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I have created a fishnet grid in Arcmap with size 512*512 meters over a certain other shapefile than I want to display on the web. With this in mind I used the Web Mercator CRS (EPSG:3395) to reference the data in.

I then loaded in the same fishnet grid in QGIS, using the same coordinate reference system. Now when I use the distance tool in QGIS, the width is about 312 metres, instead of the 512 meters defined (and measured) in ArcMap.

I was in the understanding that when using a projected CRS the values are in meters, so the coordinates would stay the same in both. Especially when using the exact same CRS.

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  • did you check if your project CRS matches the CRS of your fishnet?
    – LaughU
    Commented Aug 17, 2017 at 10:00
  • Which do you think is correct? Assuming that one of them seems to be I think you should focus your question on the procedure that you are using in the other one. If you think both are wrong ask two questions so you can describe in detail what you did with each software.
    – PolyGeo
    Commented Aug 17, 2017 at 10:07
  • Yes, in both cases in QGIS and ArcMap the project CRS matches the fishnet CRS. Both still EPSG:3395. And I know the ArcMap version is correct, as it displays the same shape as selected in the fishnet tool. I don't assume something is wrong on QGIS's side, as there I'm using the same settings and same tool as in ArcMap. So I'm wondering where/if somewhere else in the process there is something happening.
    – B.Quaink
    Commented Aug 17, 2017 at 11:22
  • Web Mercator is notorious for being inaccurate for distance. I expect you'll find the difference is map units to geodetic measurement.
    – Vince
    Commented Aug 17, 2017 at 11:23
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    1. EPSG:3395 is not Web Mercator but WGS 84 / World Mercator. If you really want "Web Mercator", it's EPSG:3857. 2. The measurements in Arc are likely planar (check the measure tool settings) while the default in QGIS is likely geodesic-based AKA ellipsoid surface distance not planar distance.
    – mkennedy
    Commented Aug 17, 2017 at 20:08

2 Answers 2

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regardless of the discrepancy between QGIS and ArcGIS (which is interesting) makes sense to reproject this workflow into a planar system that is not referencing geodetic whatsoever. As matter of course, making a fishnet grid seems best handled through a fully planar environment. It might also resolve the discrepancy in distance between the two GIS.

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As mkennedy said, it is indeed the case that Arcmap uses planar measurements to calculate the distance. Qgis uses an ellispoidal calculation thus measuring the actual distance on the globe/geoid.

QGIS distance tool

Compared to Arcmap:

Arcmap distance tool

Where you can see it's based on a planar surface.

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