A stratified sampling approach to the spatial accuracy assessment of digital cartography: an application to the Portuguese Ecological Reserve

Miguel Peixoto 1, Ana Cristina Costa 1, Marco Painho 1 and Thomas Bartoschek 1, 2 
1 Instituto Superior de Estatística e Gestão de Informação
Universidade Nova de Lisboa
Campus de Campolide, 1070-312 Lisboa, Portugal
Tel.: + 351 21 387 04 13; Fax: + 351 21 387 21 40
mpeixoto@isegi.unl.pt; ccosta@isegi.unl.pt; painho@isegi.unl.pt; tbarto@isegi.unl.pt
2 Institute for Geoinformatics
University of Münster
Robert-Koch-Str. 26-28, 48149 Münster, Germany
Tel.: +49 (0) 251 83 33083; Fax: +49 (0) 251 83 39763
bartoschek@uni-muenster.de
 

Abstract
Managing spatial data in paper maps is quite different from managing digital spatial information. Sometimes manual vectorization of scanned maps is the only way to produce digital cartography, especially when the only  source of spatial information is a paper map. Digital scanning and manual vectorization are  two processes well known for adding error to data and accuracy is a particularly important issue for users of spatial information. The National Ecological Reserve (Reserva Ecológica Nacional - REN) established in the Portuguese national law protects areas with a  diversified bio-physic structure and specific ecological characteristics. This information is often required to manage several human activities, such as mineral extraction, real estate, industry,  tourism, etc. REN maps were originally produced in paper and were vectorized to produce digital cartography. The objective of this study is to measure the spatial accuracy and to assure the conformity with the original cartography. The accuracy of the REN digital cartography was assessed through a stratified sampling scheme, with strata defined as ecological classes within each county. The global sample size and the strata samples sizes were first determined assuring proportional area representation of each class. The sampling methodology assumes a sampling error equal to 1% for the estimation of the digital cartography errors proportion in the whole study area, for a 95% confidence level. The complexity and uncertainty inherent to each ecological class was taken into consideration on the computation of those samples sizes by using the results from a previous pilot study. The final global sample size and the strata sample sizes were established assuring at least two sample units in each stratum. The sampling methodology and the accuracy assessment procedure are detailed, finally the results are discussed and some conclusions of this study are drawn.

Keywords: data quality; quality control; stratified sampling; National Ecological Reserve

In: Caetano, M. and Painho, M. (eds). Proceedings of the 7th International Symposium on Spatial Accuracy Assessment in Natural Resources and Environmental Sciences, 5 – 7 July 2006, Lisboa, Instituto Geográfico Português

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