UPDATING ELEVATION DATA BASES
- MERGING OLD AND NEW DATA
Poul Frederiksen
Associate Professor
Institute of Surveying and Photogrammetry
Technical University of Denmark
DK 2800 Lyngby, Denmark
ISPRS Commission IV
ABSTRACT
Information on data origin, quality, structure and distribution is essential to the updating as well as the use of
an elevation data base and should be stored in conjunction with the data base.
Running updating procedures on a part of an elevation data base often results in a non-homogeneous quality
and distribution of the data. This must be communicated to the user before the data base is used for analysis,
which means that new facilities for data merging and updating must be introduced in the digital cartographic
system that manages the data base.
KEY WORDS: Updating, Elevation model, Data base
INTRODUCTION
Extensive research on digital elevation models
(DEM) has been carried out over the last 20 to 25
years. The construction of the models, and in particu-
lar the study of interpolation procedures, were the
research subjects in the 1970s. A substantial number
of interpolation methods were introduced.
Later, the research focused on the quality of the mo-
dels and the relationship between data acquisition
method, point distribution and terrain characteris-
tics. It was concluded that the choice of interpolation
method had little influence on the accuracy of the ter-
rain model, while the crucial factors were the nature
of the terrain and the point density (Kubik and Bot-
man, 19776. Frederiksen, 1981).
Several procedures for control of data acquisition and
prediction of the quality of the models have been de-
veloped (Frederiksen et al., 1986. Tempfli, 1986.
Fritsch, 1988).
Today, there are a number of effective DEM program
packages on the market developed within almost eve-
ry earth science discipline. However, most of these
packages are stand alone programs and not until la-
tely has the DEM become an integrated part of the
GIS environment (Sandgaard, 1988).
Future research will focus on this integration, the
use of the models and the maintenance of the huge
amount of data that will be captured and stored in
elevation data bases.
The purpose of this paper is to identify some of the
problems that must be treated when a digital eleva-
tion data base is updated, and to propose a few solu-
tions to overcome these problems.
DEM AND ELEVATION DATA BASES
A digital elevation model is defined as a collection of
points with known x-,y- and z-coordinates. The points
represent the terrain surface with an accuracy deter-
mined by the measuring method. The distribution of
the points should vary according to the characteris-
526
tics of the terrain. The points in a DEM may be orga-
nised in a regular grid or a triangular network, may
be measured in profiles, may represent break lines or
structure lines or follow contours as a result of digiti-
sing maps or measuring contours in a photogramme-
tric stereoplotter.
An interpolation procedure is attached to the DEM
data in order to estimate the z-value of a new point at
a given planimetric position.
When the measured and/or interpolated points of a
digital elevation model are stored they make up the
terrain data of the digital elevation data base. Apart
from these, the data base is characterized by a num-
ber of parametres originating from the DEM, i.e. ac-
curacy, point distribution etc. This “historical” infor-
mation should be stored in conjunction with the ter-
rain data to constitute the complete elevation data
base and in order to make a proper updating possible.
The updating of a digital elevation data base can be
defined as replacing parts of the data base by new
points or adding new points to the data base. Similar
to the interpolation method of a DEM the updating
procedure consists of a strategy for merging new and
old data in the data base. Implicitly, the updating
should improve the capability of the data base to re-
present the terrain surface.
IDENTIFYING THE PROBLEMS
The simplest way of updating an elevation data base
is to replace the whole data base by new data. In that
case, of course, none of the above mentioned parame-
tres of the old data base have to be considered.
However, when just a single point in the data base is
replaced or a new point is added to the data base a
number of the historical parametres must be conside-
red. First of all:
* The reference system must be identical for new
and old data .
This has to be investigated before updating and ac-
tion taken to ensure the same reference level for the
entire data base.
