應用大比例尺正射影像地圖辨識沙礫質海灘的濱線

Abstract

掌岸侵淤變化可謂岸經營管理最基礎的一環，估算數年至數百年間的濱線變遷趨勢常為首要步驟，惟我國的濱線繪製作業規範亟待建立。本文以坡兖較陡的沙礫質海灘為對象，探討當採用大比例尺影像地圖(≧1:5000)估算濱線變化量時，如何選擇最適合的濱線指標(shoreline indicator)，以及如何判定濱線指標的至置。正射影地圖保留了航照拍攝當時的海灘影像，研究者可以影像上的灰度值、色彩和質地(texture)自行判釋各種濱線指標，點對於歷年濱線變化量較小的岸尤其重要。研究結果顯示，若圖資件件允許判釋出高水位線，以此線為最優選擇；若無法有效判定高水位線，則在潮差愈大且坡度愈緩的海岸，愈應優牨選擇穩定性較高者。例如，若單獨使用歷年大比例尺正射影像地圖，在臺東海岸不易判定高水位線的位置，退而求其次以乾濕線為較理想的濱線操作性定義，再其認為水線。未來將有更多高精度衛星影像或Lidar等高精度DEM應用於濱線繪製中，衛星影像自動判釋所得濱線多為水線，若能結合航照影像和Lidar將極有助於判釋出高水位線。所以若考慮已出版圖資和未來圖資所得濱線之操作性定義的一致，本文建議在經費允許下，對已出版之歷年影像地圖的濱線進行判釋時，宜同時數化乾濕線、水線、高水位線和灘內緣線四者。

Detecting shoreline change over several to several hundred years is usually regarded as the first step on coastal management for a stretch of coastline. Shoreline mapping standard in Taiwan, however, is yet to be set up. With the steep and coarse Taitung coast as the study area, this paper demonstrates how to recognizes various shoreline indicators on the large-scale orthogonals (≧1:5000). These shoreline indicators were also assessed with their continuity, stability and recognizable level. Results show that the high-water level, although usually regarded as the best shoreline indicator, is rather difficult to be identified, either in the field or from the images, especially on the gravel beaches. The waterline and the dry-wet line are with the highest continuity and more recognizable, and the dry-we line is less dynamic. Although the dry-wet line is believed as the most ideal shoreline indicator when detecting shoreline change with the existing orthogonals for Taitung coast, it is suggested to digitize the waterline, high-water level and inner edge of beach as well. Because the high-resolution satellite images and DEM (e. g. Lidar) have been (or will soon be) incorporated into the shoreline mapping practice, while the extracted shoreline is usually waterline for the former and the high-water line for the latter.