Photogrammetry
Explained
Overview
Photogrammetry is a very old and trusted
method of surveying. Most maps you look at today were created using
photogrammetry. DART uses special photogrammetry techniques to
produce drawings of anything by taking multiple photographs from
different angles of the object or area of interest. We use calibrated
digital and medium format cameras to take our photographs. Depending
on the object the photographs are taken from the ground, high lift
tripod (40ft), blimp platform, (40 to 500ft) or plane (above
500ft).
It is as, or more accurate than any other
traditional survey technique and provides numerous advantages
including lower cost, reduced field time, and an increased level of
detail picked up. In fact no other survey technique can pick up the
level of detail that photogrammetry can. Why settle for less
detail?
Ironically the more detail the site has the
easier it is to survey with photogrammetry and the more cost
effective it is. Sites with a very level of detail (like a white wall
with no detail) reduce its cost effectiveness.
If you want to know more how its done read
on.
If you want to see some examples click here.
Detailed Information
We use pictures like this
to produce maps and drawings!
From photo to scale
drawing!
Q. What can you
survey with photogrammetry?
Just about anything. If the object(s) or site
can be photographed from more than one angle, then photogrammetry can
produce a scale drawing of it!
Photogrammetry is particularly useful for
objects (sites) that are difficult to survey with traditional
techniques, for example:
- boat hulls
- buildings
- areas with difficult or
dangerous access
- industrial piping
Q. What is
Photogrammetry?
It is the science of taking scale
measurements from photographs. Simply put, a form of surveying that
uses a fully calibrated or "METRIC camera" instead of a transit or
level. Photogrammetry is usually associated with aerial photography
but with the advances in computer software capabilities it is now
widely used as a replacement for traditional surveying.
Q. Is there more than one
Photogrammetry system?
There are many different systems in use
today. For example the Ontario Base Maps (OBM) with scales from
1:2,000 to 1:10,000 are derived from aerial photographs taken with
metric cameras from planes flying at 10,000 feet. Most of the OBM
mapping work (in Ontario) was done in the 60's to 80's. That system
uses stereo pairs (two photographs that are horizontally separated)
to produce maps that are often accurate to approximately 5m. This
system requires precise camera positioning and specialized operator
skills in order to produce usable photographs and is therefore very
expensive. The cameras themselves can cost hundreds of thousands of
dollars, often have a 9 square inch negative and require large
machines with skilled operators capable of seeing in stereo in order
to produce accurate drawings from the photographs.
There are many other systems that vary
significantly in how they work, the accuracy they provide, skills
required of the operators, and control required on the photography
and so on.
Q. What specific system does
DART use?
DART uses the ROLLEI Metric Camera and the
ROLLEI Fototechnic system CDW software, developed in Germany, for its
Photogrammetry. It is used throughout the world by agencies
including: CSIS, RCMP, and the European
Space Agency. DART also uses the
system developed by EOS, which was recently used to model the Lady
Diana crash scene. Look at some recent unusual
projects
Q. What do you mean by a
Metric Camera?
ROLLEI cameras are touted as having the best
optics and precision construction available today. ROLLEI makes a
Metric version of its cameras. These cameras use a negative that is
57mm by 57 mm and have a calibrated lens; meaning the lens is
precisely measured, so the exact focal length of the camera is known.
For example a 50 mm lens may actually be 50.11mm. A special plate is
also mounted on the back of the film to keep the negative flat while
it is exposed. The plate also puts little crosses on the negative so
that any distortion of the photograph due to photo finishing lenses
can be determined. You may have noticed these cross hairs on Moon
pictures or pictures from Mars. Apart from these critical
differences, it is a standard camera. DART also uses calibrated
digital cameras when large volumes of photos are required.
Figure 2 - Example of a
typical Metric photograph - cross hairs are being highlighted for
identification. Look at some recent unusual
projects
Q. What and how do you survey
a site with Photogrammetry?
STEP 1 - FIELD WORK
Anything can be surveyed using
Photogrammetry. ROLLEI was recently used to survey the Flame on the
Statute
of Liberty prior to rehabilitation.
The only requirement being that the subject is visible and can be
photographed from three different angles. Lets take the example of
the cliff above in Figure 2. Photographs are taken from three angles
along the face of the cliff. One straight on, the second from the
left and the third from the right. A single measurement (see figure
below) is all thats required to establish the scale of the surveyed
object. That’s it.
Figure - Single distance
measurement for photogrammetry model being taken. (Targets ensure the
distance is accurately measured)
STEP 2 - PHOTO
FINISHING
Negatives are often slightly warped in one
direction. To eliminate this possible source of error they are
developed with a piece of glass laid overtop of it to keep it flat.
The size of the print used is a direct function of the size of the
detail on the object to be surveyed. For most uses, a 10 cm print is
used (like the project above) ,where very small detail is present
(like building brickwork), a 20cm print or larger is used.
The photographs are then scanned (usually at
600 dpi optical resolution) into a windows BMP file just like those
used for Windows 95 backgrounds.
STEP 3 - DEVELOPMENT OF
COMPUTER MODEL
The scanned photographs are brought into the
software and the crosses are identified by the computer. Then the
operator identifies approximately 12 points that are visible in each
of the three photographs. The camera data is input into the computer
(ie focal length and negative size). The special ROLLEI software can
then calculate the position of the camera relative to the object
surveyed. That’s it, a drawing of the site can now be created.
Figure 3 - Example of photo
measurement points being added. These same points are added to two
other photographs taken from different angles of the same area.
(Photograph taken from boat)
STEP 4 - CREATING THE
DRAWING
The site can then be mapped by simply drawing
on top of the photographs. For example, the top of the cliff is
traced on one or more of the scanned photographs and the software
creates a scale drawing of the top of bank in a standard CAD format.
The process is continued for all of the features (ie bottom of bank),
producing a complete, very accurate, completely 3
dimensional
finished drawing of the area.
Q. How accurate is it?
A good rule of thumb is 1 part in 2,000. For
example, from a distance of 10 metres the accuracy is approximately 5
mm. The accuracy can be increased for industrial applications to 1
part in 20,000 by taking more photographs, and or adding additional
control. The European
Space Agency uses special ROLLEI
cameras to measure tolerances of 30 microns on its satellites.
A relative accuracy of 1:2,000 means that for
an object with a 10m largest dimension, we can produce 3D coordinates
with 5mm accuracy at 95% (two standard deviations) probability. A
higher level of accuracy can also be provided should your application
require it.
Q. What are the benefits of
photogrammetry?
- Photogrammetry picks up a level
of detail that can not be obtained through any other surveying
technique.
- Return visits to pick up missed
detail are reduced.
- Field time is costly,
Photogrammetry dramatically reduces field time.
- Drawing production is fast,
accurate and easy because pictures of the object are always
available to identify features accurately and completely
- Sites with difficult or
hazardous access can be surveyed in complete safety. (ie bridges,
high traffic areas, or the cliff above)
- Drawings are in 3 dimensions (ie
height of cliff)
- Photogrammetry provides a
flexible level of accuracy to suit user needs.
- The more detail on the site to
be surveyed, the easier it is to survey the site.
- Photogrammetry provides a
supplemental products as part of the drawing production. For
example aerial photographs or site photographs.
Q. What are the disadvantages
of photogrammetry?
- Some features can not be picked
up with Photogrammetry. For example on sites with underground
infrastructure (ie sewer lines) the pipe elevations can not be
obtained through Photogrammetry.
- Ironically, sites with no detail
(ie open fields) can be more difficult to survey than sites with
extensive detail. DART uses ground targets on sites with limited
detail to provide common features for the system model
development.
Q. How much does it
cost?
Photogrammetry is very competitive cost wise
to total station surveys. In fact it is often less expensive to use
Photogrammetry, particularly with standard survey requirements. Three
dimensional drawings are more expensive than simple 2 dimensional
drawings due to the complexity involved creating the 3D CAD drawing.
In most cases however, only 2 dimensional drawings are required.
Photogrammetry is often the only economically viable method of
producing 3 dimensional drawings in non industrial
applications.
Q. Can photographs be taken
from plane or helicopter?
Definitely, DART owns a 30 foot blimp that flies to over 500 feet, and a tripod that extends
to 40 feet. For very large areas of land, photographs from a plane
are used to survey the site.
Figure 4 - Red line
illustrates the single distance measured for the scale of the
photogrammetric model
Q. Can I take photographs
with a non metric camera and use them?
Yes. Non metric photographs can be used as
well. There is a reduction in the accuracy by using non metric
cameras. For example a building that the ROLLEI camera can survey to
3 mm may be accurate to say 15 to 25mm with a reasonable quality 35mm
camera. Contact us if you would like to take your own pictures and we
can guide you through the process. We can also calibrate your camera
and significantly increase the accuracy of drawings created from your
photographs.
Q. What does the finished
product look like?
END
