Feature request #4252
different built-in tools calculate inconsistent polygon areas
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
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.
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
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.
#6 Updated by Magnus Homann over 12 years ago
- Assignee set to Magnus Homann
#7 Updated by Magnus Homann over 12 years ago
All except field calculator should be consitent now. Work is ongoing.
#8 Updated by Alister Hood about 12 years ago
#9 Updated by Pirmin Kalberer about 12 years ago
- Target version changed from Version 2.0.0 to Future Release - Nice to have
#10 Updated by Nyall Dawson almost 9 years ago
- Status changed from Open to Closed
Fixed in changeset 4094bae960a8521eca6dfe281408ae2e9ad773be.