# ExtractSurface extracts more than I would like

**URL:** <https://discourse.opengeosys.org/t/extractsurface-extracts-more-than-i-would-like/809>\
**Category:** Help & Support\
**Created:** [June 18, 2021, 6:25pm UTC](https://discourse.opengeosys.org/t/extractsurface-extracts-more-than-i-would-like/809 "2021-06-18T18:25:41Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Meshing](https://discourse.opengeosys.org/letter_avatar_proxy/v4/letter/m/858c86/32.png) [@Meshing](https://discourse.opengeosys.org/u/Meshing)\
**Post date:** [June 18, 2021, 6:25pm UTC](https://discourse.opengeosys.org/t/extractsurface-extracts-more-than-i-would-like/809/1 "2021-06-18T18:25:42Z")

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Hi,

I just started using OpenGeoSys with the intention of extracting the uppermost surface of an unstructured grid in .vtu format. I used the ExtractSurface tool with the following parameters:

-i MyMesh.vtu -o TopSurface.vtu

The following picture shows myMesh (top left), the TopSurface output (top right), and a temporary workaround with a ParaView threshold filter (bottom):

 ![image](https://discourse.opengeosys.org/uploads/default/original/1X/03c691689ebacdad5928df9d462f99e7b4040e4b.png)

I suppose the multiple ‘top’ slices might be theoretically correct because this is unstructured mesh in format. However, in practice the mesh is defined in layer/row/column format, there is a very clear ‘top’ to the viewer. Is there a way extract the actual top surface only? Note: this is just a simple example mesh for testing, my real meshes will be more interesting!

I seem to be able to get a temporary work-around in ParaView with a threshold filter, but ideally I’m lookign to access the global top elements of the mesh, rather than the local top elements.

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**Author:** ![thomas.fischer](https://discourse.opengeosys.org/user_avatar/discourse.opengeosys.org/thomas.fischer/32/416_2.png) [@thomas.fischer](https://discourse.opengeosys.org/u/thomas.fischer)\
**Post date:** [December 17, 2021, 8:49am UTC](https://discourse.opengeosys.org/t/extractsurface-extracts-more-than-i-would-like/809/2 "2021-12-17T08:49:17Z")

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Sorry for the late reply. Did you find the reason for the strange behaviour? If not, please, could you provide a small test case such that this behaviour can be reproduced?