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

[Matt Wilkie]

> Is there a difference between NAD83 and NAD83 CSRS ?
> (the difference between NAD83/GRS80 and WGS84/WGS84 
> is usually neglectable).

I have been operating on the presumption there must be _some_ kind of
important difference else they wouldn't have gone through the bother of
formulating and establishing a new standard. It would have been a long
process. In any case, I've done some more research and learned some things.
There's much more than I've quoted and a good graphics to visualise the
shape of the distortions. Most of the page is going to be required reading
in our shop ('twas hard to refrain from pasting the page entire!).

On WGS84 vs NAD83: "...From 1986 to 1994, WGS84 was considered the same as
NAD83. ...it turns out that "NAD83/WGS84" was mis-scaled, its origin was
offset and its axes were misaligned resulting in a coordinate shift in
Canada of 1 to 1.5 m. ... so on Jan 2, 1994, WGS84 was realigned to ITRF91
and was no longer compatible with NAD83.  This new realization of WGS84 was
called WGS84(G730) ...With low-cost recreational GPS receiver this 1.5 m
datum difference is inconsequential.  But when using higher-end GPS
receivers and/or correction techniques that provide metre accuracy and
better it must be considered."   -- http://www.geod.nrcan.gc.ca/faq_e.php#23 

The discussion on NAD83 vs NAD83(CSRS) is not as clear (to me anyway):
"...NAD83(CSRS) is consequently a more accurate and stable realization of
NAD83 ... The absolute accuracy of NAD83(CSRS) is of the order of a couple
of cm or better ... NAD83(Original) and NAD83(CSRS) are based on the same
fundamental NAD83 reference system. They are merely different realizations
or adjustments with different levels of accuracy.  Taking into account these
errors, NAD83(Original) and NAD83(CSRS) are completely compatible with one
another." -- http://www.geod.nrcan.gc.ca/faq_e.php#21

So my take on this is that it is fine to use NAD83 if the accuracy is 1m or
lower, but one should use NAD83(CSRS) (in Canada that is) when coordinate
accuracy is sub-meter. However even if the data in question is not sub-meter
if it is transformed one time with NAD83(CSRS) and another with NAD83 the
results will differ, so perhaps more important is to always use the same
datum (on data sharing a common lineage).

Now back to GDAL and related projects: does the fact that there is no proj4
NAD83(CSRS) datum parameter mean the X-to-NAD83(CSRS) transformation is not
happening? 

-matt
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