Photogrammetry - The Basics

Photogrammetry can be thought of as an enhanced stereo vision system that goes far beyond the two eyes (two images) that people use to locate objects in 3-D space. Photogrammetry uses a camera to take many images from many different perspectives. This greatly improves the accuracy of the process and actually allows us to 'see' around and behind objects.

The photogrammetry process is further enhanced by placing precision, highly reflective targets on components being measured. Images of the targeted components are taken from many different positions. Each target is captured in multiple images and there are many targets in each image. A computer program uses trigonometry to tie all the images together into a network and determine the locations of all the targets relative to each other.

Photogrammetry can measure surfaces, shafts, bores and pretty much anything else that is visible . Photogrammetry is also inherently three dimensional, so the positions of measured components are captured in 3-D space. Reference frames for the measurements can be transferred to any convenient point and orientation so measurements relate directly to the components.

Accuracy? The size of the measurement envelope does matter, but probably less than might be expected. A high resolution photogrammetry camera with good targeting and imaging techniques produces measurements accurate to better than 0.001" when the measurement area is 20 feet or less. At working distances of 100 feet, the relative accuracy of points at the extreme ends may be 0.007" while measured points closer together will retain the better relative accuracies.

A big advantage of photogrammetry is its ability to work in unstable, confined, and hostile environments. Images are taken using a high speed flash that freezes motion of the camera or of the object being measured. Stable equipment mounts are not required. Vibration of the measured object is averaged out by the many images taken for the measurement. Access to the component being measured is only required for applying targets. The measurement process only requires that targets be visible. Open spans or a dangerous environment are not a problem.

For more information about photogrammetry contact Mechtell or go to Geodetic Systems' web site. Geodetic Systems makes the photogrammetry camera and software used by Mechtell.

Small Scale Photogrammetry

Aircraft Engine Mount (just over 3' across)

Regular image on left. Photogrammetry image on right with targets.

Aircraft Engine Mount Regular Aircraft Engine Mount Photogrammetry

The photogrammetry images were taken from all around as well as above and below the mount. Once processed, the resulting 3D image looks something like this:

Aircraft Engine Mount Camera Positions

Each of the measured targets(white dots) was 'seen' from multiple camera images. Typically, the more images a target is in, the more accurately the measurements

Large Scale Photogrammetry

Steam Turbine LP and Generator (approx. 60')

Measuring larger items is a bit more challenging, but the general process is the same: Mark up the measurement area with targets and then take pictures from many different locations to tie all the targets together.

The green rays in this image show a few of the many target sightings and illustrate how the target sightings from the many camera images overlap. This target overlap in the many images enables the camera images to be combined into a 3-dimensional network of measured targets. Desired dimensions between any of the measured targets can be extracted from this network.

Turbine Generator Photogrammtry INCA3 Positions

Large Turbine-Generator (approx. 160')

Taking Images of a Turbine Generator Shell Photogrammetry Image of Turbine Shell
Photogrammetry Image of Large Turbine Generator