> For the complete documentation index, see [llms.txt](https://docs.phoenixlidar.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.phoenixlidar.com/spatialexplorer-8-and-9/user-interface/toolbars/analytics/measure/std.-dev.-along-surface-normal.md).

# Std. Dev. Along Surface Normal

This measurement computes the standard deviation along surface normal (SDASN) for a pointcloud within a polygon. The standard deviation along a surface normal is a measure of the variance of points along a surface. This is essentially a pointcloud precision measurement ("thickness" of the cloud). 

First, specify the cloudgroup and polygon geometry. Then, specify the grid stride used to create the TIN mesh. Optionally, you can specify a list of Intervals, in which case you will get one measurement per interval:

<figure><img src="/files/urJQ1JLitQa5cAHpfZW0" alt=""><figcaption></figcaption></figure>

{% hint style="info" %}
The Time Intervals option may be useful when computing intraswath precision for several flightlines.
{% endhint %}

The SDASN average value multiplied by 2 should roughly equal the pointcloud precision on a hard surface:

<figure><img src="/files/0v8uCBvsdtTdH31UCh6p" alt=""><figcaption></figcaption></figure>


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