This article introduces what are automatic tie points (ATPs), ground control points (GCPs), manual tie points (MTPs), and checkpoints (CPs) in a photogrammetry project.
GCPs and MTPs are used to improve the absolute and relative accuracy of photogrammetry projects while Checkpoints are used for quality assessment. ATPs are automatically extracted from images in the initial processing step.
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Ground control points (GCPs) are used to georeference the model. A minimum of three GCPs is required to scale, rotate and locate the model, but we recommend that each project has between 5 and 10 GCPs distributed throughout the project area. We recommend measuring more GCPs, so some can be used as checkpoints to assess the accuracy of the project. GCPs improve the relative and absolute accuracy of the model.
Automatic GCP detection (AutoGCPs) is available in PIX4Dcloud Advanced, PIX4Dengine, and PIX4Dmatic.
For the algorithm to detect the targets, the following need to be ensured:
- Good image quality (image geolocation, nadir images, sharp and high contrast).
- Targets that have the size of at least 20 times the average GSD of the project.
For more information: Automatic target detection - AutoGCP algorithm
Checkpoints (CPs) are used to assess the absolute accuracy of the model. The marks of the checkpoints are used to estimate their 3D position as well as potential errors in the clicks. This way, the relative accuracy of the area of the checkpoints may improve.
The difference between the checkpoints' initial and computed positions is displayed in the quality report, and it estimates the absolute accuracy of the model in the region.
Manual Tie Points (MTPs) are points without known 3D coordinates that are marked (clicked) by the user in the images. They can be used to assess and improve reconstruction accuracy.
MTPs are also used in PIX4Dmapper to assist merging of projects.
Automatic Tie Points (ATPs) are 3D points that are automatically detected and matched in the images and used to compute their 3D position. The number of generated ATPs highly depends on the following:
- The image resolution.
- The visual content of images.
- Processing settings.