Using gravity geoid and topography data to retrieve surface flooding scenarios

 

Speaker: Jen-Ru Liau

 

Abstract

Taiwan is located between the tropics and subtropics, where floods occur frequently in summer due to the heavy rainfall caused by typhoons. 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 arcsecond 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 (Chi-Shun Hwang, 2008). 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 use load love number to calculate the deformation of equipotential surface caused by the loading of massive water (which is neglected in the case of Taiwan because of the smaller range).

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