# RAS Coordinate System & World Coordinate system

**URL:** <https://discourse.slicer.org/t/ras-coordinate-system-world-coordinate-system/27069>\
**Category:** Development\
**Tags:** python\
**Created:** [January 6, 2023, 1:15am UTC](https://discourse.slicer.org/t/ras-coordinate-system-world-coordinate-system/27069 "2023-01-06T01:15:12Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![dsa934](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/dsa934/32/17907_2.png) [@dsa934](https://discourse.slicer.org/u/dsa934)\
**Post date:** [January 6, 2023, 1:15am UTC](https://discourse.slicer.org/t/ras-coordinate-system-world-coordinate-system/27069/1 "2023-01-06T01:15:12Z")

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Operating system: window 11  
Slicer version: 4.13.0

Hi, everyone  
I tried to mark a point on the 3d slicer through the markups tool and load the position data.

Here is the code I used.

node\_mark = slicer.util.getNode(‘F’)  
wcs\_data = arrayFromMarkupsControlPoints(node\_mark, True)

Comparing wcs\_data and node\_mark shows the same coordinates.

I thought node\_mark would belong to the RAS coordinate system and  
wcs\_data to belong to the world coordinate system, but isn’t it like this?

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [January 6, 2023, 4:09am UTC](https://discourse.slicer.org/t/ras-coordinate-system-world-coordinate-system/27069/2 "2023-01-06T04:09:18Z")

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3D Slicer’s world coordinate system is the RAS coordinate system.

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**Author:** ![dsa934](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/dsa934/32/17907_2.png) [@dsa934](https://discourse.slicer.org/u/dsa934)\
**Post date:** [January 6, 2023, 4:46am UTC](https://discourse.slicer.org/t/ras-coordinate-system-world-coordinate-system/27069/3 "2023-01-06T04:46:28Z")

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Hi Lassoan,

Thx for answering !

But I’m still confused

Since the python module I made that I want to extend with 3D slicer follows w.c.s (general world coordinate system), do RAS coordinates of 3D slicer refer to normal w.c.s?

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<div class="post-metadata">

**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [January 6, 2023, 5:08am UTC](https://discourse.slicer.org/t/ras-coordinate-system-world-coordinate-system/27069/4 "2023-01-06T05:08:34Z")

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To display any node in a custom coordinate system, add a transform node, set its transform to parent matrix, and [set this transform in the transformable node](https://slicer.readthedocs.io/en/latest/developer_guide/script_repository.html#apply-a-transform-to-a-transformable-node).

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<div class="post-metadata">

**Author:** ![dsa934](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/dsa934/32/17907_2.png) [@dsa934](https://discourse.slicer.org/u/dsa934)\
**Post date:** [January 6, 2023, 5:30am UTC](https://discourse.slicer.org/t/ras-coordinate-system-world-coordinate-system/27069/5 "2023-01-06T05:30:29Z")

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hum…

Actually, the question was whether the 3D slicer’s RAS coordinate system is the (general - people think) world coordinate system. However, if you refer to the reference of lassoan, it seems to be roughly understood. I will try and come back if successful.

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<div class="post-metadata">

**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [January 6, 2023, 6:08am UTC](https://discourse.slicer.org/t/ras-coordinate-system-world-coordinate-system/27069/6 "2023-01-06T06:08:32Z")

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> the question was whether the 3D slicer’s RAS coordinate system is the (general - people think) world coordinate system

Yes, the world coordinate system of the application is the RAS coordinate system. For example, when you load a medical image then the image is displayed so that the image origin is aligned with the world coordinate system origin; R, A, S axes of the image are the first, second, and third axis of the world coordinate system; and world coordinate system unit is millimeters.