Photogrammetry

Contents:  What is it?  How does it work?  Examples 

What is it?

Photogrammetry refers to the practice of deriving measurements from photographs. In the last five years, the variety and power of photogrammetry and related processes have increased dramatically. Structure from Motion, the process of finding the three-dimensional structure of an object by analyzing the projected 2D (retinal) motion field of a moving object, is an example of these new developments.

The Sennedjem Lintel from the Phoebe A. Hearst Museum of Anthropology at the University of California, Berkeley.
Four views of the 3D model produced from a photogrammetry image sequence of the lintel. Panels: (upper left) 3D representation with color information; (upper right) rotated detail of 3D surface information; (lower left) close up view of 3D geometry structure; (lower right) another view of the lintel's 3D surface information.

In the context of our work at CHI, photogrammetry can be applied as a tool to get mathematically sound and highly accurate textured three-dimensional (3D) geometric data from a sequence of overlapping images. Recent technological advances in digital cameras, computer processors, and computational techniques, such as sub-pixel image matching, make photogrammetry a portable and powerful technique, yielding extremely dense and accurate 3D surface data with a limited number of photos and equipment in a relatively short image-capture time.

How does it work?

Capturing photographs for photogrammetric processing can be accomplished in as few as 6 photos for a small subject. A crucial element of a successful photogrammetric process is obtaining “good” photographs, that is, a series of sharp pictures that have uniform exposure, high contrast, and fill the frame with the subject. The final accuracy of the resulting dense surface model is governed by the image resolution, or ground sample distance (GSD). The GSD is a result of the resolution of the camera sensor (higher is better), the focal length of the lens, and the distance from the subject (closer is better). The resolution of the images is governed by the number of pixels per given area and the size of the sensor.

The camera should be set to aperture priority (preferable F8) and the ISO, shutter speed, white balance, and other settings be adjusted to achieve properly exposed images. To obtain the highest order results, it is necessary to ensure that focal distance and zoom do not change for a given sequence of photos. This can be achieved by taking a single photo, at the desired distance, using the auto-focus function, then turning the camera to manual focus and taping the focus ring in place. To maintain a consistent 66% overlap, the camera must be moved a distance equivalent to 34% of a single photo field of view. To ensure the entire subject is covered by at least two overlapping photos, the photographer must position the left extent of the subject in the center of the first frame. The next step is to proceed systematically from left to right along the length of the subject and take as many photos as necessary to ensure complete coverage.

Because of the flexibility of this technique, it is possible to obtain high accuracy 3D data from subjects that are at almost any orientation (horizontal, vertical, above, or below) the camera position. However, it is important to keep the plane of the sensor and lens parallel to the subject and to maintain a consistent height (or distance) from the subject.

Camera Calibration Sequence

An additional step is this process is camera calibration to determine and map the distortions in the lens with respect to the sensor location. This can be accomplished most effectively when there are a large number of auto-correlated points in common between the overlapping images and an additional set of calibration photographs. The camera calibration photographs must be captured at the same settings as the overlapping photos. At least four additional photos are required; two taken with the camera physically rotated 90° to the previous line of overlapping photos, and two additional photos with the camera rotated 270°. The additional four camera calibration photos may be taken at any location along the line of overlapping photographs; however the best results occur in areas where the greatest number of autocorrelated points may be generated.

Adding Measurability

In addition to maintaining a proper base to height for 66% overlap and the camera calibration photo sequence, the next most important component needed to acquire geometrically correct dense surface models is the ability to introduce real-world values, or scale, to a subject. This is accomplished by simply adding an object of known dimension (meter stick or other object) that is visible in at least two stereo models (three photos). It is preferable to have two or more such objects, to ensure visibility and for accuracy assessment. These objects may then be assigned their proper length during processing.

Photogrammetry in Relation to Reflectance Transformation Imaging (RTI)

The photogrammetric image capture steps described above can be done in concert with the RTI image capture process. The camera calibration photo sequence may be used with photogrammetry software to calculate a highly accurate camera calibration. If desired, this data can be used to remove lens distortion and/or ortho-rectify the RTI image set prior to processing the RTI.

Examples

Examples coming soon.