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title: Passive scanning
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Passive scanning

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Photogrammetry

Photogrammetry is the collection and organization of reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant imagery and other phenomena.

Photogrammetry was first documented by the Prussian architect Albrecht Meydenbauer in 1867. Since then it has been used for everything from simple measurement or color sampling to recording complex 3D Motion Fields

Unlike other scanning methods that require precise orbital plans or specialized equipment, photogrammetry can be achieved simply by flying a drone in a circular pattern and capturing multiple photos. Utilizing the location data from the drone, one can construct detailed models like the example shown here: A typical medium resolution aerial photogrammetry scan of a barn. With 50-100 images a reasonably accurate model can be produced. Such models are often used in surveying and restoration projects from the scale of hand helf objects to cities. This accessibility makes photogrammetry an attractive option for various applications, with results that can be sufficiently accurate depending on the specific requirements.

However, it's essential to note that photogrammetry lacks inherent scale. Without a reference point or prior knowledge of the camera locations, the resulting model lacks a definitive scale, as cameras inherently lack absolute scale information. Therefore, incorporating at least one reference point is crucial. For example, marking a facade with visual markers or known distances, such as pieces of tape, allows for scaling within a 3D modeling program based on these references.

Stereo Matching

Stereo matching is also known as "disparity estimation", referring to the process of identifying which pixels in multiscopic views correspond to the same 3D point in a scene.

Early uses in stereophotogrammetry, the estimation of 3d coordinates from measurements taken from two or more images through the identification of common points. This technology was used throughout the early 20th century for generating topographic maps.

Cameras were applied to photogrammetry as early as 1973, such as in Imaging characteristics of photogrammetric camera systems , an early paper describing groundwork camera properties and standards for USGS (United States Geological Survey) photgrammetry surveys.

When describing photogrammetric camera systems, intrinsics and extrinsics parameters are used to characterize systems:
Intrinsic parameters describe the properties of a camera's lens and sensor.
Extrinsic parameters describe the camera's position and orientation in 3D space