International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
scale and co-ordinate system have to be known to the company
providing the aerial survey to eliminate misunderstanding.
There are different software tools available especially for the
flight planning. They facilitate to define the area, the type of
camera (focus length), overlaps between photos in the strip and
between strips, photo scale and scanning resolution.
[Ideal weather for taking aerial photos in open pit mining is not
a blue sky with sharp sunshine and sharp shadows. There are
very deep areas with nearly vertical walls in a mine or quarry.
The sharp and dark shadows make the processing (especially
the evaluation of heights) in such places more difficult.
Therefore soft light going through high clouds is preferable.
Sometimes waiting for the flight mission is quite long because
of the wrong weather conditions. Therefore, a well co-ordinated
communication between the aerial surveyor, processing
company and open pit mining company is crucial. It has to
provide a fast checking of stabilisation of ground control points
in terrain combined with a good weather forecast. Otherwise,
GCP are covered with snow or leaves or are destroyed.
2.2 Input Data
The most important input data are photos and co-ordinates of
GCP. Depending on the company, accuracy requirements in the
open pit mining are around 15 cm in position and 20 cm in
height. Therefore the scale of photos is between 1 : 4,000 and 1
: 10,000, scanning resolution usually is 1,200 dpi (21 microns).
Then the ground resolution is between 8,5 and 21 cm. The
scanning resolution can be higher but the data volume increases
rapidly. The photos can be panchromatic but colours make the
interpretation much more easy. Nowadays, also digital photos
are available. However, the quality of conventional photos is
still higher according to the author's experience. Especially the
colours of neighbouring photos are better balanced at
conventional photos.
The co-ordinates of projection centres and control points are
very critical aspects in the whole workflow. Nearly every open
pit mining site works in a different co-ordinate system. The
aerial surveyors navigate in WGS-84. This has to be taken into
account already during the planning stage of the flight mission.
The transformation between systems has to be done very
carefully with the knowledge of the co-ordinate system
definition.
For interior orientation of conventional photos the camera
calibration protocol is needed from the aerial surveyor. In the
case of digital photos, only information of the number of
columns and rows and the resolution in microns is required to
create the calibration file in the software. Some software tools
have already integrated parameters of digital cameras and then
interior orientation is done automatically. Together with the
photos, the aerial surveyors deliver also data of external
orientation (positions of projection centres and roll, pitch and
heading angles) from their GPS/INS systems. It helps to reduce
a number of control points during aerotriangulation. The aerial
surveyor usually provides also the scanning of the conventional
photos. It has to be done with special calibrated
photogrammetric scanners with not too sharp contrast without
compression which could cause the loss of information.
The position of GCP themselves has to be discussed before
the flight planning. Usually they are located around the mine or
quarry. It would be useful to have also some GCP inside the
area but there is a danger of their destruction by the mining
activity. If the data processing company does not signalise GCP
itself, it is necessary to explain all rules of the point location
and signalisation to the responsible geodetic company. Some of
the rules are known but also have to be mentioned. They
include for example that GCP should not be covered by high
buildings or trees, should be located on a contrast background.
Especially the size is very important. They must not be either
too small or too big. The size depends on the scale of the
photos. For example, at 1 : 6,000 scale a quadratic signal has
the size 20 x 20 cm, an arm of a cross signal is 60 — 80 cm long.
The colour is usually white because white subjects on a dark
background seem to be bigger than they are in reality. This
effect is called irradiation.
Figure 1. Example of signalisation of a ground control point
2.3 Software and hardware equipment
There are many software tools on the market and usually
the open pit mining company uses a different tool than the data
processing company. Therefore a detailed discussion about
required formats and accuracy is necessary. In the case of
different tools, conversions of data have to be done. Sometimes
conversion programs are made to order and have to be tested
before use. Nearly every software company withholds their
format structure, and then testing the converters is crucial to
eliminate systematic errors originating from conversion.
Variances in software use do not only occur between open pit
mining company and data processing company. Also within the
processing company different programs are used for the flight
planning, aerotriangulation, stereophotogrammetric processing,
digital terrain modelling, finalisation of vector data. To simplify
conversions known formats as TIFF for raster data and DXF for
vector data are used in the cases where it is possible. Otherwise
converters are made to order.
As a huge amount of data’ has to be processed
in photogrammetry nowadays, faster and more powerful
computers are required. Normal standard computers can have
1.5 GHz processor and | GB RAM. Computers are usually
equipped by CD/DVD burners, big hard disks and by special
back-up systems. A standard for the stereophotogrammetric
processing is Crystal Eyeware with emitter.
2.4 Human factor
The human factor still plays a very important role in
photogrammetric processing, although many procedures are
automatic. Well trained, reliable and precise people arc
fundamental for every successful work. On the one hand,
people specialising in photogrammetry should have a good
mathematical background of software tools, what they can
expect from a program, which kind of adjustment is used and
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