# Q- Surface area reported from Models: outward facing polys only?

**URL:** <https://discourse.slicer.org/t/q-surface-area-reported-from-models-outward-facing-polys-only/11795>\
**Category:** Support\
**Created:** [May 30, 2020, 2:27pm UTC](https://discourse.slicer.org/t/q-surface-area-reported-from-models-outward-facing-polys-only/11795 "2020-05-30T14:27:05Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![hherhold](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/hherhold/32/12199_2.png) [@hherhold](https://discourse.slicer.org/u/hherhold)\
**Post date:** [May 30, 2020, 2:27pm UTC](https://discourse.slicer.org/t/q-surface-area-reported-from-models-outward-facing-polys-only/11795/1 "2020-05-30T14:27:05Z")

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I have a quick question on the surface area number reported by the Models module. I did a quick exercise where I made a 5cm sphere (well, as close as I could get by hand using the ruler) and compared reported values to hand-computed values - 4_pi_r^2, etc.

The value I get for surface area is what I would expect, but I was a little confused from earlier posts ([Using surface cut of segment editor to measure area](https://discourse.slicer.org/t/using-surface-cut-of-segment-editor-to-measure-area/5990)) regarding counting front-facing and back-facing polygons in the area calculation, and needing to divide by 2.

For a “closed surface representation”, such as a segment converted to a model, it seems that the computed surface area is the outward-facing polygons only, and one does not need to divide the reported quantity by 2. Is this correct?

Thanks!!

-Hollister

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**Author:** ![jamesobutler](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/jamesobutler/32/7511_2.png) [@jamesobutler](https://discourse.slicer.org/u/jamesobutler)\
**Post date:** [May 30, 2020, 2:41pm UTC](https://discourse.slicer.org/t/q-surface-area-reported-from-models-outward-facing-polys-only/11795/2 "2020-05-30T14:41:37Z")

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I think what was meant for surface cut is that on a 2D image if you draw say a circle then your 3D surface is like a circle disc shaped object. Therefore the surface area includes both the front side and the back side so you would divide by 2 to get the area of the circle. I guess technically there is an edge to the disc which has surface area, but dividing by 2 will get you close.

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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:** [May 31, 2020, 1:02am UTC](https://discourse.slicer.org/t/q-surface-area-reported-from-models-outward-facing-polys-only/11795/3 "2020-05-31T01:02:06Z")

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You only need to divide the surface area by 2 if you want to measure area of an _open_ surface by creating a very flat _closed_ surface around it (as the closed surface wraps around both sides of the open surface).

For open surface measurements I would no longer recommend creating a closed surface and divide by 2, but instead you have better options, such as:

A. Use closed curve markups:

> [@How can I calculate an area on a CT image. I can calculate volumes (mm^3) but not areas (mm^2)](https://discourse.slicer.org/t/how-can-i-calculate-an-area-on-a-ct-image-i-can-calculate-volumes-mm-3-but-not-areas-mm-2/1549/4):
>
> In recent Slicer Preview Releases, you can draw curves and get their surface area (2D surface, as you would expect - only one side, no thickness). Select “Closed Curve” from the “Create and place” toolbar button: [image] Left-click on the image to place curve control points and right-click to finish placement: Measurements on curves are still work in progress, but you can open the Python console (Ctrl-3) and copy-paste the code below to see the surface area: curves=slicer.util.get…

B. Use cross section area measurement module:

> [@New module for measuring cross-section area of segments](https://discourse.slicer.org/t/new-module-for-measuring-cross-section-area-of-segments/11293):
>
> A new module - “Segment cross-section area” - has been added to Slicer. It can compute cross-sectional area of segmented structures along a chosen axis direction. Results are available both as a table and can be shown in a plot, too. Short demo video: For now, the module is available by installing “Sandbox” extension in the extension manager in a recent 3D Slicer Preview Release. If you have any comments and suggestions then please post it here.