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ACCURACY INVESTIGATION FOR A LARGE SCALE GIS 
T. M. Celikoyan*, M. O. Altan", G. Kemper ® 
* ITU, Civil Engineering Faculty, 80626 Maslak Istanbul, Turkey - (celikoyan,oaltan)@itu.edu.tr 
^ GGS, Kümmererstr.14, 67346 Speyer, Germany, kemper@ggs-speyer.de 
2 
ThS 9 
KEY WORDS Accuracy, Thematic, Geometric, Database, GIS 
ABSTRACT: 
In this study, the accuracy of a geographical information system is investigated. The mentioned accuracy is not only the geometric 
accuracy, but also the thematic accuracy. It is known that the standards for geographical information system are already a popular 
and topic in both practical and academic world. This study gives an example for the investigation of accuracy. 
Geometric accuracy can be derived using the errors between the geometric results and the real values. The real values can be 
calculated precise surveys or taken from maps of larger scale than the scale which is used in the study. This choice depends on the 
dimensions of the study area and the scale of the study. 
Thematic accuracy can be derived from differences of individual defined and to database added objects. These differences are the 
errors of definitions. 
The originality of accuracy investigation within geographical information system is to decide, how many objects from itself are used 
for within this investigation. Ideally, all of the objects must be used for this thematic accuracy investigation. But it is nearly 
impossible to do this investigation with all the objects if a big amount of them are used within a GIS project. This disadvantage 
forces to used sampling methods. 
In this study, an accuracy investigation of a MOLAND project for Istanbul has been done. By geometric accuracy investigation, a 
sampling class size has been defined for all the object types within the project and these samples are chosen from database 
randomly. Afterwards, the true values are taken from 1:1000 digital maps for referring samples. The errors are calculated and from 
these errors, a global geometric accuracy values are derived. For the thematic accuracy investigation, sampling class sizes are 
determined for all different landuse types. This process gives a number of approx. 3000 for sampling class. 
1. INTRODUCTION Cartographic terms). Another definition of accuracy is 
"closeness of agreement between a test result and the accepted 
The International Standardization Organization (ISO) explains reference value" (ISO/TC 211 N 1034, 2001). 
the quality in ISO 8402 as “the totality of characteristics of an 
entity that bear on its ability to satisfy stated and implied 
needs". Quality is defined as "that which makes or helps to 
make anything such as it is; a distinguish property, 
characteristics, or attribute; (in logic) the negative or 
affirmative character of a proposition (Buttenfield, 1993). 
Another definitions of quality are “the sum of all the factors 
that enable ownership satisfaction and bring customers back to 
In geoinformatic, data, which is processed, is mostly spatial 
data. Data about positions, attributes and relationships of 
features in space are termed spatial data (Morrison, 2001). 
Combining the definitions of quality and spatial data, the 
definition of spatial data quality occurs as “a statistically 
measurable concept showing the totality of indicators defining 
the negative and affirmative distinguishing specifications and 
buy a product or a service again and again" (Radwan et al., characteristics in the fields of the satisfaction of the desired 
2001) and “the integration of the terms of completeness, ^ conditions for the producer and appropriateness to the aim for 
consistency and suitability for use, which are statistically the consumer of data about positions, attributes and 
measurable” (Beard and Mackaness, 1993). More general relationships of features in space” (Celikoyan, 2004). 
definition of quality is “a statistically measurable concept 
showing the totality of indicators defining the negative and 
affirmative distinguishing specifications and characteristics in 
the fields of the satisfaction of the desired conditions for the 
producer and appropriateness to the aim for the consumer of a 
product or a service". (Tastan, 1999) 
If the quality is dealt with, accuracy must be defined, which 
affects the quality directly. The term *accuracy" is the degree of 
conformity with a standard. It relates to a quality of the results 
and is distinguished from precision, which relates to a quality of 
the operation by which the result is obtained (Glossary of 
2. BRIEF INFORMATION ON THE PROJECT AND ITS 
RESULTS 
2.1 Data 
After investigating the usable historical data, the historical time 
periods for landuse detection occur as 1940'ies, 1968. These 
years were the only periods, in which aerial photographs of 
Istanbul had been taken. For further years, 1988 and 2000 has 
been selected. Technical data contains topographical maps from 
scales of 1:25000 and 1:5000, IKONOS, IRS 1C/D images for 
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