International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
Methods of displacement monitoring is graphically shown in
figure 2.
HEIM
I 2
Physical
Monitoring
Methods
Photogrammetric
1 Geometric
Conventional Geodetic
Space Geodetic |
Figure 2. Methods for monitoring displacements ( Armenakis,
C.:1987)
Physical methods are used to measure - usually unidimensional
-relative displacements using various linear mechanical
instruments in contact with the object. Geometric methods are
capable of monitoring both relative and absolute movements
with respect to a given reference datum. Depending on the
design criteria, these methods range from photogrammetric
approaches to conventional- and satellite geodesy techniques.
The selection of a particular method is a matter of design
considerations to obtain an optimal monitoring scheme.
The type of monitoring networks affects the analysis of the
results. There are two types of geometric monitoring networks:
absolute and relative (Chrzanowski et al., 1981). If the detail
points are connected to stable (reference) points outside the
deformable body; then we have a so-called absolute or
reference network. The main concern in this kind of geometric
network is the affirmative verification of the stability of the
reference points. If however, the limits of the deformation
effects are vague,then, all points are subject to movements. This
type of geometric network is called a relative or object network.
The main problems here are the identification. of the
deformation model and the definition of the reference datum.
3.1 Photogrammetry as a monitoring method
Photogrammetric methods are suitable for monitoring
displacements, because a photograph represents a remote,
complete and instantaneous record of an object. An
instantaneous record of particular situation which may be
changing in time, together with a complete coverage, is most
appropriate for a phenomenon such as a moving object.
As a non-contact monitoring method, photogrammetry can
safely measure inaccessible or dangerous areas as well as
objects in hostile environments. The data acquisition time
ranges from short to instantaneous, thus allowing the capture of
even high frequency displacements because all points are
recorded simultaneously.
As far as the accuracy is concerned, sub-centimetre (with aerial
photogrammetry; Fraser and Stoliker, 1983) and sub-millimeter
(with close-range photogrammetry; Brown, 1980; Fraser and
Brown, 1986) accuracies are achievable.
4. CASESTUDY
To evaluate the photogrammetric method, for displacement
monitoring, rotary kilns in Abyek cement factory (Ghazvin
provience, Iran) have been chosen for study. The main purpose
of monitoring these kilns is to determine the displacement of
the center of the kiln in the place of rollers. On the other hand
we want to monitor the displacement of the kiln in order to
avoid dangers because of the horizontal and vertical
movements. Two epochs were chosen for observation, once
before changing the bricks of the kiln and the other, after
changing the bricks of the kiln,14 days later.
Here, it is necessary to describe the elementary rules and
informations about maintenance technology and monitoring the
displacements, by close-range photogrammetric method .
4.1 Rotary Kiln Maintenance Technology- Kiln Monitoring
A complete understanding of the mechanical condition of a kiln
is required to effectively schedule repair work. One of the best
ways to reach this understanding is through a monitoring
analysis. This operation must be performed every six months.
Kiln Mechanical Analysis, performed on a kiln in its normal
operating condition, has been proven reliable and completely
accurate in every application, but as a research, close —range
photogrammetry method is tested.
4.2 Design Aspects of Photogrammetric Monitoring
Network
The significant aspect of optimal design for a photogrammetric
monitoring network is underlined. This cnables us to set the
requirements for the observations in order to estimate the
unknown parameters and achieve the desired accuracy within
reasonable cost limits. The widely accepted classification
scheme of these problems is (Grafarend, 1974):
- Zero-order design (ZOD) problem. It is concerned with the
optimal definition of the reference datum. The zero-order
design problem will be solved here,by determining the datum
and having base point in a reference coordinate system.
- First-order design (FOD) problem. It is concerned with the
optimal configuration of the network. The first-order design
problem is significant for a photogrammetric monitoring
network. The configuration problem is characterized by a given
weight matrix of the observations and an ideal of desired
variance-covariance matrix of the estimated parameters and
pursues an optimal design matrix. The photogrammetric aspects
which have to be examined in order to solve this optimization
problem are those which affect the formulation and structure of
the design sub matrices. These are:
e Intersection of optical rays at the object points
e Number of camera stations
e Number of photographs on which a point appears (overlap)
e Base-to-object distance
€ Focal length in each image
e Additional parameters for interior orientation
e Density and distribution of detail- and of control points
e Target clusters
e Multi-control constraints
- Second-order design (SOD) problem. It is concerned with the
optimal designation of weights to the observations. For the
second-order design the following parameters contribute to the
improvement of the weight matrix of the observations:
e Images of high photographic quality
e Size, shape, and reflectance properties of targets
e Multiple exposures from each camera station
