I want to create some transects and a grid with specific lengths, but I realized Qgis is under measuring distances. For the transects I used "Advanced digitizing" (100m lines) and for the grid I used "create grid" with horizontal and vertical spacing of 100 m. However, instead of 100m I got in both cases 75m. All layers are with the same projection > EPSG:3857 - WGS 84 / Pseudo-Mercator.
Any ideas why this is happening?
That is happening because:
You are drawing planimetric lines in a Mercator projection far away from its true scale curve (the equator in EPSG:3857);
And the Mercator projection produces considerable deformations in distances far away from its true scale curve (curve if tangential, i.e., k_0 = 1; curves if secant, i.e., k_0 < 1; or nothing if exterior, i.e., k_0 > 1);
And you are measuring the ellipsoidal length of the drawn lines.
Advanced Digitizing tools takes the coordinates from the canvas. So the problem is in the project CRS.
If you draw a 100m line in an EPSG:3857 project (regardless the CRS of the layer in which the geometry is stored), but then measure the ellipsoidal length of the line, you will see important differences between the drawn length and the measured length, bigger the farther the line is from the equator.
If you draw a 100m line in an EPSG:3857 project, store that geometry in an EPSG:3857 layer, but then measure the Cartesian (planimetric) length of the line, the measure will return 100m.
If you want to draw lines of 100m ellipsoidal length (that is not easy in a flat surface), you could define a project CRS that does not deform the distances so much in the area in which you are drawing.
What is that CRS depends of what is that area.
The Create grid tool takes the CRS of the project by default, but you can change it in the GUI dialog. The output layer will have the CRS selected.
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