# Accuracy of "on-the-fly" CRS transformation in QGIS [duplicate]

How accurate are the OTF transformations in QGIS (2.18)?

I created a 100m by 100m square polygon in a shapefile layer using a CRS of WGS84/UTM zone 14N (EPSG:32614). The project also uses the same CRS. With OTF transformation turned off, when I measure the area and the side of the square using the measurement tool, I get 10,000 sq m for the area and 100m on each side, as expected.

If I turn on OTF transformation and use the measurement tool, I get 10066.2065 sq m for the area and 99.9941 m for each side. Not only is the area incorrect, but multiplying the lengths of the measured sides (99.9941 x 99.9941) does not equal 10066.2065.

Is there a bug or am I just not understanding what OTF does?

Currently, most of my projects use OTF because I receive shapefiles with varying CRS's. It sounds like I need to have each layer reprojected to the same CRS as the project and turn off OTF if I want to get accurate areas and distances. Is this correct and is reprojecting a layer more accurate than OTF?

• My tip is curvature of Earth. That is one of the reasons we have so many coordinate reference systems / datums. So 100 m does not have to be 100 m depending on how you look at it and same applies to area.
– Miro
Commented Sep 19, 2017 at 11:41
• I was confused by the answers given in the previous question that this question is said to be a duplicate of. Some responders seemed to think it was a bug. Others didn't seem to think so. Is it or isn't it? More basically, why does the area calculated under OTF not equal the product of the length of the sides under OTF? It is a square. Commented Sep 19, 2017 at 15:26
• The answer is do not use on the fly. Earth is not flat plane, your lines are on globe which is not perfect sphere. Some use sphere, some ellipsoid, there are different definitions and hence different transitions of whatever is on surface of Earth onto flat plane displayed in 2D window of GIS software. Try to draw a perfect 100mm x 100mm square, exact 100cm2 area on a ball and let me know how did that worked out for you.
– Miro
Commented Sep 20, 2017 at 3:11