[PROJ] Geo/Carto concepts to discuss Re: World UTM in a proper datum

[Dave Knopp]

This post is a bit from the "cheap seats" - it's been ages since I last
used Proj firsthand and it's evolved considerably (not to mention what
I've forgotten;-). However, FWIW...

I hope the following might offer some useful framework for the ongoing
discussion about what types of attribute codes may be desirable to
represent what properties.

Below is a quick (off-the cuff) high level inventory of several relevant
geodesy/cartography concepts with which the various codes interact.

Perhaps thinking/typing discussion in terms of these concepts may help
clarify which codes involve which properties.

In a ideal perfect world, one would use separate independent codes to
refer to each of the half dozen or so independent property choices.
However, back here on Earth;-), the practical question is probably how
best to keep code-to-property(ies) associations most consistent with
existing widespread current usage - while also recognizing the
continuously advancing precision and detail involved in geospatial
representations.

Dave


== H) Horizontal

H1) Earth Modeling

 H1.1) - Geometric Ellipsoid (aka Horizontal Datum) - used for
  -- LLA/XYZ conversions which in turn allow
  -- Accurate approximation for conversion between H.Datums
  (e.g. ClarkXX, GRS80, WGS84-Ellipsoid)

 H1.2) - Geoid - Only for Vertical unless for astrodetic lon/lat


H2) Cartographic Modeling

 H2.1) - Generating Surface
         (Math abstraction)
         (eg often a sphere but could be ellipsoid or even arbitrary)
         (often just consider as part of function below)

 H2.2) - Projection/developing function
         (e.g. Stereograhic, Lambert, UTM, etc)


== V) Vertical

V1) Geometry Modeling

 V1.1 - Distance from Earth Model reference surface
   (e.g. commonly the separation from same Geometric Ellipsoid in H1.1)
   (aka ellipsoidal height)


V2) Energy (Hydraulic Head) Modeling

 V2.1) - Work-integral to point from V.Datum reference surface
   (A specific Geoid is the reference surface)
   (aka orthometric height)
   (Approximately proportional to *difference* in ellipsoid altitudes)
   (geopotential model is used for the integration - if very pedantic)

 V2.2) - Alternatively - the rate at which (very precise) clocks run
   (function of ratio of clock speeds at a point vs on the Geoid)


==

Overall note that a point may be associated with *five* numbers, one is
time, three are spatial, one is energy/work - eg (t, x,y,z, w).

In most current geospatial applications, "t" is mostly discarded, and
only partial coordinate descriptions are provided as either location
(x,y,z) or hybrid pseudo-location (x,y,w). Here interpret "x,y" as
either lon/lat or map East/North.

### - dk


Lesparre, Jochem via PROJ wrote on 4/18/21 8:37 AM:
> Duncan wrote:
> 
>> As an onlooker, I have a question: what is UTM/WGS84 intended to mean? Is it (A) the UTM projection using the WGS84 ellipsoid
> 
> as a parameter, or (B)  is it meant to imply also that the coordinates
> are in a datum that is one of the several labelled WGS84? A is clear and
> completely unambiguous; B isn't, partly because it is subject to change
> as data are collected (not to mention plate motions). Data-dependence is
> also a problem with geoid models: it is why there are so many, after all.
> 
>  
> 
> Duncan, I think WGS84-UTM is normally intended to mean both A and B so
> it is using a datum of WGS84 realisation and the WGS84 ellipsoid.
> According to ISO and EPSG de ellipsoid is part of the datum, so in that
> case the answer A and B is the same as just B.
> 
>  
> 
> Jochem
> 
> 
> 
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