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Normally, an updating takes place in a limited part 
of the data base initiated by the requirement of a hig- 
her quality of the z-values. In that case: 
* The quality of the updated database is non-ho- 
mogeneous. 
The updating may be motivated by the desire for a 
higher point density in a specific area, or a grid mo- 
del is updated by merging the grid with a cartograp- 
hic data base comprising z-values of the measured 
entities such as roads and houses. This will probably 
have the effect that: 
* The point density varies in the data base 
The data acquisition method for the new points can 
determine a point distribution which leads to a data 
structure different from the original database. For in- 
stance, the data base may be organised in a grid and 
the new data may originate from the measurements 
of a technical map. Consequently: : 
* The data structure varies in the data base 
Some of these problems can be illustrated by the ana- 
lysis of the updating process of a grid data base. 
THE GRID DATA BASE 
Besides general information on reference system, ti- 
me and method of data acquisition the grid data base 
is characterized by the following parametres: 
* Grid spacing 
* Grid orientation 
* Point accuracy 
The grid spacing is often the same in both directions 
forming a square grid. However, the spacing may 
change across the data base due to data acquisition 
method or sampling stategy. Progressive sampling 
(Charif and Makarovic, 1988) and Fourier transfor- 
mations (Frederiksen, 1981) are examples of sam- 
pling and prediction procedures introduced in order 
to achieve an optimal grid spacing. In both cases, the 
grid spacing will vary according to the type of terrain 
and the accuracy requirements. 
Normally, the grid is oriented parallel to the x- and y- 
axis of the reference system, a procedure which also 
facilitates the use of the data base as regards interpo- 
lation and extraction of data. 
Basically, the accuracy of the z-values is related to 
the accuracy of the planimetric positions of the acqui- 
red points, but in this context, the quality of the elev- 
ation data base is considered to be dependent on the 
z-values only. 
The most evident advantage of a grid data base is the 
data structure. Data can be compressed, for instance 
by run-length coding, and only the z-values need to 
be stored in conjunction with the above mentioned 
characteristics. This holds as long as the grid size do- 
es not change. If the grid size varies from one part of 
the data base to another, the compressed data stora- 
ge can only be preserved if additional information 
specifying the locations of the changes of the grid size 
is stored. 
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Figure 1. 
New data captured to update the elevation data base 
in a limited area may be organised in a grid with ori- 
entation, spacing or accuracy differing from the origi- 
nal data base. Irregularly distributed elevation data 
must be considered as well. 
If the grid orientation of the new data differs from 
the grid of the data base, or if the new data are irre- 
gularly distributed, an interpolation procedure must 
be introduced to accomodate the grid data structure, 
either in order to preserve the original grid spacing of 
the data base or to define a grid with a new spacing 
in the updated area. 
OLD AND NEW DATA 
Independently of the structure of the new data it, 
must be decided whether the new data will replace 
the old data or whether both data sets will contribute 
to the updated data base. 
In case the new data are sampled at the same posi- 
tions as the points of the existing data base, two pos- 
sibilities exist: 
* The new z-values can replace the old data at 
the grid points or 
* New and old data can be merged to a new esti- 
mate of the height. 
In case the new z-values are sampled in a grid with a 
spacing differing from the data base but with the sa- 
me orientation the old data can be rejected and the 
updated area will simply consist of the new grid data. 
In any other case, an interpolation procedure has to 
be introduced to preserve the grid structure. A num- 
ber of strategies are available for the updating: 
*« New data can be interpolated to the data base 
grid and the old data rejected. 
« New data can be interpolated to the data base 
grid and merged with old data.