Stereo compilation is a technique used within photogrammetry to create maps or scale drawings from photographs, leveraging the depth perception provided by viewing images stereoscopically.
Understanding Stereo Compilation
At its core, stereo compilation is a method employed in the field of photogrammetry. Photogrammetry, as defined by the USGS, is "the process of making maps or scale drawings from photographs, especially aerial photographs" (USGS-000276).
Stereo compilation utilizes a pair of overlapping photographs taken from slightly different viewpoints – known as stereographs. When these stereographs are viewed using specialized stereo compilation glasses, the observer perceives a three-dimensional model of the area depicted in the images. This 3D view is crucial for accurately measuring heights, identifying features, and drawing contours or objects directly from the photographs.
How Stereo Vision Aids Compilation
The human brain's ability to perceive depth relies on receiving slightly different images from each eye. Stereographs mimic this by capturing the same scene from two distinct positions. Viewing them with stereo compilation glasses isolates the left image for the left eye and the right image for the right eye, tricking the brain into seeing a 3D model.
This 3D model allows photogrammetrists to:
- Accurately measure coordinates: Determine the precise ground location (X, Y, Z coordinates) of features depicted in the photographs.
- Trace features: Draw linear features like roads, rivers, and property boundaries directly in their correct horizontal position.
- Collect elevation data: Identify and measure height points, enabling the creation of topographic maps with contour lines.
- Identify ground objects: Distinguish different types of terrain, vegetation, and structures based on their shape and height.
Tools of the Trade
Historically, stereo compilation was performed using analog or analytical stereoplotters, complex mechanical or optical-mechanical instruments that allowed the operator to trace features while viewing the 3D model. The reference specifically mentions the use of stereographs and stereo compilation glasses within this photogrammetric context (USGS-000276).
Today, digital photogrammetry software often performs similar tasks, using digital image processing to create digital elevation models (DEMs) and extract features, though the fundamental principle of using overlapping images to derive 3D information remains central.
In essence, stereo compilation is the process of extracting geographic information and creating cartographic products by leveraging the three-dimensional view obtained from overlapping stereoscopic photographs.
