essential. We take it either to be the time-span of the measurement, or a narrower time-span which covers the contributing time-critical observations. In essence, this defines a declarative spacetime box (volume), which again is an approximation. It appears to me that such an approximation would normally be used to determine parts of the extent of some instance of Presence by overlap, coverage or containment. 
> 
> Sxxx Position Measurement
> 
> Subclass of:        E16 Attribute Assignment
> 
> Scope note:     This class comprises activities of measuring positions in space and time. The measured position is intended to approximate a part or all of the extent of the presence (instance of E93 Presence) of an instance of E18 Physical Thing or E4 Period of interest, such as the outer walls of an excavated settlement, the position of a ship sailing or the start and end of athlete’s run in a competition. Characteristically, a theodolite or GPS device may be positioned on some persistent feature. Measuring the position of the device will yield an approximation of the position of the feature of interest. Alternatively, some material item may be observed moving through a measured position at a given time. 
> 
> A position measurement is an evaluation of a combination of measurement of multiple associated distances and/or angles (instances of E54 Dimension) from a particular spot to certain reference points of previously known position in the same reference space. Often, the observed constituting dimensions are not documented, or hidden in an electronic device software.The measured position is given as an E94 Space Primitive corresponding to a declarative place. Together with the measured time-span covering the time-critical observations it forms a spacetime volume, which should normally overlap with the spatiotemporal extent of the thing or phenomenon of interest. 
> 
> Properties:
> 
> Oxx1 determined position (was determined by): E94 Space Primitive
> 
> Oxx2 has validity time-span (is position validity for): E52 Time-Span
> 
> We may now formulate the approximation to the things of interest, e.g. 
> 
> Oxx3 overlaps with presence: E93 Presence.
> 
> But the time=span of this presence is already implicit in the time-span of validity.
> 
> If we use:
> 
> Oxx3 overlaps with presence of: XXXX, we need a property for E18 and another for E4…
> 
> Another use case is when someone wants to determine if she is at a particular declarative place: Fisherman now mark positions in the sea with GPS, in order to return to the same spot...
> 
> --------------------------------------------
> 
> Opinions?
> 
> To be discussed!
> 
> Martin
> 
> -- 
> ------------------------------------
>  Dr. Martin Doerr
>               
>  Honorary Head of the                                                                   
>  Center for Cultural Informatics
>  
>  Information Systems Laboratory  
>  Institute of Computer Science             
>  Foundation for Research and Technology - Hellas (FORTH)   
>                   
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>  
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