International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
CDs or DVDs with photos. These media are not so reliable and
often files are not readable. If the amount of data is huge,
replaceable and more reliable hard disks are used.
The first step in the photogrammetric software is setting up the
new project. The calibration protocol file has to be created in
a form according to the software needs. Then other input data
have to be set, e.g. an attachment of the photos, scanning
resolution and scan position of photos, the average flight
altitude or the average terrain height, the file with co-ordinates
of GCP with accuracy, the file with co-ordinates of projection
centres and roll, pitch and heading from GPS/INS system with
accuracy as well, definition of strips and blocks, corrections
like atmospheric refraction, earth's curvature and lens distortion
and information about used units.
Usually many photos are taken in a project even in several
strips. Therefore the method of aerotriangulation is used to save
a number of ground control points. Neighbouring photos are
connected by so-called tie points which are searched for in the
triple overlap of the photos. Therefore the overlap between two
photos should be 60% at a minimum to get a suitable triple
overlap (2096) between every three following photos. The tie
points can be searched for also automatically. The information
about co-ordinates of projection centres and rotations is a very
big help in the automation of tie points searching. The GCP
have to be picked up exactly on their position on the photos. All
co-ordinates of measured GCP and tie points with external
orientation parameters from GPS/INS, whether automatically
found or manually measured, are used in adjustment — usually
the bundle block adjustment method. A result from
aerotriangulation includes adjusted values of external
orientation parameters, adjusted co-ordinates of GCP and tie
points with mean square errors to consider an accuracy.
Afterwards, the epipolar pairs are created to reduce the effect of
vertical parallax and enable better stereoviewing. The work in
stereomode includes drawing new lines and points, deleting old
ones, moving, splitting lines, trimming and other operations.
A correct interpretation can be worsened by shadows and a low
contrast between crest and toe and plane areas. Therefore,
software tools include usually a function of gamma corrections
and a change of brightness and contrast at photos “on the fly”.
Figure 4. Example of shadows on orthophoto combined with
vector data
For the creation of orthophotos, the raster points from automatic
correlation are needed to form the digital terrain model. This
correlation facilitates to search for corresponding pixels in the
overlap of neighbouring photos and to get their third co-
ordinate — altitude from their horizontal position on the photos.
The digital terrain model is made from correlated points and
points, lines and areas evaluated in stereomode. Sometimes
buildings and plants are drawn in the height of their roof even
though they should be on the ground. Belt conveyors are also
not on the ground. Therefore, for the purposes of the digital
terrain model the objects which are not on the ground are
deleted. Only for the purposes of a real orthophoto, the
buildings should be made as cubes or blocks to remove also
distortion of them. However, buildings and plants are not so
important in the case of the open pit mining.
The colours and contrast can differ from one photo to another.
Therefore, a colour balancing is necessary. The last step
includes masking and mosaicing the colour balanced
orthophotos.
After delivering all outputs to the open pit mining company
a very important step follows. This is the sorting of all data and
saving all important information on CDs, DVDs, hard disks
and/or servers.
4. CONCLUSIONS
Even though a suitability of the use of photogrammetry in the
open pit mining compared to the conventional measurement is
still discussed, many examples and projects speak about its
advantages, e.g. the economic view especially in saving time
for processing, and sufficient accuracy aspects as well. Also the
purchase costs have a more favourable rate with the geodetic
equipment. Compared to past times when big analogous and
analytic photogrammetric instruments were very expensive,
nowadays only fast and powerful computers with software are
needed, that could often be obtained at decreasing prices in the
last years. Taking photos is still quite expensive, though. But
costs can be reduced by an economic flight planning which
combines near located open pits in one flight mission. The costs
for human power are in photogrammetry lower as well. In the
terrestrial measurement usually two people are needed, and
during photogrammetric processing only one person.
Therefore photogrammetry is a suitable method even in the case
of large scale photos and high accuracy requirements. All these
cost reducing aspects suggest the use of photogrammetry in the
open pit mining evaluation not only for bigger organisations but
also for smaller open pit mining companies.
ACKNOWLEDGEMENT
The author is grateful for the support of the company “SST,
Prof. Dr.-Ing. Stoll and Partner, Ingenieurgesellschaft mbH"
while writing this paper. The company Kalkwerke H.
Oetelshofen, GmbH & Co., Wuppertal, Germany is gratefully
acknowledged for providing the photos and data for
the presentation. Furthermore the author would like to thank
Dipl.-Geogr. Gerhard Kemper from the GGS company for his
commentary.
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