Show me the Code, Distance Matrix

: In this lesson, we will illustrate the importance open source code.

The Distance Matrix algorithm documentation says nothing about how it calculates distances!

Watch a video of the issue.

Reading code to understand an algorithm

• Go to the QGIS GitHub repository github.com/qgis and search for “distancematrix” within the repository
• distance matrix algorithm source code
  • line 179: Transforms the target features into the source CRS (coordinate reference system)
  • line 182: CRS set to source CRS
  • line 183: Ellipsoid set to context ellipsoid
• distance area class source code
  • line 413: converts to meters
  • the only information passed to qgisdistancearea is the context ellipsoid and transformation, and the source crs
• The API (applicaton progrmaming interface) guide can also be a quick and helpful reference to QGIS variables qgis.org/api
  • there is a context.ellipsoid() and a context.distanceUnit(), but the distance matrix algorithm never uses distanceUnit.

Conclusions

• Distance Matrix appears to calculate distance in meters, no matter what.
• Target features are automatically reprojected to match the CRS of input features
• If the input layer uses a projected coordinate system, that system is used or planar distance calculation.
• If the input layer uses a geographic coordinate system, the project ellipsoid is used for ellipsoidal distance calculation.

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