Surface point inspection different from surface profile inspection

[[Je...]]

I have defined a Surface Point and a Surface Profile in Gom Inspect.

The GD&T position of the Surface Point is 0mm. (This is because the point is used in the construction of a custom coordinate system using the 3-2-1 approach)
I also defined a Surface Profile. I was expecting the surface profile in the close area around the Surface Point to have a value similar to the Surface Point (0mm in this case).
However, there is a deviation of roughly 1mm. 
How is this possible? I understood the Surface Point is projected onto the actual Surface. How can it then have a different value than the Surface itself?

[[Be...]]

Hi,

is this a 'Surface profile' or a 'Surface deviation on actual'? Educated guess: For a surface profile you can define an optional datum system. If you don't specify a datum system the surface profile calculation will try to find a transformation that the maximum deviation of all values is minimized. If you specify for instance the current alignment as datum system the deviation should be the "same" as the discrete  point deviation (due to point sampling it could differ a little bit). 

The 'Surface deviation on actual' respects automatically the current alignment and therefore the values shouldn't differ.

Hope this helps?!

Regards,

Bernd

[[Je...]]

Hi Bernd,

Thank you for your answer.

I am not sure I fully comprehend your response.

However, I noticed that I first constructed a plane, before calling the GD&T surface profile. However, it turns out that the patch on the CAD is not strictly a plane. So I changed it to first creating a surface, instead of a plane, and that seemed to fix it.

Regards,

Jeroen