[GRASS-dev] Different CRS matching in 7.2 and trunk

[Helena Mitasova]

> On Nov 10, 2017, at 7:20 PM, Markus Metz <markus.metz.giswork at gmail.com> wrote:
> 
> 
> 
> On Sat, Nov 11, 2017 at 12:22 AM, Helena Mitasova <hmitaso at ncsu.edu <mailto:hmitaso at ncsu.edu>> wrote:
> >
> > Please note, that  in spite of the fact that the datum.table shows same transformation parameters,
> > different nad83 datums are different (it is not just a different name for the same datum) and there are different EPSG codes associated
> > with the relevant CRS.
> > You can find more about it here (or just google it)
> > https://confluence.qps.nl/pages/viewpage.action?pageId=29855153 <https://confluence.qps.nl/pages/viewpage.action?pageId=29855153>
> >
> > Supposedly, the shift from NAD83(86) to NAD83(HARN) can be done with the NGS NADCON program (I have not checked it out)
> > (http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.shtml <http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.shtml>).
> 
> There are 12 (!) different definitions for NAD83. What to do about this? Regard these 12 definitions as different datums? In this case I would need to reinstate the datum check and we need to add more entries to datum.table.

Markus, we absolutely need to have a datum check - although the differences between different NAD83 datums are less than a meter, differences between NAD27 and NAD83 are over hundred meters in some areas. Anything that has a different EPSG should be treated as different CRS exactly for the reasons you mentioned in your answer to Vasek.

Helena

> 
> Markus M
> 
> >
> > Helena
> >
> > On Nov 10, 2017, at 5:30 PM, Markus Metz <markus.metz.giswork at gmail.com <mailto:markus.metz.giswork at gmail.com>> wrote:
> >
> >
> >
> > On Fri, Nov 10, 2017 at 8:46 PM, Vaclav Petras <wenzeslaus at gmail.com <mailto:wenzeslaus at gmail.com>> wrote:
> > >
> > > On Fri, Nov 10, 2017 at 2:21 PM, Markus Metz <markus.metz.giswork at gmail.com <mailto:markus.metz.giswork at gmail.com>> wrote:
> > > >
> > > > > >
> > > > > > 7.2 considers this OK while trunk considers this different. I'm not sure which one is correct.
> > > > >
> > > > > In this case, 7.2 is correct, but 7.2 does not compare datums, i.e. projections would be regarded as matching even with e.g. nad27 and nad83.
> > > > > The comparison of datums is new in 7.3 and needs some refinement.
> > > > >
> > > > > I will fix the comparison of swapped lat_1 and lat_2.
> > > >
> > > > Fixed in trunk r71656.
> > > >
> > > > The test for different datum names has been disabled again in trunk r71657. There are several different datum names in lib/gis/datum.table that apparently mean the same: same ellipsoid and same transformation parameters. Apparently, GRASS does not provide a datum name when converting projection information from GRASS to proj4 for reprojecting data.
> > >
> > >
> > > Thank you so much, Markus. This saves me a lot of trouble.
> >
> > About avoiding trouble, the motivation of the stricter CRS comparison is to avoid subsequent geolocation errors. If data have been imported and differences in the CRS were not detected, it can become very difficult later on in the processing to figure out the reason for geolocation errors (different data not matching each other spatially). I'm not sure what to do about different datum names. The current check relies on the test for differences in ellipsoids.
> >
> > Markus M
> >
> > >
> > > (I'm working on a Jupyter Notebook where I need trunk.)
> > > https://github.com/wenzeslaus/Notebook-for-processing-point-clouds-in-GRASS-GIS <https://github.com/wenzeslaus/Notebook-for-processing-point-clouds-in-GRASS-GIS>
> > >
> > > >
> > > >
> > > > Markus M
> > > >
> >
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> >
> >
> > Helena Mitasova
> > Professor at the Department of Marine, 
> > Earth, and Atmospheric Sciences
> > and Center for Geospatial Analytics
> > North Carolina State University
> > Raleigh, NC 27695-8208
> > hmitaso at ncsu.edu <mailto:hmitaso at ncsu.edu>
> > http://geospatial.ncsu.edu/osgeorel/publications.html <http://geospatial.ncsu.edu/osgeorel/publications.html>
> >
> > "All electronic mail messages in connection with State business which are sent to or received by this account are subject to the NC Public Records Law and may be disclosed to third parties.” 
> >

Helena Mitasova
Professor at the Department of Marine, 
Earth, and Atmospheric Sciences
and Center for Geospatial Analytics
North Carolina State University
Raleigh, NC 27695-8208
hmitaso at ncsu.edu
http://geospatial.ncsu.edu/osgeorel/publications.html <http://geospatial.ncsu.edu/osgeorel/publications.html>

"All electronic mail messages in connection with State business which are sent to or received by this account are subject to the NC Public Records Law and may be disclosed to third parties.” 

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