台灣大地基準參考框架討論

On the Reference Frame of Taiwan Geodetic Datum

「大地基準」是包含定義地球形狀、方向、位置、尺度的測量基本，為實施國土測量所必須，亦與民生工程息息相關。台灣地區，自日據時期起建立的「大地基準」，包含日本?地測?部之陸測系統、地籍測量系統、民國58年改算之三度分帶、TWD67、TWD97，堪稱歷史豐厚。各個「大地基準」均對應到不同的參考框架，在GNSS發展之前，「大地基準」乃為區域性自訂。因為以橢球體為基準的測量在實務上無法達成，全球各地的聯接，僅能依靠在「地平坐標」系統中進行的天文測量。由美國GPS衛星運作起，以橢球體為基礎的觀測便利性大增，在大尺度範圍，其作業的效率、經濟效益、觀測精度，甚至優於角邊測量。爾後，在有大量GPS及其他GNSS觀測量的支持下，ITRF精度大幅提升，已為國際作為參考對應的共同選擇。由於地球表面處於動態變遷之中，大地基準的持續演進、更新，實為可期。本文陳述台灣「大地基準」之演遞，並參考國際案例，就未來發展提出探討。

The Geodetic Datum includes information on the earth’s shape, orientation, position, and scale. This datum is of fundamental necessity for national spatial infrastructure, and also essential to any civil development. Starting from the era of Japanese occupation, there have been several Taiwanese Geodetic Datum created. Namely, the Terrestrial System of the Surveying Department, Japanese Army, the cadastral coordinate system, the re-computed three-degree band Transverse Mercator Coordinate System, TWD67 and TWD97. Each geodetic datum is referred to as a reference frame. Prior to the GNSS era, all the geodetic datum implemented by all countries were either regional or local by nature. This largely resulted from the inability to survey directly based on ellipsoid. The only way to approach“global connection”, was through the astronomic observations conducted in the local horizontal coordinate system. Since the operation of GPS, the capability of direct surveying based on ellipsoid has become available. In addition, for large area surveying, GPS based surveying is superior to the traditional terrestrial angle and distance based surveying technology not only in improved efficiency, but with higher, more uniform accuracy and is more economical. In recent years, with the large amount of GNSS observations, the quality of ITRF is rapidly increasing. Consequently, ITRF becomes the common choice to be referenced for the national geodetic datum of each country. Due to thechanging nature of the earth’s surface, the evolution and update of geodetic datum is unavoidable. This article briefly reviews the history of Taiwanese geodetic datum, and discusses future development with reference to several international entities.