Using PIX4Dcatch with your mobile device, it is possible to measure Ground Control Points (GCPs). Accurate GCPs increase the global accuracy of the project.

Create a new GCP collection

1. Launch PIX4Dcatch .
2. Tap New GCP collection to open the GCP management dialog.
3. Select New GCP collection to create a new GCP collection folder where the GCPs will be stored.
   • Name the new GCP collection.
   • Select the desired coordinate reference system.
     • Choose the horizontal coordinate reference system.
     • Choose the vertical coordinate reference system.
     • Select the geoid when available.
     • Select return on the keyboard or tap Save.

       

       Selecting a Coordinate reference system for the GCP collection.

Measure GCPs

There are two methods available to measure GCPs using PIX4Dcatch, with or without using the laser.

Collect GCPs using a pole

• Select the desired collection where the GCPs will be saved.
• Tap on New GCP.
• Enter:
  • Antenna Height / Altitude Correction (Optional).
  • Description (Optional).
  • Measurement Duration. (15-30 seconds recommended)

  

  Collect a new GCP by setting the Antenna Height and the Measurement Duration.

• Tap on Measure.

The measurement will begin once the circles are aligned and the device is kept perfectly vertical. The green dot needs to remain in the center to start and continue the GCP measurement.

Measuring a new GCP.

All points measured will be saved and accessible in their GCP Collection. It is possible to add new measurements and or reference photos to the collection as needed.

Collect GCPs using laser

Toggling the Use Laser feature allows the collection of GCPs using the downward-facing laser.

• Select the desired collection where the GCPs will be saved.
• Tap on New GCP.
• Toggle Use Laser on.
• Enter:
  • Description (Optional).
  • Measurement Duration. (15-30 seconds recommended)

  

  Toggle Use Laser on.

  Note: When using the laser feature the vertical offset will be considered internally and is not necessary to input manually.
• Tap on Measure.

Aim the laser to the center of the control point to measure. The measurement will begin once the circles are aligned, and the device is kept perfectly vertical throughout the measurement.

Measuring a GCP using the laser feature.

All points measured will be saved and accessible in their GCP Collection. It is possible to add new measurements and or reference photos to the collection as needed.

Reference photos

GCP reference photos

It is possible to attach reference photos to each GCP to aid identification. A reference photo can be taken before measuring a New GCP or after the point has been measured and saved.

How to add reference photos

1. There are two ways to access the reference photo camera.
   • Before measuring the GCP, from the New GCP dialog, tap the Camera icon in the Reference Photos (Optional) tab.

     

     New GCP Reference Photos (Optional) tab

   • After measuring the GCP in the list of saved GCPs, press and hold the desired single-point measurement and select Add photos from the dropdown menu.

     

     Long-press a saved single-point measurement to add reference photos

2. Frame the GCP and tap to capture a reference photo. Take multiple photos as needed. Delete unwanted photos by tapping the icon.

   

   Capturing a reference photo

3. Tap to accept the photos or to cancel when finished.

How to export GCPs

Once the GCPs are measured, they can be exported in predefined file formats.

To do so:

1. Select the desired GCP collection from the GCP Management dialog

2. Tap Export at the top of the screen.

• Several files are generated.
• Project_name_complete.csv: Generic GCP file with all information.
• Project_name-PIX4Dmatic.csv: File that's compatible with PIX4Dmatic.
• Records folder containing individual GCP .json files (for internal use).
• Project_name_Metadata.json file (for internal use).

Files exported from PIX4Dcatch of a GCP collection.

After the files are exported, they can be copied or shared. This way they can be used in PIX4Dmatic or other third-party software.

How to import GCPs to PIX4Dmatic

With the exported PIX4Dmatic.csv it can now be imported directly into PIX4Dmatic.

Importing a GCP file into PIX4Dmatic.

To import the PIX4Dmatic.csv into PIX4Dmatic:

• Open project in PIX4Dmatic
• Click Tie Points to open the Tie Points table.
• Drag and drop, or use the Select from disk option to specify the PIX4Dmatic.csv file with GCPs.
• Define the Horizontal and vertical coordinate reference system.
• (Optional) Define the Geoid or Geoid height.
• Click Apply.

After the Tie Points are imported, they are displayed in the Tie points table, the 2D and 3D view.

For more information: Tie Points (GCPs, MTPs, and CPs) in PIX4Dmatic.