Height Modernization Issue

The height above sea level (HASL),a general term of orthometric height,normal height and dynamic height,is extensively used in engineering project.The conventional method for determining the HASL is the geometric leveling,its primary drawbacks are labor-intensive,non-cost-effective and non-real-time in nature,whereas GPS HASL survey can overcome these drawbacks to a great extent.The GPS HASL survey is in essence the height transferring from the GPS surveyed ellipsoidal height to the height above sea level in terms of geoidal height.It has been proven feasible.This paper is devoted to the GPS HASL survey. The concept of height modernization was put forward by the U.S.National Geodetic Survey,in recognition of the situation that on one hand the line-of-sight leveling does not meet the needs of today's mobile and technology-driven society in regard to efficiency and accuracy,and on the other hand,the GPS has been capable of providing in real time accurate height needed for a wide range of applications.Its main idea is to use the GPS rather than the geodetic leveling to measure the height above sea level and to strengthen the aspect of the national spatial reference system.The height modernization involves the research and development activities seeking to improve the ellipsoidal height determination and to refine the geoid and the activities leading to improved height determination both for the static survey and for the moving navigation.The technical feasibility of GPS HASL survey depends on the accurcay of ellipsoidal height and the availability of high accuracy and high revolution geoid.The prerequisite for the ellipsoidal height determination is the existence of a GPS network of points of sufficient density,which should contain continuously operational reference stations and A and B order points.While for constructing the geoid in a large area with the centimeter level it is necessary to have banks of dense gravity points and a great quantity of GPS/leveling points well distributed throughout the territory.From the GPS HASL survey point of view,our national spatial geodetic network needs to be densified,much attention needs to be paid to the height determination in GPS positioning,and more importantly,great efforts have to be made to refine our geoid nationwide.It is proposed that a 5 cm geoid covering the most of territory be constructed step-by-step by an effort worth ten years or even longer.To this end the following tasks should be accomplished:① to observe additionally more than 120000 gravity points,thereby ensuring to have a specified number of gravity points in each 5'×5' grid;② to set up about 7000 GPS/leveling points,averaging one point er 35km×35km;and ③ to carry out repeated leveling and leveling ties necessary to develop GPS/leveling.