[QGIS-Developer] Length measurement weirdness (again)
andreaerdna
andreaerdna at libero.it
Thu May 3 09:43:13 PDT 2018
Have you noticed this in the measure tool info box part?:
- if OTF is off:
The calculations are based on:
Project CRS transformation is turned off. Distance is calculated in degrees,
based on project CRS (WGS84).
Ellipsoidal calculation is not possible with CRS transformation disabled.
Distance is roughly converted to degrees by using scale at equator (1 degree
= 111319.49 meters).
- if OTF is on:
The calculations are based on:
Project CRS transformation is turned on and ellipsoidal calculation is
selected.
The coordinates are transformed to the chosen ellipsoid (WGS84), and the
distance is calculated in meters.
Moreover the User Guide (paragraph 8.8) is more clear:
QGIS provides four means of measuring geometries:
• the interactive measurement tools ,
• measuring in the Field Calculator,
• derived measures in the Identify Features tool,
• and a vector analysis tool: Vector→Geometry Tools→Export/Add Geometry
Columns
Measuring works within projected coordinate systems (e.g., UTM) and
unprojected data. The first three measuring tools behave equally to global
project settings:
• If “on the fly” CRS transformation (see Define On The Fly (OTF) CRS
Transformation) is enabled, the
default measurement metric is - different from most other GIS - ellipsoidal,
using the ellipsoid defined in
File → Project properties → General. This is true both when geographic and
projected coordinate systems
are defined for the project.
• If you want to calculate the projected / planimetric area or distance
using cartesian maths, the measurement ellipsoid has to be set to “None /
Planimetric” (File → Project properties → CRS). However, with a geographic
(= unprojected) CRS defined for the data and project, area and distance
measurement will be ellipsoidal.
• If “on the fly” CRS transformation is disabled, the measurement metric is
planimetric when the project coordinate system is projected and ellipsoidal
when the project coordinate system is unprojected / geographic.
However, neither the identify tool nor the field calculator will transform
your data to the project CRS before measuring. If you want to achieve this,
you have to use the vector analysis tool: Vector → Geometry Tools
→Export/Add Geometry Columns. Here, measurement is by default planimetric
except if you choose the ellipsoidal measure.
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Andrea Giudiceandrea
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