# Utilizing Geoid Models

Many applications of LiDAR data require the transformation of ellipsoidal height values, as recorded through GNSS, to an orthometric datum. This vertical datum adjustment can easily be made in Phoenix LiDAR’s SpatialExplorer software during data export through the creation of a custom output coordinate system that includes a geoid model.

More information about the .gtx format is available from the [VDatum Transformation Grid Formats](https://vdatum.noaa.gov/docs/gtx_info.html) page on NOAA's website.

## How to use geoid grids in SpatialExplorer

### Obtain the necessary geoid files in .gtx format.

{% hint style="info" %}
This is the same format used by and installed with NOAA’s Vdatum software.

Sources include:

* <https://vdatum.noaa.gov/download.php>
* <http://download.osgeo.org/proj/vdatum/>
  {% endhint %}

### Make .gtx files available to SpatialExplorer.

{% hint style="info" %}
Store geoid grid .gtx files in:

* C:\Users\\%userprofile%\Documents\Phoenix LiDAR Systems\CRSData
  {% endhint %}

![Storage location for geoid grid files](/files/-Lpdmb7Dhvk3WayJTxdh)

### Create a custom output coordinate system in SpatialExplorer during data export.

Add the option &#x200B;**+geoidgrids=​the\_name\_of\_your\_geoid\_file.gtx*****​*** ​to the parameters of your coordinate reference system.

![Add geoid option to custom proj4 string](/files/-LpdoMu_2Mmf37XY6qYu)

Once added​, clock the "Save as...​" button to define and save it as a new custom coordinate system definition.


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