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Work with Tie Points for Cell Tower Reconstruction

Jim Peters ZDAgent
  • Updated

In this article:

Introduction

Even carefully planned flight missions that include orbit flights and dedicated routes around structures such as cell towers may sometimes produce a 3D mesh in which some parts of the tower are not fully or accurately reconstructed. In the example below, reconstruction artifacts are visible, including vertical shifts in the antennas (red arrows), mesh separation at the top of the left antenna (yellow arrow), and misalignment of the support pipes (blue arrows).

This typically occurs when there is insufficient photo overlap in a specific area. In PhotoMesh, tie points can be added to strengthen the aerotriangulation (AT) solution in such weaker areas. Tie points establish correspondence between project photos by identifying the same physical point in three or more photos.

A tie point was added to the top-left corner of the antenna as shown by the red dot in the figure above. After rebuilding the project to update the AT solution with the newly added tie point, the resulting 3D mesh reconstruction shows that the antenna alignment issues have been corrected (see figure below).

Creating Tie Points

  1. Open the relevant cell tower project in PhotoMesh.
  2. In the Project Panel, select the Control and Tie Points group. The Control and Tie Points list opens under the 3D Window. (Initially, this panel is empty. The example below shows a project in which one tie point was already added prior to photo sampling.)
  3. Create a tie point near the area where shifting or deformation is visible:  
    1.  On the Home tab, in the Add group, click the arrow under Control Points and select Create Tie Point from Terrain.
    2. Click in the 3D Window to define the tie point’s position. The Control Point Editor is displayed with the Photos panel populated by photos whose frustum intersects with the tie point created.
  4. In the Control Point Editor, set the Photos filter to: All in Frustum (Calculated).
  5. In the Photos panel, sort the Distance column in descending order so that the shortest distances (typically 1–5 meters) appear at the top.
  6. Browse through the photos until you find ones that clearly show the tie point location. Use the mouse wheel to zoom in and out, and hold down the left mouse button to pan within the photo window. 

Sampling the Tie Points

  1. Press S on the keyboard to activate sampling mode. A white square with a “+” symbol attaches to the cursor. Use the left mouse button to sample the tie point in the current photo. A red “+” appears at the sampled location.
  2. Press N to move to the next photo. Repeat sampling in 10–20 photos from a variety of angles.
    After adding or updating tie points, save the project (Ctrl+S).
  3. As photos are sampled, the Control and Tie Points panel begins to populate. Sampling Error (px) displays values once at least two photos are sampled.
  4. Interpreting the Sampling Error:
    1. The first two sampled photos in the example below produced sub-pixel (less than 1 pixel) errors.

    2. To effectively correct the shifting or deformed mesh, sample 10–20 photos where the yellow “X” (projected triangulation) is not centered on the original tie point location selected in the 3D Window. 

    3. As additional photos are sampled, you will often see some Sampling Error values greater than 1 pixel. These larger errors indicate that the current AT solution is not fully consistent at this location. In the 3D Window, you will also see lines extending in multiple directions from the tie point. After running aerotriangulation again, the solution adjusts to minimize these discrepancies, resulting in improved alignment and a more accurate 3D mesh reconstruction.

    4. After completing the tie point sampling process, save the project.

Running an AT-Only Build and Performing QA on the Tie Points

  1. Build the project with the AT Only step.
  2. Review the tie points in the Quality Report. For a cell tower project, acceptable results typically show RMS errors below 1 pixel at each tie point location. If a tie point shows higher error values, revisit and resample a few photos to reduce the error.
  3. Save the project after making adjustments.

Performing a Full Build

  1. When satisfied with the AT results, set the build steps to Complete Project.
  2. Run the build to process the updated AT solution and generate a corrected 3D mesh reconstruction.     

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