Oblique Photogrammetry: Examining Angle and Overlap

A project log for Photogrammetry and Image Acquisition

An exploration into image acquisition techniques and its effects on the quality of 3D models using Pix4D Mapper Pro software.

travis-broadhurstTravis Broadhurst 07/11/2016 at 12:180 Comments

Oblique photogrammetry was by far the method that I used the most. It is also the method that has the most degrees of freedom as Nadir imaging does not have any flexibility on the angle of the lens relative to the object. Oblique photogrammetry, on the other hand, can be performed even with changes in FOV, overlap of the images, and angle of the camera relative to the object. I wanted to see how each of these affected the quality of the model.

To do this, I selected a basic object of a paper mache giraffe about 0.8 meters in height. I placed the giraffe in a local playgound on a background of mixed sand and gravel to allow for a large number of keypoints. In this way, I could be sure that background shift or a deficiency of keypoint matches would not factor into this experiment. I then tested the angle of the images as well as the percent overlap, while keeping the entire giraffe in the FOV of the camera. I was able to create rings in the sand of varying radii and had the camera on an extendable arm to make sure the height above the ground was constant. I then changed the distance from the object to alter the angle of the camera, and captured images at varying degrees in order to test the overlap. I performed 9 trials, testing 3 different angles (15, 45, 75 degrees) and 3 different amounts of overlap (50%, 75%, 90%). Processing was done with Pix4D Mapper and each model was scaled so that information on the density of triangles could be recorded and compared.

Overall the quantitative and qualitative data both confirmed that the model improves as the percent overlap is increased. Additionally, the best models were from the images taken at 45 degree, so that seems to be the optimal angle (at least in this case). More analysis of the data is currently in progress,but further testing that could include multiple angles or multiple levels of oblique images could prove to be useful.

Although it was shown that the quality of the model was best at a high overlap and at 45 degrees, the level of accuracy and precision of a model is completely determined by the user and depends on personal preference. Thus, although 90% overlap and 45 degrees provide the best model, it is unreasonable for me to recommend this as 75% overlap and 15 degrees might provide the necessary quality for someone else in my position.

The data for this experiment is included in the project files under "Overlap_Angle_Test"

Thank you.