[QGIS-trac] Re: [Quantum GIS] #1219: OTFT on, layer's and project's CRS the same: measure tools and Identify Features return impossible measurments

[Quantum GIS]

#1219: OTFT on, layer's and project's CRS the same: measure tools and Identify
Features return impossible measurments
--------------------------------------------------------------+-------------
        Reporter:  smizuno                                    |         Owner:  nobody           
            Type:  bug                                        |        Status:  new              
        Priority:  critical: causes crash or data corruption  |     Milestone:  Version 1.0.0    
       Component:  Projection Support                         |       Version:  HEAD             
      Resolution:                                             |      Keywords:  measure area line
Platform_version:                                             |      Platform:  All              
        Must_fix:  Yes                                        |   Status_info:  0                
--------------------------------------------------------------+-------------
Comment (by smizuno):

 After a day away from all of this and more testing, I realized two things:

 1. that Identify and the Measure Distance/Area tools are using plane
 calculations when OTFT is off. When OTFT is on they use calculations on an
 ellipsoid. (confirmed by looking in QgsDistanceArea) This is not indicated
 in the User Guide (0.9.1 release) which mentions only choosing an
 ellipsoid for measurements. I am aware that ellipsoid calculations may
 have gross errors on very small angles (and 1m is a really small angle)
 unless designed for such use.

 2. when OTFT is enabled the ONLY valid measure for Identify and Measure
 tools is meters. I have tried this with UTM, long/lat, and a TMERC
 projection in feet. Measuring lines or polygons with any of these CRSs as
 the map canvas CRS would yield reasonable values only when the Measurement
 units is set to Meters.

 In addition, the Scale bar is correct only when the Measurement units is
 set for units matching the map canvas CRS.


 These make for a very confusing user interface.

 I recommend that the Identify, Measure tools and the Scale bar should show
 measurements in either meters or feet (scaling as necessary) that are
 calculated on an ellipsoid, regardless of the CRS or OTFT. Note that
 Degree measurement are generally of little use. Also note, scale bar units
 in feet or meters don't have much meaning if the map canvas CRS is
 long/lat. Perhaps there could be a switch to use plane calculations, but
 this might complicate things too much.

 I believe this would be much more useful to most users.

 The User Guide should mention any limitations in measurements, such as may
 be present  on very small distances or areas.

-- 
Ticket URL: <http://trac.osgeo.org/qgis/ticket/1219#comment:6>
Quantum GIS <http://qgis.org>
Quantum GIS is an Open Source GIS viewer/editor supporting OGR, PostGIS, and GRASS formats