由於地球暖化導致溫度顯著上升，南、北極與格陵蘭冰層融化使得海水面上升，近海地勢較低之地區首當其衝面臨到被淹沒之困境。藉由靜態淹水模擬可了解較易受淹沒地區之分佈範圍，並可將此結果當做防災規劃之依據。水往低處流中“低”所表示的並非地形低而是物理上能量較低處，也就是重力位能低，所以實際上淹水與否應取決於重力大地水準面 (geoid) 而非地形高程。然而長久以來的淹水模擬都是直接使用數值高程模型 (DEM) 當基準，但高程參考面主要可分為橢球體與大地水準面是兩種因而也有兩種高程系統，但淹水模擬時應選用正高系統之模型。我們研究中所用之數值高程資料為2000年太空梭雷達製圖任務 (SRTM) 台灣地區之資料，此任務利用太空梭上不同位置的天線接收雷達回波進行干涉得地表高程。SRTM高程系統建於EGM96上，此模型於台灣地區資料性質不佳，因此我們將其參考面轉換至空間解析度為100公尺之重力大地起伏值 (Huang et al., 2007)。此外我們亦有農委會林務局於1986年利用航空測量的資料，是以傳統解析立體測圖儀掃描量測而得40公尺解析度的數值高程模型為正高資料。最後我們使用SRTM進行橢球高與正高之淹水模擬並比較其差異。同樣的概念亦可用於大尺度模擬問題：火星古海洋存在之樣貌，高程資料取自於MGS (Mars Global Surveyor) 任務中MOLA (Mars Orbiter Laser Altimeter) 所得0.25度解析度的測高資料；重力場資料則用多次火星任務中軌道参數推算所得。模擬過程中將地球與火星視為剛體忽略彈性體與黏滯體變形之影響。

Global warming causes the temperature increased significantly and ice sheet melting caused sea level rise. So the low-lying area near coast will be submerged. If we calculate and retrieve surface flooding scenarios, we can have a warning to reduce the damage. Generally, we use Digital topography model (DTM) as a model to simulate the flooding scenarios. Before we simulate, we have to know which height reference system we use. There are two relevant types of heights: orthometric height and geometric height. Flooding flows according to the gravity geoid, so the orthometric height is the appropriate one to use. We have Taiwan’s 40m orthometric DTM, and the Shuttle Radar Topography Mission (SRTM) geometric DTM. Taiwan 40m DTM was obtained from 1986 aerial photographs with 40 meter space resolution refers to the coordinate system TWD67 and orthometric height system. SRTM interferometric radar data, collected during an 11-day space-shuttle mission in February 2000, at 3 second resolution were used to generate DTM for latitudes smaller than 60°, with coordinate system WGS84 and geometric height system. To convert SRTM’s height system from geometric to orthometric height, we use the geoid data with 100 meter space resolution (Hwang et al., 2007). We can then simulate the surface flooding scenarios with different water heights. We can use the same approach to simulate the ancient ocean on Mars. Martian DTM was obtained from the Mars Orbiter Laser Altimeter (MOLA) on the Mars Global Surveyor (MGS) mission at 0.25 degree resolution, while the Martian geoid from radio tracking data of the spacecraft orbits from various missions. We consider the Earth and Mars as rigid bodies instead of the elastic and viscous bodies.

Burrough, P.A. and R.A. McDonnell, Principles of geographical information systems, Oxford University Press, Oxford, 1998.
Farr, T. G., P. A. Rosen, E. Caro, R. Crippen, R. Duren, S. Hensley, M. Kobrick, M. Paller, E. Rodriguez, L. Roth, D. Seal, S. Shaffe , J. Shimada, J. Umland, M. Werner, M. Oskin, D. Burbank, and D. Alsdorf, The Shuttle Radar Topography Mission, Rev. Geophys., 45, RG2004, 2007.
Hansen, J., L. Nazarenko, R. Ruedy, Mki. Sato, J. Willis, A. Del Genio, D. Koch, A. Lacis, K. Lo, S. Menon, T. Tovakov, Ju. Perlwitz, G. Russell, G.A. Schmidt, and N. Tausnev, Earth’s energy imbalance: confirmation and implications, Science, 308, 1431–1435, 2005.
Head, J.W., H. Heisinger , M.A. Ivanov, M.A. Kreslavsky, S. Pratt, and B.J. Thomson, Possible Ancient Oceans on Mars: Evidence from Mars Orbiter Laser Altimeter Data, Science, 286, 2134-2137, 1999.
Hofmann-Wellenhof, B., and H. Moritz, Physical geodesy, Second Ed., U.S.A., Springer, 2006.
Hwang, C., Y.S. Hsiao, H.C. Shih, M. Yang, K.H. Chen, R. Forsberg and A. V. Olesen, Geodetic and geophysical results from a Taiwan airborne gravity survey: Data reduction and accuracy assessment, J. Geo¬phys. Res., 112, B04407, 2007.
Lemoine, F. G., S. C. Kenyon, J. K. Factor, R.G. Trimmer, N. K. Pavlis, D. S. Chinn, C. M. Cox, S. M. Klosko, S. B. Luthcke, M. H. Torrence, Y. M. Wang, R. G. Williamson, E. C. Pavlis, R. H. Rapp and T. R. Olson, The development of the joint NASA GSFC and NIMA Geopotential Model EGM96, NASA Tech. Rep., 1998-206861, 1998a.
Lemoine, F., N. Pavlis, S. Kenyon, R. Rapp, E. Pavlis, and B. Chao, New high-resolution model developed for earth''s gravitational field, Eos Trans. AGU, 79 (9) , 113, 1998b.
Li, X., and H.-J. Gotze, Tutorial: Ellipsoid, geoid, gravity, geodesy, and geophysics, Geophysics, 66, 1660-1668, 2001.
PDS Geosciences Node: http://pds-geosciences.wustl.edu/default.htm
Vanicek , P. and N.T. Christou, Geoid and its geophysical interpretations, U.S.A., CRC Press, 1994.
Yu, S.B., H.Y. Chen, and L.C. Kuo, Velocity field of GPS stations in the Taiwan area, Tectonophysics, 274, 41-59, 1997.
上河文化股份有限公司，台灣地理人文全覽圖，上河文化股份有限公司，2003。
王鑫，地形學，聯經出版事業股份有限公司，1988。
行政院農委會：http://www.coa.gov.tw/show_index.php
呂誌強，DEM 解析度對大地起伏模式之影響，交通大學，碩士論文，2004。
賴子銘、史天元，SRTM/TOPSAR高程數據比對，航測及遙測學刊，11(4)，447-459頁，2006。