1

I created Buffer (with Vector, Geoprocessing Tools) with 500 M distance from a point in the map (let's say Point A). I made another point (Point B) and it is not inside the 500 m Buffer. The Distance matrix tool shows that the points are separated about 413 M from each other. I noticed that this difference comes (using the Measure Line tool), because the buffer has Cartesian distance, yet the distance in Distance Matrix shows the ellipsoidal distance values.

How to either make the buffer in ellipsoidal distance, or let the distance matrix shows Cartesian distance?

Note: The Cartesian distance of point A and B is about 661 Meters for each other.

  • 2
    Please state which CRS you used for which operation, as well as where the points are situated in relation to your CRS (close to central meridians or way off-center). – Erik Dec 17 '19 at 15:44
  • WGS 84/Pseudo-Mercator with Authority ID:EPSG: 3857 I put the points somewhere in Germany; as i am using OSM datas.. (How do i check is it off center or not? I am.. quite new to this) – necrohiero Dec 17 '19 at 16:24
  • Use an equi-distant CRS. Distance measurements in WGS 84/Pseudo-Mercator will always be wrong (except in rare exceptions, like how a broken clock is right twice a day). – csk Dec 17 '19 at 18:19
  • EPSG:3857 is a Mercator projection, cylindrical, direct (not transverse) and tangential to the equator. So it doesn't deform distances along the equator, but deforms distances a lot far away from it. – Gabriel De Luca Dec 18 '19 at 2:50
0

Buffer tool uses the CRS of the layer.
We can't have an "ellipsoidal distances" projection.
So, if you want to draw buffers with the ellipsoidal distance in QGIS, the most similar that you can do is project the layer to a CRS that do not deform the distances so much in your work area.
Also, that is the best practice.

Distance matrix tool uses the ellipsoid definition of the project to make measurements.
In Project menu, Porperties, General tab, Measurements section, you will find the ellipsoid definition for the project and the units used for ellipsoidal measurements.
But you can define there the strange "None / Planimetric" ellipsoid, which turns all ellipsoidal measurement tools to planimetric.
So, you can measure planimetric (Cartesian based in the CRS of the layer) distances in Distance matrix tool.

  • I have tried both ways; The first one works, but of course wrong (in a way that the distance that is shown in GMaps is actually 400 meters, and with planimetric is 600 meters), But at least, the distance matrix shows the same distance. Now, if i want to use a new CRS, which one doesn't deform the distances, which one would be the right one? I just tried DB_REF/ 3-Degree Gauss-Kruger zone 4 (ID: EPSG:5684). And this one still skew the distance by about 100 meters. – necrohiero Dec 20 '19 at 10:20
  • In topography we are used to use custom projections, because we work in small areas and need good accuracy in distances. What projection to use depends on your work area. – Gabriel De Luca Dec 20 '19 at 10:24
  • Ah, ok, i forgot to add, that i am working in the middle of Germany. Maybe any ideas? – necrohiero Dec 20 '19 at 11:48
  • Yes, if the area is small. create a custom transverse mercator centered in your area. If not try a UTM zone. Or look official cartography in your country and work in officilal projecion. – Gabriel De Luca Dec 23 '19 at 2:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.