# Ground Control Points (GCPs)

### **Ground Control Points (GCPs)** (CSV/TXT/GCP)

LiDAR derived products use surveyed ground points to control and test absolute vertical accuracy. 

* You can find more information on how to collect and utilize survey ground control [here](https://www.phoenixlidar.com/wp-content/uploads/2020/11/Survey-Ground-Control-Recommendations.pdf): 

There are two ways to differentiate high accuracy survey control points - Ground Control Points (GCPs) and Survey Checkpoints (SCPs).

* GCPs and SCPs are generally collected at the same time, using the same methodology. 
* GCPs are surveyed control points used for data adjustments, and SCPs are surveyed control points used for accuracy reporting. 
* GCPs utilized for data adjustments should not be used to validate the accuracy of the data product. 
* You can find more information on GCPs and SCPs [here](https://www.phoenixlidar.com/wp-content/uploads/2020/11/Survey-Ground-Control-Recommendations.pdf)

Step 1: File > Open File > Navigate to GCP file > Open

Step 2: Adjust the "Start import at row" Integer value to exclude column headers from data import

Step 3:  Select the correct type of column separator for your GCP file

Step 4:  Use the Column Assignments to match the proper column header to the corresponding data column in the file 

![](/files/-M_7aLQcKG3MsPyd5vQz)

* By specifying the CRS of the GCPs, SpatialExplorer will then transform the control points to the project datum.  

Step 5:  Click the "..." button at the top right to specify the GCP file coordinate system.

![](/files/-M_7VtkqB-gGPE8zijma)

* Select desired horizontal CRS, vertical CRS, units, and Epoch and then click "OK"
* **Note: If your vertical CRS is in orthometric heights, the "Select..." button will appear.** 
* **Click on the "Select.." button to download/select a geoid model** 

![](/files/-M_7WZY6edcvnfdnjFsW)

* Search for a geoid file name, select desired geoid, and click on the folder icon to the left of the name to download the file. 
* After the file is downloaded, the folder icon changes to a recycling bin icon.
*  Select the desired Geoid, and click "OK" 

![](/files/-M_7XrCkSULB1Kav_SW3)

* The geoid model will be displayed below the vertical CRS

![](/files/-M_7YCLHAKTRpcnqizOH)

* Click "OK" to finish

Step 6:  Visually verify the input survey control is properly georeferenced.

![](/files/-M_7b0AbhsQmS3hJzg8l)

Step 7:  Click on the GCP file name within the Project window under "Ground Control"

* A table of GCPs will be dislayed at the bottom left of the screen
* Within this window, specify the "Type" of surveyed point between "control", "checkpoint" and "unused" for each GCP by left clicking on the type. 
* "Control" is a GCP, where "check" is an SCP, "unused" points will be ignored during the processing.  

Click [here](https://docs.phoenixlidar.com/lidarmill-desktop/post-processing/workflow-kk/ground-control) to jump to the "Ground Control Assessment" section of the workflow


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