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1
1
After extracting the changes, all subsequent operations
in the semantic realm are as in the original mapping.
Flow of geometric data
Data flow
The main stages in this flow are the preparation of input,
mensuration, transformations, editing and generaliza
tion, and plotting (fig. 10). The operations specific for
updating by means of DMP occur in the preparation
stage, mensuration- and transformation stage.
I CONTROL and
I CORRECTION DATA.
rprepure b analys'
I identify/select t
H>
From fig.6
lu/ts i
fro» S I
tle/check points!
(murk index poll
photo t nap
I Measure on photo:
I fiducial.control.tie/check pts|
Measure on map:
controltie/check pts|
Measure/digitise on photo
x.y strings/points for all
changed data;t4h height:
Pre-edit geometry]
[Transform (bcorrect) from]
"digltlser to photo system
i (bcorrect) from
er to nap system I
-CE
External orlei
is form (bcorrect )l
i photo to »odel I
[Transform (bcorrectfrom]
model to map (or field)
[Estimate accuracy
geomet ry
Mensuration
Transformations
Editing and
generallsatlon
Fig. 10. Flow of geometric data.
Basic versus control data
In the data flow, two different ingredients are involved,
i.e. the „basic" and the „control" data. The basic"
data locate the elements of the semantic information in
the different co-ordinate systems, whereas the con
trol'data are used merely for the internal process con
trol. Users of the information products are in principle
not interested in the control data unless they upgrade
these products further.
Examples of the control data input are the locations of
fiducial points, principal distance, control points (rota
tion matrix), tie points (for local transformations), cor
rection data (for deterministic errors) and the DTM. The
latter, however, serves in DMP merely for process
control.
Process control in DMP can be differently synthesised,
depending upon the type of the control data input.
Normally, the external orientation is determined by
means of the control points by resection in space. In
the case of aerial triangulation, however, the rotation
matrices may be directly available, though a new aerial
triangulation is not justified for updating.
Operations in the preparation stage concern only the
control data. The basic data are generated in the mensu
ration stage, though some control data are also mea
sured. The differential heights of new man-made objects
are used for locational corrections and thus belong to the
control data. Geometric pre-editing of the basic data is
associated with measurements, i.e., verification of the
geometric correctness and consistancy, and the cor
responding corrections.
The transformation stage concerns mainly the basic da
ta, although the control data are also involved (i.e., in
transformations of the control points, tie points and in
external orientation). Moreover, check points can be
used to assess accuracy. Other subsequent stages in up
dating, i.e., editing, generalization, and plotting are as in
the original (digital) mapping (fig. 10).
In the following, attention is given only to those stages
which imply the operations specific for updating by
means of DMP.
Operations
The preparation stage concerns primarily identification
of the control and tie points. Special care is required in
identifying conjugated points on generalized maps on
which some detail is displaced. Redundant control and
tie points should therefore be provided, which permit a
least squares fit and detection of gross errors. These
points should be marked, and indexed. New object
points with differential heights should also be marked
and indexed if parallaxes are to be measured separately.
The mensuration d\Q\{\s\r\Q stage concerns both the
„new" and the „old" data sources. The control data are
measured on both sources, whereas the basic data are
measured on the new photographs. If the DTM is not a
priori given, the contour lines and/or spot heights should
be sampled locally on the existing topographic maps,
i.e., in the zones of the changes. Differential heights can
be derived from the x-parallaxes. These may be mea
sured either simultaneously with the changed plani
metrie features, or separately (e.g., by means of a ste
reoscope and parallax bar). Simultaneous measurement
is preferable because it is faster and more reliable than
separate measurement [8]. The pre-editing of the geo
metric data is as in the original mapping.
The transformation stage represents the nucleus of
DMP. It comprises three successive transformation
steps, i.e., from the digitiser to the photoco-ordinate
system, then to the model co-ordinate system, and
finally to the map (or field) co-ordinate system. Each
transformation step may be accompanied by the correc
tion routines for deterministic errors. The control data
are needed to determine the transformation coefficients
in each step.
In the first step, affine or similarity transformation is
usually applied to both the control and the basic data.
In the second step, the basic data and tie points are
transformed into the model space (by the collinearity of
another transformation). The process is iterative (fig. 11
[81.
CONTROL DATA
I Correction Rotation
at ion I
BASIC DATA
Collinearity
(or other)
transformation
XYZ
Transformation
to map (or field)
Fig. 11. iterative photo- to model transformation.
Usually three to four iteration cycles per point suffice.
The third transformation step serves for local merging of
the changed data into the existing data base. Thus the
basic data are transformed into the co-ordinate system
of the data base, whereby corrections for deterministic
errors (e.g., shrinkage of map) and cartographic projec-
Interpolation
CONTROL DATA
Heights Ah DT»P
298
NGT GEODESIA 82
