# 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="https://2222094320-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FdEevfLZRIk38LUPwDa4V%2Fuploads%2FNtodmbAtNoMaHUzWN2LP%2Fimage.png?alt=media&#x26;token=c039e21a-be6f-46b0-9bb4-26f046e6f2c4" 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="https://2222094320-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FdEevfLZRIk38LUPwDa4V%2Fuploads%2FHcUHH52EoQ4JFUBePkQY%2Fimage.png?alt=media&#x26;token=ab7515e4-26c9-4c6d-b5a3-920c474ac5c6" alt=""><figcaption></figcaption></figure>