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Anke Trittenbach
Hello all together,

I have a problem and I hope you can help me with this.

My intersection operation for geometries doesn’t return the correct result when the geometries have many decimal places.
There is a method for checking if a point lays on a straight line. In this method the slope and the intersection with Y-Axis of the straight line is calculated. With this and the x-coordinate of the point the method calculates the y-coordinate which should the point have if the point would lay on the straight line. Then this and the real y-coordinate will be compared. If they are equal the point lays on the line. But this doesn’t work correct. Sometimes the method returns that the point does not lay on the straight line, because the calculated values differ with a value of 0.000000000004. But in reality the point lays on the straight line.

Does somebody know if the calculation is wrong? Or how did you avoid such kinds of problems? Does this problem maybe deals with the precision or the tolerance?

With kind regards and thanks for your help.

Anke Trittenbach

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Fernando González
I'm not an expert on this but I think floating point arithmetic is not absolutely precise. For example my computer gives this result 0.1*0.1 = 0.010000000000000002. I think it has to do with the fact that some numbers (like 0.1) are periodic when expressed in binary and computers can only store a finite number of decimals. Maybe you can work with a tolerance.


On Tue, May 19, 2009 at 2:03 PM, Anke Trittenbach <[hidden email]> wrote:
Hello all together,

I have a problem and I hope you can help me with this.

My intersection operation for geometries doesn’t return the correct result when the geometries have many decimal places.
There is a method for checking if a point lays on a straight line. In this method the slope and the intersection with Y-Axis of the straight line is calculated. With this and the x-coordinate of the point the method calculates the y-coordinate which should the point have if the point would lay on the straight line. Then this and the real y-coordinate will be compared. If they are equal the point lays on the line. But this doesn’t work correct. Sometimes the method returns that the point does not lay on the straight line, because the calculated values differ with a value of 0.000000000004. But in reality the point lays on the straight line.

Does somebody know if the calculation is wrong? Or how did you avoid such kinds of problems? Does this problem maybe deals with the precision or the tolerance?

With kind regards and thanks for your help.

Anke Trittenbach

--
Neu: GMX FreeDSL Komplettanschluss mit DSL 6.000 Flatrate + Telefonanschluss für nur 17,95 Euro/mtl.!* http://dslspecial.gmx.de/freedsl-surfflat/?ac=OM.AD.PD003K11308T4569a
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Fernando González Cortés
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http://fergonco.scienceontheweb.net

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Anke Trittenbach
I agree with you. I wanted to figure out how jts deals with this problem. Jts delivers the correct results but I didn’t find it out. I thought that someone could tell me or could give me some tips how jts does.
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