<div class="gmail_quote">On Wed, Jul 29, 2009 at 9:48 AM, Matt Wilkie <span dir="ltr"><<a href="mailto:maphew@gmail.com">maphew@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
Hi Robert, thank you for contributing your insight, it helps!
<br><div class="im"><br>> cs2cs -v is quite clear about what it's doing, OGR not so much.
<br>>
<br>> Using the "straight" Proj EPSG defs on my system:
<br>> $ cs2cs -v +init=epsg:3579 +to +init=epsg:4617
<br></div>...
<br><div class="im">> No datum transform will happen (ellipses match, and +towgs84 is ommitted (ie ==0)).
<br><br></div>Then I would say it is improper for ogr2ogr to write NAD83(CSRS) in
<br>the output .prj. Proj4 and dependant projects may think WGS84, NAD83
<br>and NAD83(CSRS) are synonymous but from my reading over the last
<br>couple of weeks it is clear that while the first two were
<br>interchangeable at one time, they are not now.
<br><br>In future a person or program using data defined as NAD83(CSRS) will
<br>reasonably expect sub-meter datum accuracy, for that is the purpose of
<br>the datum. if this is not the case, it should not call itself such.
</blockquote><div><br></div><div>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.</div>
<div><br></div><div>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).</div>
<div><br></div><div>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.</div><div><br></div><div>Rob :)</div><div><br></div></div>