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I'm using ST_SnapToGrid: ST_SnapToGrid(somePoint, 0.0000001) and I get strange result on many points, such as: 

- POINT (21.619820510769063 41.94186153740355) -> POINT (21.6198205 41.941861499999995)  
- POINT (22.49594094391644 41.20357506925623) -> POINT (22.495940899999997 41.203575099999995)

What am I doing wrong?

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  • Expecting every floating-point value to have an evenly divisible mantissa? There are just some values which can't be manifested by IEEE representation. It would be nice if ST_AsText had an optional precision operand, but it doesn't. – Vince May 6 at 11:07
  • Try the floating point converter in h-schmidt.net/FloatConverter/IEEE754.html with decimal value 0.0000001 and you will see gets changed into 1.0000000116860974230803549289703369140625E-7. – user30184 May 6 at 11:13
  • 1
    See Why are floating point numbers inaccurate? – CL. May 6 at 11:24
  • Yes, I know about floating point imprecision problem. I thought (or wrongly assumed) that it will try to round the number in such cases. So I think what I want to do is to round all geometries to given precision in addition to SnapToGrid. Or maybe not even use it, only round the numbers. I not sure now. – kujaw May 6 at 14:57
  • you could try with a ST_AsTWKB roundtrip. – ThingumaBob May 6 at 15:27
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Turns out that all of the geometries that couldn't be divided by ST_SnapToGrid had form of xx.xxxxxxx9999999x, where x is a digit. That means, that if I want its precision to be centimetric, it's safe to treat such geometries as a number and just round it mathematically, all of them will round to the higher number.
After such process, they matched with the geometries I should get after reducing precision. So, first I snapped the X and Y geometries separately, then rounded them and recreated geometry from concatenated string. 

ST_GeomFromText(concat(
'POINT (', 
round(ST_X(ST_SnapToGrid(point, 0.0000001))::numeric, 7), 
' ', 
round(ST_Y(ST_SnapToGrid(point, 0.0000001))::numeric, 7),
')'
))
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