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I measure distance (km) in QGIS (Ctrl+Shift+M)

enter image description here.

In Ellipsoidal, it using CSR EPSG:7030.
In Cartesian, they didn't say what projection is used.

If it using ellipsoidal option, does it mean measured using great circle formula in ellipsoid model?
If it using projection, what projection it used?

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    EPSG:7030 refers to the projection, which is based on the Clarke 1880 (RGS) ellipsoid. Distances on the ellipsoid are calculated using an implementation of Vincenty's formular (inverse), cartesian distances are Euclidean distances (Pythagoras) using units and coordinates of the planar projection.
    – geozelot
    Nov 11, 2021 at 8:30
  • thanks, it's good references. but since the map is in indonesia, i think EPSG:7030 prefer to be this one epsg.io/7030-ellipsoid . it's already in meter unit but i dont know how the unit can be meters scale. and why there's 2 coordinate system with same epsg?
    – astaga
    Nov 12, 2021 at 8:05
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    EPSG codes are unique per table, not universally. There is an ellipsoid epsg.org/ellipsoid_7030/WGS-84.html? and transformation epsg.org/transformation_7030/…? It is best to use the official epsg.org site when it is essential to get right information. Site epsg.io is maintained by MapTiles company and it has partly wrong and outdated data. And QGIS is using the CRS of the project for calculating cartesian distances.
    – user30184
    Nov 15, 2021 at 8:18
  • I am still not clear why the area calculation tool is using EPSG=7030 if the original shapefile is in a different projection. Any help on this?
    – Sharad
    Aug 10, 2022 at 13:30

1 Answer 1

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If it using ellipsoidal option, does it mean measured using great circle formula in ellipsoid model?

Yes. I tested this by drawing a line with the measure tool from Nome, Alaska to Longyearbyen, Svalbard, with the project projection set to EPSG:3857 (Web Mercator). When using only two points, the ellipsoidal distance measured roughly 4,200km. The cartesian distance measured roughly 20,700km. When adding many intermediate points between the two, the cartesian distance remained the same, but the ellipsoidal distance increased to 6,500km. By adding the intermediate points, I forced the ellipsoidal distance to follow the Web Mercator version of the shortest line between the two, whereas without the intermediate points, the ellipsoidal distance calculation uses the great circle line instead.

If it using projection, what projection it used?

The projection you currently have set for QGIS display is what will be used for cartesian calculations. This can be checked in the very bottom-right corner of the QGIS window.

It's important to note that no map projection preserves accurate distances between all points. Equidistant projections preserve distance between the contact point (typically the center of the map) and any other point. Some other projections preserve accurate distance along the contact line(s). Therefore, the Cartesian mode of the measure line will be inaccurate in many cases at small scales (like a map of a nation or larger area), and Ellipsoidal should be preferred in most cases.

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