> 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 -- View this message in context: http://n2.nabble.com/gdal-ogr-proj-and-NAD83%28CSRS%29-tp3311110p3316518.html Sent from the GDAL - Dev mailing list archive at Nabble.com. _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev