[Proj] datum matters

[Richard Greenwood]

On Sat, Mar 28, 2009 at 7:23 PM, Noel Zinn <ndzinn at comcast.net> wrote:
> Interesting subject of surprising sensitivity.  Let me offer some
> terminology that helps me parse the issue, with an acknowledgement of EPSG
> for some of it.
>
> Conversions are mathematically exact (within the quality of the algorithm
> and machine precision) changes among coordinate systems (in the EPSG sense,
> not the Richard Greenwood sense), for example, lat/lon to projected
> Northing/Easting, or lat/lon to ECEF geocentric Cartesians.
>
> Transformations are changes in datums, what Richard calls coordinate
> systems, a term that I have redefined above.  Transformations are always
> inexact since the parameters are always empirically derived (measurements
> have errors) and because datum relationships vary spatially and (probably)
> temporally, for example, NAD27 lat/lon to WGS84 lat/lon, NGVD29 normal
> orthometric heights to NAVD88 Helmert orthometric heights.
>
> Some of us prefer the mathematically purity (and entertainment) of
> conversions and others prefer the dirty-fingernail (and remunerative) task
> of quantifying relationships among real-world datums.  Courses for horses,
> as they say in the UK.
>
> Now the EPSG (or maybe the ISO before them) have conflated these concepts
> into the coordinate reference system (CRS), for example, Geographical CRS
> for the lat/lon coordinate system of a particular datum, or Projected CRS
> for a particular projection on a particular datum (what Richard calls a
> coordinate system).  I am not convinced that this taxonomy is an advance
> (agreeing with the recently departed), but I am so in awe of the enormous
> compendium of useful, real-world information that this taxonomy supports
> that I happily accept it as the de facto standard.
>
> Noel Zinn
>
> PS - Shouldn't pj_transform() be pj_convert() if you accept the definitions
> above?

I'm thinking of projections and coordinate systems as nouns, things,
definitions. And I see conversions and transformations as verbs,
actions that one performs on data. I agree that conversions are
rigorous and transformations not necessarily so. I believe that
pj_transform() supports 3 types of datum transformation in addition to
conversions.

-- 
Richard Greenwood
richard.greenwood at gmail.com
www.greenwoodmap.com