地形暈渲效果之比較研究

Abstract

暈渲是表現地形起伏的常見方式，透過地圖讓讀者直覺感受地形特徵。在電腦地圖發展之前，繪製暈渲圖是一項非常專業技巧，只有少數的地圖專家有能力繪製。近年來，電腦繪圖快速發展，使用數值地形模型來繪製暈渲圖已經非常普遍。電腦繪製的暈渲圖主要是透過坡度和坡向的計算，模擬光源投射在地表所產生的陰影，本質上是以局部地區的地形分析計算為基礎，和手工繪製的暈渲圖有顯著差異，而各繪圖軟體所使用的演算法不同，所產生的暈渲圖彼此間也存有差異。如何利用數值地形資料自動化生產接近手工暈渲品質的效果，是一項值得探討的課題。電腦暈渲的效果的影響因子主要有演算法(軟體)、地形類型、數值資料解析度。本研究從這兩個因素出發，比較不同地形類型、不同演算法產生的電腦暈渲，希望找出這些電腦暈渲演算法的特性。MDOW、Swiss-style暈渲是為改進電腦暈渲的一些缺點而研發出的新演算法，其灰度值對比要比其他演算法來得小，具有柔和的表現效果。Surfer軟體各演算法的灰度值對比強烈，灰度值標準差大，比較不適合於一般地圖呈現。其中，Lambertian Reflection方法的對比尤為強烈，如果要顯示清楚的地形特徵，本方法將是一個良好的選擇。

Shaded relief is a common way to represent terrain relief for allowing readers to identify topography intuitively. Before the development of computer mapping, only very few professional cartographers can perform shaded relief illustration. Thanks for the advancement of computer applications, analytical shaded relief maps are becoming popular using digital elevation model (DEM) and computer mapping packages. Analytical shaded relief usually relies on the calculations of slope and aspect and simulates the shadows of light shinning on the surface. It is based on calculation of terrain analysis in local terrain, and results in great difference with manual shading. Furthermore, the underneath algorithms of various mapping packages are different, so as their results. How to make analytical shading compatible to manual shading is worthy of investigation. The factors affecting analytical shaded relief effect are algorithm (software), landform classification and DEM resolution. This research starts with comparisons of algorithm (software) and landform classification. It is anticipated to find out the characteristics of several available algorithms. MDOW hillshade and Swiss-style hillshade are new algorithms which improves the shortcomings of previous analytical shaded relief methods. The gray value contrast is very strong in four algorithms of Surfer software with rather high standard deviation. Although Lambertian Reflection algorithm has the highest gray value contrast and very detailed relief presentation, it is a good choice to use Lambertian Reflection algorithm if clear and subtle representations of topographical features are desired.