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

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.