How to collect GCPs - PIX4Dcatch

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

 
Note: Currently this feature is available using iPhone or iPad (Supported Devices). We recommend using this feature with the viDoc RTK rover for cm accuracy. If using a viDoc RTK rover you will need to connect it before collecting GCPs. For more information: Connect PIX4Dcatch to the viDoc RTK rover.

Create a new GCP collection

  • Launch PIX4Dcatch.
  • Tap gcp_mtp_manager.svg New GCP collection to open the GCP management dialog.
  • Select New GCP collection to create a new GCP collection folder where the GCPs will be stored.
    • Name the new GCP collection, select return on the keyboard, or tap on Save.

Measure GCPs

There are two ways to measure GCPs using PIX4Dcatch, using a rod/monopod, or using the laser(iPhone viDoc RTK rover).

 
Tip: For Higher accuracy:
  • Stay clear of buildings.
  • Maintain a clear view of the sky.
  • Hold the device steady while the leveling bubble is in the center.
  • Use viDoc RTK Rover for RTK-level accuracies.

For more information see the GNSS and viDoc RTK rover best practices article.

Collect GCPs

  • 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)

    Ready to measure a new GCP

    Collect a new GCP by setting the Antenna Height (165cm) and the Measurement Duration (30 sec).

 
Tip: How to set the Antenna Height / Altitude Correction:
  • When using a viDoc RTK rover, the Antenna Height / Altitude Correction is measured from the base of the antenna to the ground centered on the control point.
  • When capturing GCPs using the GNSS/GPS of the device, Antenna Height / Altitude Correction is measured from the center of the device to the ground centered on the control point.
  • If a surveying rod is screwed into the viDoc RTK rover, the Antenna Height should be measured between the ground centered on the control point to the base of the antenna and include the height of the survey pole.
  • Enter the height in cm.

Illustration to correctly measure vertical offset when collecting GCPs

Measuring the Antenna Height / Altitude Correction (red line) when using a viDoc RTK rover.

  • 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.

Collect_GCP_02.PNG

Measuring a new GCP.

Collect GCPs using laser

 
Note: The laser feature is available on the viDoc RTK rover iPhone models and not with the iPad.

Toggling the Use Laser feature allows the collection of GCPs using the downward-facing laser on the viDoc RTK rover iPhone models.

  • 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)

    useLaser.jpg

    Toggle Use Laser on.

     
    Note: When using the laser feature the vertical offset will be measured by the laser 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.

 
Tip: Use a rod/monopod, and roughly align the laser to the spot before starting to measure.

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.svg 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

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.

PIX4Dmatic_set_GCP.gif

Importing a GCP file into PIX4Dmatic.

To import the PIX4Dmatic.csv into PIX4Dmatic:

  • Open project in PIX4Dmatic
  • Click gcp.svgTie 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 see Tie Points (GCPs, MTPs, and CPs) in PIX4Dmatic article.

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