On Wed, Jul 29, 2009 at 9:48 AM, Matt Wilkie <[email protected]> wrote:
> Hi Robert, thank you for contributing your insight, it helps! > > > cs2cs -v is quite clear about what it's doing, OGR not so much. > > > > Using the "straight" Proj EPSG defs on my system: > > $ cs2cs -v +init=epsg:3579 +to +init=epsg:4617 > ... > > No datum transform will happen (ellipses match, and +towgs84 is ommitted > (ie ==0)). > > Then I would say it is improper for ogr2ogr to write NAD83(CSRS) in > the output .prj. Proj4 and dependant projects may think WGS84, NAD83 > and NAD83(CSRS) are synonymous but from my reading over the last > couple of weeks it is clear that while the first two were > interchangeable at one time, they are not now. > > 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. 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). Proj4 just converts numbers. Where the numbers are defined from (EPSG definitions or your own) and what they mean is a whole other kettle of fish. Rob :)
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