Choice of Ellipsoidal or Cartesian Calculations Needed in Measure Tool
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Bill Chiarchiaro
Unlike QGIS, the QField measure tool does not allow the choice of ellipsoidal or cartesian distances. Apparently, it calculates ellipsoidal distances. For projects that are based on a cartesian grid (e.g. UTM), this can be a source of error or confusion, or at least cause the user to have to do extra calculations involving a scale factor.
There should be a setting to choose whether ellipsoidal or cartesian calculations are performed.
Along with affecting the calculation of distances, the setting should also affect the calculations of the measure tool's other results as well (azimuth, perimeter, and area).
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Mathieu Pellerin
Bill Chiarchiaro, the measurement tool will respect your project's ellipsoid which is configured when preparing the project in QGIS through the project properties dialog's general panel. If you set the ellipsoid to "none/planimetric", you will end up with cartesian measurements.
Are you suggesting you'd want the ability to dynamically swap between the two modes?
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Bill Chiarchiaro
Mathieu Pellerin,
Hello, Mathieu.
Thank you for working on this feature request. Being able to set the ellipsoid to “none/planimetric” can be a useful feature for some small, isolated projects. However, use of planimetric coordinates or calculations is a separate issue from whether a project has an associated ellipsoid.
The great majority of QField projects use GNSS-derived data, which are referred to some geodetic reference frame, which has an associated ellipsoid, and thus it would be incorrect to set such projects’ ellipsoid to “none/planimetric”. Geodetic coordinates (latitude and longitude) are dependent on the ellipsoid, and geodetic (ellipsoidal) distances and azimuths are computed on the surface of the ellipsoid. Furthermore, a project can have an associated projection to a plane or grid, such as UTM. The conversions between geodetic and grid coordinates are dependent on the reference frame’s ellipsoid, and thus grid distances (“cartesian” distances in QGIS-speak) and azimuths are too.
If a user has selected a CRS that incorporates a plane or grid projection, for example UTM on some version of NAD 83, that’s a strong indication that the user is interested in grid (“cartesian”) calculations. That doesn’t mean that the user is interested exclusively in grid calculations. QGIS allows the user to select either ellipsoidal or cartesian calculations. QField should offer the same choice. Certainly, not offering grid calculations when the user is working with grid coordinates is a shortcoming.
As noted in the initial statement of this feature request, the setting should affect the calculation of distance, azimuth, perimeter, and area.
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Zsanett Medvei
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in progress
Such a great input, thank you for sharing. We put this on our backlog.