一個完整正確的資料庫是地理資訊系統的核心。基於地理資料的本質和測量及數化工作的限制，所有地理資料的準確度皆屬有限，亦即一定含有誤差。爲了確保地理資訊系統分析結果的可信度，資料品質是相關研究的重要課題。疊圖是地理資訊系統中非常普遍的一個功能，這個功能對於資料品質的優劣極爲敏感，可以清楚地顯示資料品質的瑕疵。所以它除了被廣泛應用在區位選址、變遷偵測等分析工作之外，也是資料品質檢核的一項利器。本研究利用電腦模擬疊圖操作以探討資料品質問題。研究工作包括兩個部份：第一部份，利用S-Plus軟體產生數種不同類型的誤差，再將這些誤差加到原始圖檔多邊形的坐標中，以模擬點圖過程的誤差，然後再將誤差圖層和原圖套疊；第二部份，針對疊圖後的結果，分析多邊形的數量和幾何特性，以試圖找出slivers並加以消除。本研究分別以多邊形之面積和形狀指數進行分析，結果顯示slivers的形狀不見得如傳統所認定的細長。此一發現有待進一步研究證實。

A complete and correct database is fundamental to reliable applications of GIS. Due to the nature of geographic data and limitation of measurement, all geographic data are of limited accuracy. In order to assure the credibility of GIS analysis, the quality of data is an important issue within GIS research. Overlay is a common operation of GIS, which is very sensitive to the quality of data involved. As a result, besides of its widely use in site selection and change detection, overlay is often adopted for error detection of GIS data. This research uses overlay as a tool to investigate errors in two steps. First, a number of erroneous map layers are created using the S-Plus software and ARC/INFO. These maps contain errors of various types and magnitude. Through overlay, this research investigates the relation between data quality and the results of overlay. Second, this research uses spatial factors, such as acreage and shape index, to identify and eliminate sliver polygons. Results show that slivers are not thin and long as we used to think. This controversial finding is subject to further research.