[Gdal-dev] gdal/ogr/proj and NAD83(CSRS)

[Matt Wilkie]

>> In future a person or program using data defined as NAD83(CSRS) will
>> reasonably expect sub-meter datum accuracy, for that is the purpose of
>> the datum. if this is not the case, it should not call itself such.
>
> The accuracy of any data is irrelevant to the projection it's in though. I
> can digitize data from satellite images that are up to 100m off
> horizontally, or from paper maps at 1:500K with cartographic license taken.
> Whether I do it in NAD83(CSRS) or any other projection/datum - that doesn't
> impact the data quality - that's based off where the information comes from
> and what I've done to it.

Yes exactly, that's why I said *datum* accuracy, as distinct from
coordinate or projection accuracy.

> Whenever i convert datums I will lose information, that's reality. People
> tend to convert datums via magic commands and assume it's all perfect but it
> needs to be pretty carefully understood what it means (which i think you now
> do).

Well I certainly have a lot more facts and figures in my head. It
remains to be seen whether it has resulted in greater understanding!
:)  I'm okay with magic commands losing information when it's just
part of the way things work and documented. I'm not so comfortable
when loss is not apparent, or worse obscured, which is what I see
happening here and why I'm continuing to pick at it.

I'd like to return to something touched on earlier but not explored
fully. In the background of ogr2ogr, cs2cs, etc. what parameters are
actually used in the conversion/transformation? One can use any of
proj4 strings, EPSG:nnn, OGC or ESRI WKT and maybe some others on the
command line, but as this thread reveals there are parameters within
these syntax which are silently passed by. What or where is the
authoritive list of supported parameters? (or the list of ignored ones
if that is shorter)

thanks,

-- 
-matt