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The missing string in the projection parameters is : +ellps=clark80
But the projection parameters in Qgis 0.8.0 have another problem (see ticket #598). Either lon_0 should be non null, either the primer meridian should be set to Paris, but not both.

As the milestone for this bug resolution is postponed to the next release whenever there is a new GIS release, I have decided to solve it myself, because it is very boring to have to set manually the projection parameters each time you open a file.

The error message is generated by the function QgsSpatialRefSys::createFromProj4 in the file qgsspatialrefsys.cpp :

QRegExp myEllipseRegExp( "\\\\+ellps=\\\\S+" );
  myStart= 0;
  myLength=0;
  myStart = myEllipseRegExp.search(theProj4String, myStart);
  if (myStart==-1)
  {
    [[QgsLogger]]::warning("QgsSpatialRefSys::createFromProj4 error proj string supplied has no +ellps argument");

    return mIsValidFlag;
  }
  else
  {
    myLength = myEllipseRegExp.matchedLength();
  }

According to the PROJ.4 documentation (available at
ftp://ftp.remotesensing.org/proj/OF90-284.pdf), the +ellps parameter is not mandatory (see page 9, paragraph "Specifying the Earth’s Figure").

The "+ellps" parameter is only a convenient method of specifying standard ellisoidal constants.

The standard way is to use two constants (only one for a sphere) :
The first and required value +a=a where a is the semimajor axis of the ellipse or equitorial radius.
The second parameter can be in any one of the following standard forms:
• semiminor axis or polar radius +b=b,

• flattening +f=f ,

• reciprocal flattening, +rf=1/f ,

• eccentricity +e=e, or

• eccentricity squared +es=e2 .

In the case reported upper, the PROJ4 string is :

+proj=lcc +lat_1=46.8 +lat_0=46.8 +lon_0=2.337229166666664 +k_0=0.99987742 +x_0=600000 +y_0=2200000 +a=6378249.2 +b=6356515.000000472 +units=m +no_defs

It uses the standard form to specify the ellipsoid, so QGIS should accept it.

Simple patch proposal : remove the obligation for the +ellps parameter.

diff -ur qgis_0.9.0.orig/src/core/qgsspatialrefsys.cpp qgis_0.9.0/src/core/qgsspatialrefsys.cpp
--- qgis_0.9.0.orig/src/core/qgsspatialrefsys.cpp       2007-09-16 04:45:42.000000000 +0200
+++ qgis_0.9.0/src/core/qgsspatialrefsys.cpp    2007-11-29 12:48:01.000000000 +0100
@@ -514,17 +514,11 @@
   myStart= 0;
   myLength=0;
   myStart = myEllipseRegExp.search(theProj4String, myStart);
-  if (myStart==-1)
-  {
-    [[QgsLogger]]::warning("QgsSpatialRefSys::createFromProj4 error proj string supplied has no +ellps argument");
-
-    return mIsValidFlag;
-  }
-  else
+  if (myStart!=-1)
   {
     myLength = myEllipseRegExp.matchedLength();
+    mEllipsoidAcronym = theProj4String.mid(myStart+ELLPS_PREFIX_LEN,myLength-ELLPS_PREFIX_LEN);
   }
-  mEllipsoidAcronym = theProj4String.mid(myStart+ELLPS_PREFIX_LEN,myLength-ELLPS_PREFIX_LEN);
   //mproj4string must be set here for the rest of this method to behave in a meaningful way...
   mProj4String = theProj4String;

Second patch proposal : checks that the proj4 string contain either the +ellps parameter, either the +a parameter :

diff -ur qgis_0.9.0.orig/src/core/qgsspatialrefsys.cpp qgis_0.9.0/src/core/qgsspatialrefsys.cpp
--- qgis_0.9.0.orig/src/core/qgsspatialrefsys.cpp       2007-09-16 04:45:42.000000000 +0200
+++ qgis_0.9.0/src/core/qgsspatialrefsys.cpp    2007-11-29 13:03:28.000000000 +0100
@@ -488,10 +488,13 @@
 {

   //
-  // Example:
+  // Examples:
   // +proj=tmerc +lat_0=0 +lon_0=-62 +k=0.999500 +x_0=400000 +y_0=0
   // +ellps=clrk80 +towgs84=-255,-15,71,0,0,0,0 +units=m +no_defs
   //
+  // +proj=lcc +lat_1=46.8 +lat_0=46.8 +lon_0=2.337229166666664 +k_0=0.99987742
+  // +x_0=600000 +y_0=2200000 +a=6378249.2 +b=6356515.000000472 +units=m +no_defs
+  //
   mIsValidFlag=false;

   QRegExp myProjRegExp( "\\\\+proj=\\\\S+" );
@@ -514,17 +517,22 @@
   myStart= 0;
   myLength=0;
   myStart = myEllipseRegExp.search(theProj4String, myStart);
-  if (myStart==-1)
+  if (myStart!=-1)
   {
-    [[QgsLogger]]::warning("QgsSpatialRefSys::createFromProj4 error proj string supplied has no +ellps argument");
-
-    return mIsValidFlag;
+    myLength = myEllipseRegExp.matchedLength();
+    mEllipsoidAcronym = theProj4String.mid(myStart+ELLPS_PREFIX_LEN,myLength-ELLPS_PREFIX_LEN);
   }
-  else
+
+  QRegExp myAxisRegExp( "\\\\+a=\\\\S+" );
+  myStart= 0;
+  myLength=0;
+  myStart = myAxisRegExp.search(theProj4String, myStart);
+  if (myStart==-1 && mEllipsoidAcronym.isNull())
   {
-    myLength = myEllipseRegExp.matchedLength();
+    [[QgsLogger]]::warning("QgsSpatialRefSys::createFromProj4 error proj string supplied has no +ellps or +a argument");
+
+    return mIsValidFlag;
   }
-  mEllipsoidAcronym = theProj4String.mid(myStart+ELLPS_PREFIX_LEN,myLength-ELLPS_PREFIX_LEN);
   //mproj4string must be set here for the rest of this method to behave in a meaningful way...
   mProj4String = theProj4String;

Note :
With one of the above patches applied, and when there is no +ellps argument, new warnings appear :

Warning: [[QgsSpatialRefSys]]::getRecord failed :  select * from tbl_srs where parameters='+proj=lcc +lat_1=46.8 +lat_0=46.8 +lon_0=2.337229166666664 +k_0=0.99987742 +x_0=600000 +y_0=2200000 +a=6378249.2 +b=6356515.000000472 +units=m +no_defs'
Warning: [[QgsSpatialRefSys]]::findMatchingProj will only work if prj acr ellipsoid acr and proj4string are set!...
Warning: [[QgsSpatialRefSys]]::getRecord failed :  select * from tbl_srs where parameters='+proj=lcc +lat_1=46.8 +lat_0=46.8 +lon_0=2.337229166666664 +k_0=0.99987742 +x_0=600000 +y_0=2200000 +a=6378249.2 +b=6356515.000000472 +units=m +no_defs'
Warning: [[QgsSpatialRefSys]]::findMatchingProj will only work if prj acr ellipsoid acr and proj4string are set!...

I don't think it is a problem, the map is correctly displayed and referenced.

Milestone Version 0.9.1 deleted