[QGIS-Developer] QGIS3: reminder of an annoying bug

Daan Goedkoop dgoedkoop at gmx.net
Wed Jan 24 04:20:56 PST 2018


Hello,

I just noticed that this issue apparently also surfaces when opening
projects from QGIS 2.18 in QGIS 3.0

E.g. a project with the CRS set to 25833 (ETRS89/UTM zone 33N) in 2.18
will open with CRS 3006 (SWEREF99 TM) in the current QGIS 3.0.

Kind regards,

Daan

2018-01-20 18:05 GMT+01:00 Jürgen E. Fischer <jef at norbit.de>:
> Hi Tobias,
>
> On Sun, 14. Jan 2018 at 15:09:30 +0100, Tobias Wendorff wrote:
>> I was able to observe many users over the year, which do not correct
>> the projection neither for work within QGIS, nor for saving (they are
>> not aware of the problem) Worse still: when saving, EPSG:3044 is
>> written to the PRJ file (for shapefiles). Thus, the error manifests
>> itself permanently. There are also other
>
> I just tried and the EPSG is only written to the qpj, but not to the prj.
> That's why we write .qpj, because otherwise we may run into the same problem
> even within QGIS.  GDAL doesn't write the full WKT into the prj - it "morphs to
> ESRI" for compatibility reason.  The qpj carries it.
>
> If there is no qpj, we rely on GDAL to detect the correct CRS from the prj,
> which can have issues with some prjs (if there is a qpj too - but we feed it
> the full WKT from the qpj).
>
> GDAL does detect the correct EPSG from prj it wrote itself. Apparently from the
> different name of the projection, because the rest of the prj is the same.
> There is no information about the different axis order in the prj (ie. the only
> actual difference between EPSG:3044 and EPSG:25832).
>
> I guess you see the problem with prj from third parties that use different
> projection names.  And hence there may be no clue left in the prj that could
> help with identifying whether it's EPSG:3044 or EPSG:25832.  How do your .prj
> actually look like?
>
>> Possible workaround:
>> We should check all SRS/CRS, where the parameters overlap. Then we
>> should order those "duplicates" by the frequency of use (for exaple
>> official projections first, follwed by historical projections).
>> Maybe  there's only a handful of projections that are affected.
>
> Unfortunately not.
>
> sqlite> select count(*),count(*)-count(distinct parameters) from tbl_srs;
> 6759|1633
>
> 1633/6759 crs have duplicates.
>
> sqlite> select count(*),parameters from tbl_srs group by parameters having count(*)>1 order by count(*) desc limit 10;
> 138|+proj=geocent +ellps=GRS80 +units=m +no_defs
> 62|+proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs
> 51|+proj=longlat +ellps=intl +no_defs
> 36|+proj=longlat +ellps=GRS80 +no_defs
> 25|+proj=geocent +ellps=WGS84 +units=m +no_defs
> 17|+proj=longlat +ellps=clrk80 +no_defs
> 15|+proj=longlat +ellps=bessel +no_defs
> 14|+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +no_defs
> 13|+proj=longlat +ellps=clrk66 +no_defs
> 9|+proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +vunits=m +no_defs
>
> So some even have 138 duplicates.
>
> sqlite> select srid,description from tbl_srs where parameters='+proj=utm +zone=32 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs';
> 3044|ETRS89 / ETRS-TM32
> 25832|ETRS89 / UTM zone 32N
> 6707|RDN2008 / UTM zone 32N (N-E)
> 7791|RDN2008 / UTM zone 32N
>
> The parameters of EPSG:25832 exists 4 times.
>
> https://trac.osgeo.org/gdal/ticket/4345 looks relevant.  With GDAL trunk
> gdalsrsinfo -e on a EPSG:25832/3044 prj with the projection name changed to
> unknown finds EPSG:25832 and EPSG:3044 with a confidence of each 90%.
>
> I guess we better let the user decide with CRS to use, when we don't find an
> accurate match.  Even if there wouldn't be so many duplicates, the frequency of
> use may be different for different areas of application and any automatism might
> still fail and appear as a bug.
>
>
> Jürgen
>
> --
> Jürgen E. Fischer           norBIT GmbH             Tel. +49-4931-918175-31
> Dipl.-Inf. (FH)             Rheinstraße 13          Fax. +49-4931-918175-50
> Software Engineer           D-26506 Norden             http://www.norbit.de
> QGIS release manager (PSC)  Germany                    IRC: jef on FreeNode
>
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