# Adjusting Automatically to GCPs (Vertical Only)

This section covers how to compute and apply a single vertical translation to the point cloud based on GCP elevations.&#x20;

{% hint style="info" %}
You can always choose not to apply the computed adjustment, so this workflow may also be useful to users who'd like to quickly check absolute accuracy.&#x20;
{% endhint %}

Point clouds can be automatically adjusted to fit control points vertically. Residuals between the GCP and the point cloud can be computed either **1) in respect to a mesh** (or surface) created from the point cloud around the GCP, or **2) in respect to the nearest single point**. &#x20;

Use the **Align to GCP** tool to auto assign vertical residuals to each GCP.

Users can specify the point classes that should be used when comparing GCP elevations to point cloud elevations. If ground points have already been classified, it may be sensible to compare GCPs only to ground points, however any and all classes can be used to determine GCP-to-cloud residuals:&#x20;

<figure><img src="https://2222094320-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FdEevfLZRIk38LUPwDa4V%2Fuploads%2FQQbIzxIXWRIpeKHN0va5%2Fimage.png?alt=media&#x26;token=f22b9107-ab31-4588-a6fe-17657f773f39" alt=""><figcaption></figcaption></figure>

Once the vertical residuals have been assigned, they will be visible in both the map and profile view. Residuals will always be displayed as a positive value. Note in the example below that points 3 and 4 have no residual listed, as they were set to **Check** - only points set to **Control** will be compared to the cloud when using the **Auto-Assign Residuals** tool.

<figure><img src="https://content.gitbook.com/content/dEevfLZRIk38LUPwDa4V/blobs/47nEb8P4xN9UDMp2j02k/ahhh_resid_map_prof.PNG" alt=""><figcaption><p>Points 2 and 5 are automatically assigned vertical residuals</p></figcaption></figure>

Residuals have been computed but no adjustment has been performed. To apply an adjustment based on the automatically computed residuals, click the **Apply** button located in the **corrections menu** (bottom right corner of SpatialExplorer by default):

<figure><img src="https://2222094320-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FdEevfLZRIk38LUPwDa4V%2Fuploads%2FAHTx9O8AkNlLBxVNoOiE%2Fimage.png?alt=media&#x26;token=343a7e69-5360-445b-b800-c7f35304c4e2" alt=""><figcaption></figcaption></figure>

{% hint style="info" %}
Applying a single vertical shift to a dataset is an effective method for removing bias cause by a variety of potential issues that have a direct linear impact on geo-referencing, such as slight error in a reference station elevation.
{% endhint %}

A single vertical shift to the trajectory will be computed based on the average GCP-to-cloud residual. If instead you'd like to apply a variable adjustment, rather than a single global adjustment, you can use GCPs as part of a [LiDARSnap trajectory optimization](../lidarsnap-v4) routine. In the example below, a 1 cm vertical shift will be applied to the entire trajectory. Note that no horizontal shift has been computed as this must be computed manually (covered in next section).&#x20;

<figure><img src="https://2222094320-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FdEevfLZRIk38LUPwDa4V%2Fuploads%2Fe4035VTY379Mk2gLwnNm%2Fimage.png?alt=media&#x26;token=87115b82-468b-40f2-8dde-852bdecd76ae" alt=""><figcaption></figcaption></figure>

After clicking **OK,** a new trajectory will be produced with the vertical adjustment applied. This trajectory will automatically be loaded into the project and set as the active trajectory. The point cloud will be recomputed based on this new, adjusted trajectory, and GCP-to-cloud residuals will then be recalculated.&#x20;
