QGIS » QGIS Application

You said that "The problem is that OGR's IsSame() function is not robust enough to make a difference between this and the specified ones." In fact, there is close to no difference besides the towgs84 part which would "just" be needed to be transferred to the projection found by QGIS in it's DB.

I don't know if failing on that completely and not recognize any projection
is the best solution here. I think that the IsSame() heuristics work quite
well. Substracting the input and the output (from IsSame()) projection string will show just towgs84 as difference which is perfect.

Best,
Markus

Well, it seems that towgs84 parameter is quite important. Maybe in this case there's really small difference. But when looking to the QGIS's database of projections, there are (many) projections that specify towgs84 parameter, so I guess that it might be quite important - removing it temporarily would be incorrect.

A solution might be to use original proj string even if another projection has been declared as the same. But I don't know if this won't break other things or create some inconsistencies.

Hi,

I think we have a missunderstanding here. I meant to say
that the towgs84 parameter should be taken out while
parsing the string (to match the QGIS SRS DB), then
reinserted, after having found the matching EPSG (which usually lacks the towgs84 parameter.

In case that a towgs84 parameter is already present in the QGIS SRS DB, it should use the user provided one (maybe pop up a notice that it was doing so).

Hope that's clarified now what I tried to say.

Markus

Replying to [comment:5 [email protected]]:

I think we have a missunderstanding here. I meant to say
that the towgs84 parameter should be taken out while
parsing the string (to match the QGIS SRS DB), then
reinserted, after having found the matching EPSG (which usually lacks the towgs84 parameter.

I understand your idea. But the problem is in the way how QGIS copes with spatial references. Once proj4 string is read, QGIS searches for a SRS ID in SRS database. SRS ID is unique for every projection. Thus even if two projection are nearly same (e.g. with or without towgs84) they must have different SRS ID to enable QGIS understand the difference.

That's why taking out towgs84 temporiraly makes no sense - another SRS ID would be chosen instead.

Rescue from this situation is to add custom projection. Once added, your projection has its own SRS ID and will be detected correctly.

Martin

Hi Martin,

I am again falling into this trap:

Warning: QgsSpatialRefSys::getRecord failed : select * from tbl_srs where parameters='+proj=utm +zone=32 +ellps=intl +towgs84=-87.000,-98.000,-121.000 +units=m +no_defs'

Displaying these two maps with on-the-fly projection activated, they are shifted by around 100m:

PROJCS[[UTM Zone 32 Northern Hemisphere"GEOGCS["international"DATUM["D_European_1950"SPHEROID["International_1924"6378388297]TOWGS84[-87000-98000-121000]]PRIMEM["Greenwich"0]UNIT["Degree"0017453292519943295]]PROJECTION["Transverse_Mercator"]PARAMETER["latitude_of_origin"0]PARAMETER["central_meridian"9]PARAMETER["scale_factor"09996]PARAMETER["false_easting"500000]PARAMETER["false_northing"0]UNIT["Meter]]

PROJCSTransverse Mercator"GEOGCS["international"DATUM["D_Monte_Mario"SPHEROID["international"6378388297PRIMEM["Greenwich"0]UNIT["Degree"0017453292519943295]]PROJECTION["Transverse_Mercator"]PARAMETER["latitude_of_origin"0]PARAMETER["central_meridian"9]PARAMETER["scale_factor"09996]PARAMETER["false_easting"1500000]PARAMETER["false_northing"0]UNIT["Meter]]

In my opinion at least a visible warning should pop up that reading the .proj file failed.

thanks,
Markus

This can't be done unless we re-do the entire projections system, earliest at 0.9.

In current SVN trunk i(pre 1.0.0) t continues to fail:

qgis viapri.shp
Python support ENABLED :-)
Loading python plugins
Loaded : Plugin installer (package: plugin_installer)
Critical: [[QgsSpatialRefSys]]::createFromWkt -- theWkt is uninitialised, operation failed
Warning: [[QgsSpatialRefSys]]::getRecord failed :  select * from tbl_srs where parameters='+proj=tmerc +lat_0=0 +lon_0=9 +k                                               =0.9996 +x_0=1500000 +y_0=0 +ellps=intl +towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68 +units=m +no_defs'
Warning: [[QgsSpatialRefSys]]::findMatchingProj will only work if prj acr ellipsoid acr and proj4string are set!...

It works when I manually remove the TOWGS84 string from the WKT (.prj) file.

Suggestion: Ignore the TOWGS84 string when matching the projection, and issue a warning (not a complete failure) that TOWGS84 differs.

Note that most (in theory all) Italian QGIS users have this problem. Likewise for other countries.

thanks,
Markus

Replying to [comment:14 homann]:

Talked to timlinux, and the idea with have is if the specfied projection does not have an exact match (including +towgs84= )in the database of projections, it will ask the user if he/she wants to create a custom projection.

Do you think that will solve the issue?

I think so. But:

This wont happen until 1.1.0 I'm afraid, needs GUI changes and has potential for large changes of projection code needed.

Understood. But as suggested above: Please issue at least a warning that TOWGS84 differs (if not happening already).

Markus

I'm sorry, but the warning is still a GUI change. That towgs differs is just a special case of the fact that the projections differs. As soons as GUI freeze is over i'll try to fix this.

There is a problem in PROJ4: there are several datum for EPSG 3003 (and
others), but in PROJ4 the epsg file does not allow a relation 1:n with
present format (thanks Markus for clarifying this).
Currently, users need to define their special SRS, which is cumbersome
and scary for newbies. GRASS solved it recently:
http://grass.osgeo.org/screenshots/gui.php#wxpython
-> Location wizard (geodetic datum selection for EPSG 3003 - Italy mainland)
Would it be feasible to copy the approach and port it to QGIS?