運用投影法於球面全景攝影圖建立樹木形數

Abstract

為了減少和防止破壞性的取樣方式和達到更快速的估算林分材積，以大幅降低人工及時間的成本，本實驗透過球面全景攝影並利用投影法的影像轉換方式來建立樹木形數(Tree form factors)。為了解Equirectangular projection、Cylindrical projection、Mercator projection、以及Miller projection四種投影法的變形方式，利用臺大森林系系館外牆的柱體，將拍攝之三維立體影像轉換成二維平面影像後，以柱體上相等間距的磁磚來建立影像中柱體不同高度的柱寬點位座標，並透過點位座標分析結果，了解實際物體在不同拍攝距離以及不同的影像高度中，所產生的垂直向與水平向的拉伸以及壓縮的狀況。另以球面全景攝影拍攝實際林地中的柳杉樣樹，同樣將拍攝之三維立體影像轉換成二維平面影像後，利用網格套疊影像，取得柳杉樣樹在不同高度上的樹徑點位座標，建立四種投影法所呈現的形數數據以及樹徑與樹高比率關係圖，進而與樣樹所裁切下來的圓盤進行比對。從分析結果得知，不論在柱體或是柳杉樣樹的影像分析，Cylindrical projection都有較佳的結果呈現，與實際值最為相符。而從拍攝距離與影像關係的結果中，了解拍攝距離在6m以上開始，逐漸與實際數據吻合，可能與標的物在影像中所佔的比例相關。試驗結果顯示，利用投影法來分析影像時，僅能取得二維的平面的數據，而無法模擬三維的真實狀況，因此無法用於尖削度模式(Taper)或形率(Form quotient)，不過卻能夠有效的用來分析形數數據。

By building tree forms of many tree species in Taiwan, it is possible to better assess stocking information of forests. However, building tree forms often requires destructive sampling. In order to prevent or reduce the destructive sampling such as stem analysis, this study attempts to use spherical panoramic photography to reconstruct tree form by applications of map projection methods. This study applies four types of projection methods, which are Equirectangular projection, Cylindrical projection, Mercator projection, and Miller projection, First, we used spherical panoramic photography to image a pillar of a building that has fixed width and height. The spherical panoramic image was then transformed from 3-dimensional to 2-dimensional with the four projection methods. Then, coordinates of the edges of the pillar in the four projected images were extracted using geographical information system. The extracted coordinates allowed us to understand characteristics of vertical and horizontal deformation of each projection method. Secondly, the same process was carried but with a Japanese cedar tree in a forest stand. With the projected images of the Japanese cedar trees, a grid was overlaid on them to extract edges of the tree stem. The extracted coordinates were analyzed and breast height form factor was calculated for the Japanese cedar tree. The form factor of the Japanese cedar tree was estimated for four different projection methods and with its stem analysis data to determine the best projection that could reconstruct tree form. The results showed that Cylindrical projection was the best to reconstruct the shape of the pillar and the Japanese cedar tree. In addition, photography distance over 6 meters has the best results. The methodology developed in this study allows us to reconstruct tree form factors effectively, but it is limited when attempting to estimate tree taper and tree form quotients.