What is photogrammetry?
Before describing the
operation of the VSTARS system, a brief introduction to photogrammetry is
provided for those who are unfamiliar with the technology. Photogrammetry,
as its name implies, is a 3dimensional coordinate measuring technique
that uses photographs as the fundamental medium for metrology (or
measurement).
The fundamental principle used by photogrammetry is triangulation. By
taking photographs from at least two different locations, socalled "lines
of sight" can be developed from each camera to points on the object. These
lines of sight (sometimes called rays owing to their optical nature) are
mathematically intersected to produce the 3dimensional coordinates of the
points of interest. Triangulation is also the principle used by
theodolites for coordinate measurement. If you are familiar with these
instruments, you will find many similarities (and some differences)
between photogrammetry and theodolites. Even closer to home, triangulation
is also the way your two eyes work together to gauge distance (called
depth perception).
Photography  The First
Part of Photogrammetry
Taking photographs is, of course, essential for making a photogrammetric
measurement. To obtain the high accuracy, reliability and automation the
system is capable of, photographs must be of the highest quality.
Fortunately, because of the design of the system, photography with VSTARS
is actually simpler than film photography.
Metrology  The Second Part
of Photogrammetry
Photography in its broadest sense is a process that converts the real
3dimensional world into flat 2dimensional images. The camera is the
device that makes this transformation or mapping from 3 dimensions to 2
dimensions. Unfortunately, we cannot map the 3dimensional world onto two
dimensions completely so some information is lost (primarily the depth).
Triangulation
Triangulation is the principle used by both photogrammetry and theodolites
to produce 3dimensional point measurements. By mathematically
intersecting converging lines in space, the precise location of the point
can be determined. However, unlike theodolites, photogrammetry can measure
multiple points at a time with virtually no limit on the number of
simultaneously triangulated points.
