e-GPS水準測量精度研究

Study on the accuracy of e-GPS leveling

由e-GPS測量之點位橢球高h，係自橢球面起算至點位之垂直距離；然而，在台灣地區採用高程系統為正高H，係自基隆平均海水面（大地水準面）起算至點位之垂直距離。就某一點而言，由於橢球面與大地水準面不重合，所以兩面之間有一段分離量，稱為大地起伏N；N值可以由橢球高h與正高H計算之（N=h-H）。內政部曾經在台灣全島2065個一等水準點上實施GPS測量與水準測量；因此，這些水準點的橢球高h與正高H皆已知，因此可以計算2065個水準點的大地起伏N。再利用此2065個水準點的大地起伏，可以建構區域大地起伏模式。如果區域大地起伏模式建構完成，可以根據點位的平面坐標，內插其大地起伏N，則該點的正高H值，可以由e-GPS所測之橢球高h與大地起伏N相減求得（H=h-N）；此即e-GPS水準測量的意義。因此，本文將以內政部所提供的一等水準點資料以及台南市政府所提供的一等水準點上之e-GPS測量成果，探討e-GPS水準測量精度有關的議題：（1）建構區域大地起伏模式及有關精度分析，（2）在一等水準點上實施e-GPS水準測量，得到正高估值，再與已知正高值比較，分析e-GPS水準測量的精度。 The ellipsoidal height (h) determined by e-GPS is referred to the ellipsoid surface. However, the orthometric height (H) used in engineering application is referred to local geoid. Since the ellipsoid surface and local geoid are not coinciding, the separation between these 2 surfaces for a specific point is defined as undulation N. In the Taiwan region, a new national vertical datum, TaiWan Vertical Datum 2001 (TWVD2001), was established using the varied observations of 2065 benchmarks between 2000 and 2003.Each benchmark has two types of heights, namely orthometric height H and ellipsoidal height h. Then, it is possible to generate a regional geoid model using data of 2065 benchmarks. Since e-GPS surveying work is relatively time-saving. Hence, if a regional geoid model can be generated from these 2065 benchmarks with an adequate degree of accuracy, then, it is possible to transform the ellipsoidal height h from the e-GPS to the orthometric height H. This procedure is defined as e-GPS leveling. In this paper, issues relating to the accuracy of e-GPS leveling are studied, such as the establishment of regional geoid models, the achieved accuracies of e-GPS leveling compared to the announced orthometric heights by Minister of Interior. The detailed test results will be presented in this paper.