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QGIS is a free, open source, cross platform (lin/win/mac) geographical information system (GIS)
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Area/Length measure tool returning substantial difference on small sample between Cartesian and Ellipsoidal #58059

Closed oucil closed 3 months ago

oucil commented 3 months ago

What is the bug or the crash?

I am using the measure tool to get the area, in acres, of a small parcel of land. When switching between Ellipsoidal vs Cartesian method, I'm getting a difference in results that is far too large. In the first image, the Cartesian area is calculated as 1.167ac which seems about right for a space that is roughly 250' x 160'...

Cartesian

When I switch to Ellipsoidal, I'm getting a value of only 0.563ac, only half as large...

Ellipsoidal

My ellipsoidal setting is set to 'WGS 84 (EPSG: 7030)

Projection

The way I understand it is ellipsoidal uses the curvature of the earth in its measurement, and cartesian is flat. There is no way that an area could be different by such a degree over a field smaller than a soccer pitch. I've tested with just the measure length tool as well and get similarly off results.

UPDATE:

I have the $length variable in use in a number of labels for my line layers and they seem to show the correct value.

Steps to reproduce the issue

  1. Use the measure tool to mark of an area about the size of an acre.
  2. Switch between Cartesian and Ellipsoidal.

Versions

3.36.3-Maidenhead (Rev. 2df96554)

Supported QGIS version

New profile

Additional context

No response

nyalldawson commented 3 months ago

What crs is your project using?

oucil commented 3 months ago

@nyalldawson WGS 84 Pseudo-Mercator

nyalldawson commented 3 months ago

@oucil that's not a projection designed for accurate planar length/measurement calculation. You'll get nonsense values like this back using Cartesian calculations in that projection.

Closing as not a qgis issue.

oucil commented 3 months ago

@nyalldawson I switched the project to WGS 84 World Mercator and I'm getting similar results. I'm doing a smaller map area of only about a square km. Is there a different CRS you would suggest?

nyalldawson commented 3 months ago

@oucil that depends entirely on the geographic area you are working in. I'd suggest searching up your local city / state / territory for recommended local projections to use.