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Why the process of reprojection from geographic coordinates to projected and finally to geographic coordinates, produces a very tiny displacement from the original information.

  • 13 February 2023
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rubeng
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If I reproject geographic information from geographic coordinates to projected coordinates, and before I convert to geographic coordinates again, the final information has a little displacement regard the original information. I don´t have any other process in between. The displacement is very tiny, but I need to know why is it produced. I´ve make the same process in Arcgis and it doesn´t appear this displacement.

I can see the displacement in QGIS because this application allows to make a very big zoom to the information. I think that ArcGis doesn´t allow to make such a very big zoom.

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Best answer by rubeng 14 February 2023, 07:24

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caracadrian
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It all comes down to mathematics, geometry precision (number of decimals used) and decimal to binary (and reverse) conversion.

A computer stores decimal numbers as binary (1 and 0) with a given precision (integer, float, double, signed/unsigned). When you convert from dec to bin and back to dec, if you use enough decimals, you get a different number. Generally the last decimal is different.

The same thing applies to CRS reprojection. Each operation introduces a very small conversion error, and they accumulate fast.

rubeng
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But it is a big problem not to have the posibility of manage decimals used into the reprojection transformer, because if you need a high precision you are in trouble.

 

And I also have tried a similar translation with Arcgis and it doesn´t happen. The library that uses Arcgis to reproject the information seems to work with fewer decimals then.

 

Thanks

redgeographics
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Userlevel 5
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Why do you transform from geographic to projected and back to geographic? Can you maybe get away with using a GeometryExtractor to store the geometry in an attribute first, then transform to projected, do whatever you need to do there and then use a GeometryReplacer to replace the projected geometry with the original (geographic) one, thereby avoiding the second transformation alltogether?

rubeng
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I can´t do that, beacause in between the projection transformations I will change the geometry splitting many features. And after splitting features I need to get back to geographic coordinates.

 

But I think I´ve found the solution. There is a transformer called "esrireprojector" that works good, without displacement after the reprojections . This transformer seems to use Arcgis reprojection library, and as I said in Arcgis there wasn't any problem with the sequence of reprojection.

 

Thanks.

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