Analysis and discussion of GPS elevation replacing the five grade leveling

XIE Xiangcheng，FENG Guangyin，YANG Jun

（Jingjiang Survey Bureau of Hydrology and Water Resources, The Hydrological Bureau of Changjiang Water Resources Commission,Jingzhou,Hubei,434000,China）

Abstract： In recent years，the Global Positioning System is widely applied in the fields of Surveys． The horizontal precision in GPS surveying has been high，which can satisfy the demand of precision in some horizontal control network．But the precision of height in GPS surveying needs a further improve .so the study of GPS height transform system has important meanings for the application of GPS .This arises by the disagreement between GPS height system and level height system ,so the results of GPS surveying must process the transform of height system，which Can satisfy the need of engineering．The paper mostly studies height transform and particularly discusses mathematic fitting model， At last in this paper, fitting of a regional GPS elevation and comparative analysis of the accuracy of the standard height is proposed the use of GPS observations plus a known elevation point of seeking to be determined point of elevation fitting way to describe grade GPS network geometry fitting height can reach the standard of ordinary measurement accuracy.

Keywords：GPS height；height anomaly；height transform；elevation fitting；precision analysis

為了滿足工程建設和地形測量的需要，測區都要進行平面和高程控制測量。近幾年以來，GPS衛星定位技術已在許多領域得到了廣泛的應用，目前使用GPS進行測量，能容易得到WGS-84系統下的三維坐標，將觀測數據通過處理可以很容易得到觀測點的國家或地方三維坐標，平面精度一般都能達到精度要求，但是得到的高程成果不一定滿足工程建設要求，特別在山區誤差更大。GPS得到的是大地高程（橢球高），實際應用中所采用的高程為海拔高程(正常高)，兩者之間存在高程異常值的差異，本文結合工作實際，從高程異常擬合函數、轉換常數解算及實踐應用等幾個方面，來論述GPS測量中的擬合高程值代替等外水準的可行性以及應用。

1  GPS擬合高程的方法及精度

GPS測量得到的大地高程H與實際應用中所采用的海拔高程h之間存在高程異常值的差異，即h = H - ξ，ξ為該點的高程異常值(又稱大地水準面差距)，如圖1.1所示。高程異常隨地形變化和地區地層的密度等有很大關系，要想獲得精確的正常高就要獲取準確的高程異常值。目前，高程異常值的獲取方法大致有以下幾種：

①、從國家高程異常值圖上查取或者從全球高程異常模型中得到，如國際上廣泛采用的

OSU91A、EGM96模型等；這兩種方式的精度不高，一般在分米到米級之間，不能滿足實際生產的需要。

圖1.1.大地高與正常高

Fig 1.1. Geodetic height and normal height

②、從局部地區的精化大地水準面模型中得到；這種方法精度較高，在幾個厘米至十幾個厘米

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