film, new topography is annotated on the photograph. 
While the topographers are collecting field changes, 
photogrammetry is applied to provide a solid geometric 
base for the digitizing process. The original photographs 
are subject to aerial triangulation and block adjustment. 
Ridges of roofs are used to establish the relation between 
ground coordinates and photo-coordinates. The fiducial 
marks are also measured in photo-coordinates in order to 
determine their imaginary ground coordinates. 
When the topographers return from fieldwork, the 
enlarged aerial photographs are scanned with 1 meter 
resolution. The pixel-coordinates of the fiducial marks are 
measured. Using the ground coordinates of these points, 
determined in the  photogrammetric process, the 
transformation parameters for rectification can be 
determined. The control points of the triangulation 
process are not marked on the enlarged photograph. 
Therefore the fiducial marks are used. Combining the 
transformation parameters with a height model (25 m 
grid), the scanned photograph is transformed into a 
digital orthophoto. The digital orthophoto contains the 
annotations made by the topographer. 
Meanwhile, a negative of the existing map is made. The 
marked features on the positive are masked on the 
negative. This negative then contains the unchanged 
topography. The negative is scanned and digitized in 
vector format. This work is contracted out. As a result, 
a vector-file containing unchanged topography is 
produced. 
Heads-up digitizing 1s applied to add the new topography 
to the vector-file. The digital orthophoto serves as a 
background image. The operator digitizes the annotations 
from the orthophoto. The operator does not need to 
interpret the orthophoto itself, because all features to be 
digitized are annotated with special symbols. These 
symbols represent the codes which should be attached to 
the points, lines and centroids. When digitizing is 
finished, software is applied to check the data-structure 
and to built the areas as closed polygons. TOP10vector is 
then finished, although it needs to be checked. A check- 
plot is made, which is compared with the markings on the 
film and the annotations on the photograph in order to 
check whether all features are correctly digitized. When 
the corrections are completed, TOP10vector-production 
is finished. 
The TOP10vector-dataset is used for map production at 
scales 1:10,000 and 1:25,000, and for the production of 
TOPS0vector. TOPSOvector is the digital topographic 
database at scale 1:50,000 and is produced through the 
interactive generalization of TOP10vector. TOP10vector 
is also made available to the user community. 
At the beginning of 1996, 60% of The Netherlands is 
894 
digitally covered in the TOP10vector-structure. The first 
edition of TOP10vector will be completed by the end of 
1997. Then, all 650 1:10,000 map sheets will be 
available in digital format. All these datasets have to be 
updated regularly. 
3. REVISION: GENERAL ISSUES 
When an updating system has to be designed, one should 
consider the following: 
revision cycle: continuous or cyclic 
revision cycle depending on area or object type 
. production method: data capture and digitizing 
. type of update delivery. 
sens 
3.1. Revision cycle 
One can decide to have a continuous update or cyclic 
update. Most cadastral maps and large scale base maps 
are continuously updated, often by means of land 
surveying methods. The date of capture or digitizing is 
often input. Topographic maps of smaller scales are often 
revised through aerial photography, which make them 
more suitable for cyclic revision. However, as large scale 
databases are being built, the digital format makes it 
possible to apply the large scale data for continuously 
updating smaller scale databases. 
3.2. Areas or objects 
In case of cyclic revision, one can choose to vary the 
cycle for densely populated areas, where more changes 
occur, and for sparsely populated areas. It is also possible 
to update some feature types more often than others, for 
example to update buildings and roads more often than 
other topography. 
3.3. Production method 
When the same system of data collection is used, there is 
no need to change the production method. However, 
digital updating offers new opportunities, because it is 
relatively easy to incorporate digital data from other 
sources, e.g. changes from larger scale databases. It is 
also possible to store historic data, more or less invisible 
in the dataset. An example is the updating system of the 
Ordnance Survey of Northern Ireland (Gray, 1995). 
3.4. Type of update delivery 
Updated datasets can be delivered to users. However, it 
is also possible to deliver change-only datasets to users. 
The last option is promising for GIS-users, who have 
collected additional user-specific attribute data. When 
change-only datasets are delivered, there is no need to 
enter attribute data for the unchanged features for the 
second time. Such update delivery requires an unchanged 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
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