How to manage Point Collections (GCPs) - PIX4Dcatch RTK

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PIX4Dcatch allows performing single point measurements, known as Ground Control Points (GCPs). Accurate GCPs increase the global accuracy of the project and can also be used as checkpoints for assessing the accuracy of the 3D reconstruction.

Create a new Point Collection

Before measuring points, it is necessary to first create a new point collection (also defined as GCP collection). This point collection is the placeholder for storing the measured or imported points.

Note: We recommend using this feature with a GNSS receiver (e.g. Pix4D-adapted viDoc) for centimeter accuracy.
  1. Tap the PIX4Dcatch_Tools_Menu_inverted.svg icon from the main screen to open the Tools dialog.
  2. Select PIX4Dcatch_Point Management_inverted.svg Point Management. All point collections are stored and can be accessed in this section.
  3. Tap the New Collection PIX4Dcatch new collection button button to create a new point collection for storing the measured or imported points.
  4. Type a name for the new point collection.
  5. In the Point CRS tab, select the desired output coordinate reference system:
    • projected Coordinate System icon Choose the horizontal coordinate reference system.
    • altitude vertical icon Choose the vertical coordinate reference system.
    • globe geoid icon Select the geoid (if available).
  6. Tap Save.
  7.  
    Note: When a local geoid is not available, it is possible to enter the geoid height over the ellipsoid to obtain orthometric heights for the points measured.

    PIX4Dcatch single point enter geoid height over ellipsoid

    Enter the average geoid height over the ellipsoid for the area measured

Note: When creating a new point collection, the horizontal and vertical coordinate reference system fields are automatically filled out depending on the current region and configuration information.
  • With GNSS/GPS:
    • Horizontal: WGS 84 (EPSG 4326)
    • Vertical: Ellipsoidal height
  • With Pix4D-adapted viDoc RTK rover:
    • Horizontal: depends on the NTRIP CRS entered. Examples: ETRS89 (EPSG 4258), NAD83(2011) (EPSG 6318), JGD2011 (EPSG 6668)
    • Vertical: Ellipsoidal height

Measure Points

After creating a point collection, it is now possible to measure points. For RTK measurements, 2 methods are available: with a survey pole/tripod or hand-held using the ground laser (downward-facing) of the Pix4D-adapted viDoc RTK rover.

Measure points using a survey pole/tripod

An important pre-requisite for achieving high-accuracy measurements of GCPs is setting the correct antenna height (Z axis).

    1. Create or select the desired point collection where the points will be stored.
    2. Tap the New Point New Point button button.
    3. The point name will be auto-generated based on the point collection name, but it can be modified. If the point collection will be used for Auto Tag Detection, then the point name suffix has to include the same number as the number visible on the tag.
    4. Make sure you calculate the antenna height and add the correct value before you start measuring. The antenna height value can be entered in meters or feet.
      • When using a Pix4D-adapted viDoc RTK rover, the antenna height is measured from the base of the antenna to the ground centered on the point.
      • When using the GNSS/GPS of the device, the antenna height is measured from the center of the device to the ground centered on the point.

      antennaHeight altitudeCorrection diagram

      Measuring the Antenna Height using a Pix4D-adapted viDoc RTK rover (red line).

    5. (optional) Add reference images and write a description.
    6. Set the measurement duration.
    7. Tap the Measure Measure Button small button.

      PIX4Dcatch_new_point_dialog_pole.jpg

      PIX4Dcatch New Point window (antenna height)

    8. It is recommended to wait for RTK Fixed and optimal HDOP/VDOP values before the measurement starts.
    9. The button Start measuring will be active once the device is aligned with the ground and the bubble turns green. The green bubble needs to remain as centered as possible to correctly perform the point measurement.

      RTK_bubble_not_aligned.PNG

      RTK_bubble_aligned.PNG

      RTK_measure_30sec.PNG

    10. All points measured will be saved and accessible in their point collection. Continue measuring new points following the same workflow. It is also possible to edit the name and description, add reference photos, and delete points. For more information: Edit points.
Tip: To achieve high accuracy, please read the GNSS and Pix4D-adapted viDoc RTK rover best practices

Measure points with the Pix4D-adapted viDoc ground laser

Toggling on the Use Laser button allows the measurement of GCPs using the ground (downward-facing) laser on the Pix4D-adapted viDoc RTK rover.

Note: The laser feature is unavailable on the Pix4D-adapted viDoc RTK rover iPad model.
Tip: When using the laser, the vertical offset will be calculated automatically and it does not need to be measured manually.
  1. Create or select the desired point collection where the points will be stored.
  2. Tap on the New Point New Point button button.
  3. Toggle on the Use laser button. When using the laser, the vertical offset from the antenna to the ground will be automatically computed internally and there is no need to set it manually. The Antenna Height / Altitude Correction should be greyed out.
  4. (optional) Add reference images and write a description.
  5. Set the measurement duration.
  6. Tap the Measure Measure Button small button.

    PIX4Dcatch_new_point_measurement_laser.jpg

    PIX4Dcatch New Point window (laser on)

  7. It is recommended to wait for RTK Fixed and optimal HDOP/VDOP values before the measurement starts.
  8. The button Start measuring will be active once the device is aligned with the ground and the bubble turns green. The green bubble needs to remain as centered as possible to correctly perform the point measurement.

    RTK_bubble_not_aligned.PNG

    RTK_bubble_aligned.PNG

    RTK_measure_30sec.PNG

  9. Aim the laser at the center of the point.
  10. All points measured will be saved and accessible in their point collection. Continue measuring new points following the same workflow. It is also possible to edit the name and description, add reference photos, and delete points. For more information: Edit points.

Import points

PIX4Dcatch is also capable of importing GCP coordinates into an existing point collection.

  1. If the desired point collection does not exist yet, it needs to be created first.
  2. Click on Import Points. 
  3. Select the file with the GCP measurements. Ensure that the coordinates have to be in the same coordinate reference system as the point collection.
  4. Select the correct unit of measurement (meters, feet, US survey feet).
  5. Select the axes orientation (Northin/Easting, Easting/Northing). For more information about the GCP formats, please refer to Tie points (GCPs, CPs) import format - PIX4Dmatic

List points

Select a point collection and view the list of points that were measured or imported.

PIX4Dcatch_GCP_collection_dialog.jpg

PIX4Dcatch single point collection window

  • export.jpg Export Point Collection: the complete information, including PIX4Dmatic-compatible files and logs are exported.
  • Point Collection name
  • The number of measured points currently in the point collection.
  • PIX4Dcatch_GCP_list_map_view_toggle.png Toggle list and map views
  • Collected points.
    • PIX4Dcatch_measured_GCP.jpg


      PIX4Dcatch single point measurement window

    • Point name.
    • RTK/GPS tag.
    • Horizontal and vertical accuracies.
    • Coordinates measured.
    • calendar.png Date of measurement.
    • timer.png Measurement duration.
  • New_GCP_button.png Measure a new point.
  • Output coordinate reference system for the point collection.

Edit points

Select a point collection and edit the points that were measured or imported. The following editing actions are possible during and after the point measurements:

  1. Add and edit reference photos
  2. Rename points/point collections
  3. Delete points/point collections
Note: Descriptions can be added during the measurement but they cannot be added or edited after the measurement.

Reference photos

For adding more details about the site conditions, capture reference photos for each point. A reference photo can be taken before or after measuring a new point.

PIX4Dcatch reference photo tab

PIX4Dcatch single-point reference photos

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

      PIX4Dcatch_add_reference_photo_new_point.jpg

      New Point Reference Photos Tab

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

      PIX4Dcatch add reference photo after measurement

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

  2. Aim at the point and tap next button to capture a reference photo. Take multiple photos as needed. You can also delete photos by tapping the delete.jpg icon.

    PIX4Dcatch capture reference photo

    Capturing a reference photo

  3. Tap checkmark icon to save the photos or tap cancel icon to cancel when finished.

Delete Points

It is possible to delete single points from a point collection or entire point collections.

Catch_point_delete.png

PIX4Dcatch Delete Point window

  1. Tap and hold the point to be deleted.
  2. From the menu, Tap Delete  delete red mobile icon.
    • Confirm by tapping Delete  red checkmark icon
    • Abort by tapping Cancel  cancel.png

Rename Points

It is possible to rename single points and entire point collections.

PIX4Dcatch point collection edit menu

PIX4Dcatch edit Point Collection window

  1. Tap and hold the point collection or single-point to be renamed.
  2. From the menu, Tap Rename  edit pencil icon. Type the new name and select Rename.

Manage Point Collections

List Point Collections

PIX4Dcatch_PointManagement.png

PIX4Dcatch Point Management window

The Point Management window shows the following:

  • The number of collections.
  • PIX4Dcatch_GCP_list_map_view_toggle.png Toggle between list and map views.
  • A list with the existing point collections.
    • coordinateSystem_icon.jpg Input coordinate reference system.
    • gcp.png The number of measured points.
  • New_GCP_collection.png Start a new point collection.

Export Point Collections

Note: PIX4Dmatic and PIX4Dcloud can import GCPs and markings automatically. The following steps are not required for most of the cases.

Once points have been measured, they can be exported in predefined file formats.

PIX4Dcatch_GCP_collection_dialog.jpg

PIX4Dcatch point collection window

  1. Select the Point Collection from the Point Management window.
  2. On the top right corner, tap export icon Export.
  3. The exported GCP collection contains the following files:
      • Images folder: Contains the Reference image files.
      • Records folder: Contains individual point .json files and logs (for internal use).
      • project_name_Metadata.json file (for internal use).
      • project_name-complete.csv: Generic point file with all information.
      • project_name-PIX4Dmatic.csv: File that's compatible with direct import in PIX4Dmatic.

    PIX4Dcatch point collection exported files

    Files exported from PIX4Dcatch of a GCP collection

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

Visualize Point Collections in Augmented Reality (AR)

Point Collections with projected and geographic coordinate reference systems can be visualized in AR. More information can be found in the article AR Points - PIX4Dcatch.

Delete Point Collections

A Point Collection can be deleted from the Point Collection management screen.

Catch_pointCollection_delete.png

PIX4Dcatch edit Point Collection window

  1. Tap and hold the Point Collection to be deleted.
  2. From the menu, Tap Delete  delete red mobile icon.
    • Confirm by tapping Delete  red checkmark icon.
    • Abort by tapping Cancel  cancel icon

How to import points to PIX4Dmatic

Note: PIX4Dmatic and PIX4Dcloud can import GCPs and markings automatically. The following steps are not required for most of the cases.

After exporting the GCP collection, the file project_name-PIX4Dmatic.csv can be imported directly into PIX4Dmatic.

PIX4Dmatic import tie points

Importing a GCP file into PIX4Dmatic

  1. Open project in PIX4Dmatic
  2. Click gcp iconTie Points to open the Tie Points table.
  3. Drag and drop, or use the Select from disk option to specify the PIX4Dmatic.csv file with GCPs.
  4. Define the Horizontal and vertical coordinate reference system.
  5. (Optional) Define the Geoid or Geoid height.
  6. Click Apply.
  7. After the 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.
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