product for new applications. It is quite likely that
specifications have to be adapted to new user-
requirements. These requirements may, for example, be
formulated within the TOP10vector-user-forum.
Changes in topography: It is evident that these changes
appear in the change-only datasets.
Corrections/improvements: This category is of great
concern. When the new orthophoto is placed behind the
existing dataset, mismatches may be visible. These
mismatches may be errors, which have to be corrected.
They also may be irrelevant, as they are caused by the
statistical processes of the data capture. In our practice,
it is very difficult to give clear guidelines to distinguish
between errors and statistical noise.
Edge-matching: Edge-matching is the process of
connecting features at the border of two adjacent datasets.
Connecting datasets produced in different years will result
in changes. In the near future, the number of changes
caused by edge-matching will drop significantly, because
edge-matching has its largest effect at the initial dataset
production. However, the problem of edge-matching will
always arise at the border of areas with different revision
dates.
Structural changes: Due to the topological structure,
features will be split up or merged when a connecting
feature is added or deleted respectively. The structural
changes (split or merge) will appear in the change-only
datasets, although they do not represent changes in the
field.
It is impossible to produce change-only-datasets where the
changes are classified by the five causes of change,
described above. Only the structural changes can be
automatically filtered. Another valuable difference can be
made between geometrical changes and attribute value
changes. Therefore, the change-only extraction
distinguishes the following types of changes:
features-deletions: change in geometry
features-additions: change in geometry
features-deletions: change in attribute values
. features-additions: change in attribute values
features-deletions without structural changes (subset of
a.)
features-additions without structural changes (subset of
b.)
© ao os
As said earlier, all file-types are compared. Some types
of change are not applicable for certain file-types, due to
the content and structure of these files. Table 5 shows
which files are produced after comparison of two
TOP10vector-versions.
The required content of change-only datasets depends on
the application. For the TOP10vector change-only check-
897
plots, the following specifications have been formulated:
All changes have to appear on the plots except the
following:
1. Structural changes (they are logic and do not need to
be checked): both BAS-files FILTERED appear on the
plot instead of the BAS-files GEOMETRY.
2. Small geometrical differences, caused by line-
smoothing algorithms, should not be plotted.
Therefore, a tolerance value of 40 cm is used.
3. Deleted areas (they do not need to be checked): the
AREA-files GEOMETRY (del) and ATTRIBUTE(del)
do not appear on the plot.
GEOMETRY | ATTRIBUTE FILTERED
del add del add del add
Ener sl
B AS * * * * * *
AREA * »* * *
BLD * *
SYM * *
PAT * *
Table 5 Files produced by comparison
However, for delivery of change-only datasets to users
other specifications may be valid. In order to preserve
consistency in the topological structure, they may require
all files, except the FILTERED-files, produced without
a tolerance value.
Some differences between change-only information for
check-plots and delivery to users are visualized in figure
6. The nodes are visualized in TOP10vector to indicate
the start- and end-points of line-features. Topographic
edges are line features to separate two topographic areas,
for example roadsides. In this example four changes are
detected:
- a new building
an attribute-change of an area (cropland into grassland)
an attribute-change of a line (topographic edge into
ditch)
one area is split into two (grassland split up in
grassland and cropland).
Each change will be discussed in more detail, reference
is made to table 5 and figure 6.
The new building appears in the BLD-file
GEOMETRY(add). It appears both on the checkplot B2
and the change-only file C2 for delivery. |
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
