## Theory of Photogrammetry

"Science and art whose subject is the study of photography with the intention to collect data leading to refunds dimensions and determine the shape and position of an object in space."

Based Remote Sensing Terminology
and Photogrammetry, 1997.

### 1 - The principle of photogrammetry

Intersection of lines views

Photogrammetry is a technique used in metrology to perform measurements of coordinates in three dimensions from photographies.Le basic principle of photogrammetry is the triangulation. Consider a point on the object, photographed from two different positions. The ray connecting that point to the camera is called "line of sight". The intersection of "sight lines" (or triangulation) to determine the position in space (3D) point shot.

In practice, it will take more than two photographs at the point on the object, thereby improving the accuracy of the measurement. The calculation is called photogrammetry calculating compensation by the method of bundles (bundle block adjustment in English). It is a comprehensive assessment to determine simultaneously the position and orientation of the camera (1) during the shooting, the camera parameters and the position of measured points.

Calculation of the position and orientation of the camera

Mesurment example

Photographs around the object

Triangulated points from camera's positions

Point of view from one camera's position

Set od lines views

### 2 - Materialization of measurement points

Different techniques are used to materialize the measuring points on the object: target retro-reflective optical targets projected probes and natural points.

#### Retro-reflective targets

The characteristic of retro-reflective paper is that it reflects the light to its source with a high yield. For example, a retro-reflective target is 100 to 1,000 times more effective than targeting traditional black and white back light.
A flash of the camera can illuminate retro-reflective targets during shooting. The images of targets in the photographs will then be more easily detected and measured more precisely. Another very important advantage of retro-reflective target is that the exposure of targets becomes completely independent of the ambient light. Indeed, pictures can be taken in a very light or in complete darkness without the exposure of targets to be amended.
Thus, it is not necessary to be a "photographer" to make a precise measurement system with V-STARS.

On the left, retro-reflective target unlit

On the right, retro-reflective target illuminated

A large variety of retro-reflective targets are available to meet the needs of each measure. See our catalog of tools for optical measurement.

#### Projected optical targets

To project targets optical, we use the projector light PROSPOT2. This quality unit can be projected up to 40,000 targets very high optical resolution.

Prospot 1 projected targets

#### Feelers

Items not visible or hidden in the fields of vision devices can be measured by sensors of different forms adapted to the configuration of the object.