Feature request #4252: different built-in tools calculate inconsistent polygon areas - QGIS Application - QGIS Issue Tracking

Feature request #4252

different built-in tools calculate inconsistent polygon areas

Added by Alister Hood about 13 years ago. Updated over 8 years ago.

Status:	Closed
Priority:	Normal
Assignee:	Magnus Homann
Category:	Projection Support
Pull Request or Patch supplied:	No	Resolution:
Easy fix?:	No	Copied to github as #:	14197

Description

- When using the "measure area" tool, there is an "ellipsoidal" checkbox option. If this is unchecked, a slightly different area is reported, which I think I have seen called the "planimetric" area.
- The "Identify results" for polygons include a "Derived" area. This area is the "ellipsoidal" area.
- The field calculator can also report the area of a polygon. This area is always the "planimetric" area.

I think this behaviour should be tidied up:
- either ellipsoidal or planimetric (I'm not sure which one) measurements should be default everywhere.
- assuming both types of measurement are actually useful, then it should be possible everywhere to use either type. This could be done by a single global or project preference which controls the results for all three tools, or it could be implemented separately for each tool. I don't know which approach would be best.

Associated revisions

Revision ddbdcf8a
Added by Nyall Dawson over 8 years ago

Fix project unit confusion (pt 2): add project distance unit setting

Adds a new option in project properties to set the units used for
distance measurements. This setting defaults to the units set in
QGIS options, but can then be overridden for specific projects.

The setting is respected for length and perimeter calculations in:
- Attribute table field update bar
- Field calculator calculations
- Identify tool derived length and perimeter values
- Default unit shown in measure dialog

Also adds unit tests to ensure that length and perimeter calculated
by attribute table update bar, field calculator and identify tool
are consistent wrt ellipsoidal calculations and distance units.

(refs #13209, #12939, #2402, #4857, #4252)

Revision dfdcec89
Added by Nyall Dawson over 8 years ago

Fix project unit confusion (pt 3): add area unit settings with a
ton of available area units (eg m2, km2, mi2, ft2, yd2, ha, ac,
etc)

Adds a new option in both the QGIS setting and project properties to
set the units used for area measurements. Just like the distance
setting, this defaults to the units set in QGIS options, but can
then be overridden for specific projects.

The setting is respected for area calculations in:
- Attribute table field update bar
- Field calculator calculations
- Identify tool derived length and perimeter values

Also adds unit tests to ensure that area calculated by attribute table
update bar, field calculator and identify tool are consistent wrt
ellipsoidal calculations and area units.

TODO: make measure tool respect area setting

(refs #13209, #4252 and fixes #12939, #2402, #4857)

Revision 479d90a5
Added by Nyall Dawson over 8 years ago

Make measure tool respect project area unit setting

Also add unit tests for measure tool length and area measurement
to ensure they return the same results as field calculator
and identify tool

Refs #13209, #4252

Revision 4094bae9
Added by Nyall Dawson over 8 years ago

Respect project ellipsoid and unit settings in virtual fields

Fix #12622, #4252

History

#1 Updated by Paolo Cavallini almost 13 years ago

#2 Updated by Giovanni Manghi almost 13 years ago

Target version set to Version 1.7.4

#3 Updated by Alister Hood over 12 years ago

Also see #3296 I think.

#4 Updated by Giovanni Manghi over 12 years ago

Target version changed from Version 1.7.4 to Version 2.0.0

#5 Updated by Alister Hood over 12 years ago

Note that the "export geometry columns" feature in ftools is now capable of calculating ellipsoidal areas, so I guess people should be encouraged to use that tool. It is a lot slower though.