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ACCURACY OF GIS DATABASES - 
THE DTM COMPONENT AND DERIVED APPLICATIONS 
S. Filin & Y. Doytsher 
Technion - Israel Institute of Technology 
Haifa, Israel 
ABSTRACT 
The paper describes a method to estimate accuracy of objects, by using the error propagation 
technique to determine the "real" (precise) accuracy of elements derived from aerial photographs. 
The paper discusses various aspects of the proposed method and demonstrates the mechanism on 
applications in different levels of complexity, dealing especially with those related to the altimetric 
component of the terrain. Within those applications are direct measurements, computational func- 
tions and DTM applications. 
1. INTRODUCTION 
Handling the accuracy of GIS databases 
is almost an untreated subject. Although its 
significance in aspects of proper data manage- 
ment, data analysis, and in aspects of planing 
usually it is left without any evaluation. 
When accuracy is do evaluated, usually a 
fixed value (derived from the source of which 
the data were captured) is determined. For ex- 
ample the maps' quality determines the data 
set's accuracy in case of data collected from 
maps, or, the aerial triangulation standard de- 
viation (SD) determines the accuracy when 
data are collected from aerial photographs. In 
better cases, accuracy is estimated by referring 
the data set to a more accurate one, and by 
using statistical tests, the database accuracy is 
determined. As improved as those methods are, 
yet their indirect approach, supplying only a 
global value for the whole data set, can not 
deal with or solve the accuracy of the observa- 
tions themselves. 
The paper describes a new “direct” ap- 
proach for evaluating the accuracy of spatial 
data, based on aerial photographs. This direct 
approach follows the photogrammetric solution 
steps and by error propagation technique 
evaluates the SD of every data element. 
The following chapters describe the im- 
plementation of this mechanism for applications 
in different levels of complexity, dealing espe- 
cially with those related to the altimetric com- 
ponent of the terrain. 
2. ERROR PROPAGATION IN THE 
PHOTOGRAMMETRIC PROCESS 
The photogrammetric orientation aim is 
to enable coordinate transformation from the 
image coordinate system to the ground coordi- 
nate system. Several transformation models can 
be found, and different transformation models 
are implied for solving different geometry of 
photographs such as panoramic, spot images, 
etc. Frame stereo-pair photographs, for in- 
stance, can be solved on one hand in a two step 
procedure - relative orientation and then 
model's absolute orientation, or, on the other 
hand, by computing the exterior orientation for 
each photograph separately. For every trans- 
formation model a proper error propagation 
mechanism can be developed. 
The paper demonstrates the error propa- 
gation mechanism applied on the commonly 
used stereo-pair frame photographs transfor- 
mation model, based on the relative and abso- 
lute orientations. This model is considered as a