How to collect GCPs with PIX4Dcatch and viDoc RTK rover

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

IN THIS ARTICLE

Create a new GCP collection
Measure Ground Control Points
Collect GCPs using a pole
Collect GCPs using laser
Reference photos
Export Ground Control Points
How to import GCPs to PIX4Dmatic

Note: We recommend using this feature with the viDoc RTK rover for cm accuracy. When 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

Tap 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 the desired coordinate reference system.
  - Choose the horizontal coordinate reference system.
  - Choose the vertical coordinate reference system.
  - Select the geoid when available. If the local geoid is not available, it is possible to enter the geoid height over the ellipsoid.
    
    Note: When a local geoid is not available it is possible to enter the geoid height over the ellipsoid to obtain geoidal heights for the points measured.
    
    Enter the average geoid height over the ellipsoid for the area measured.
  - Select return on the keyboard or tap Save.
    
    Selecting a Coordinate reference system for the GCP collection.

Note: When creating a new GCP collection PIX4Dcatch will prefill the horizontal and vertical fields depending on the current region and configuration information.

With viDoc RTK rover:
- Horizontal: Depends on the NTRIP CRS entered. e.g. ETRS89 (EPSG:4258), NAD83(2011) (EPSG:6318), JGD2011 (EPSG:6668), etc.
- Vertical: Ellipsoidal height
With GPS:
- Horizontal: WGS 84 (EPSG:4326)
- Vertical: EGM 96 (EPSG:5773)

Measure GCPs

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

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 a survey pole or tripod/monopod for stability.
Use viDoc RTK Rover for RTK-level accuracies.

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

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.

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, the 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 meters/feet.

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.

PIX4D acquiring geolocation survey pole

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

Note: The laser feature is only available on the viDoc RTK rover phone model. Android compatibility to be added in the near future.

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.

PIX4Dcatch acquiring geolocation laser

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.

Note: Android compatibility coming soon.

How to add reference photos

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
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
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

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.

General

Create a new GCP collection