Chek Lap Kok Airport 1

When the rock was blasted, most of these provisional reference points disappeared, and work on a stable permanent network could not begin until that task was completed. Benchmarks solidly anchored deep in the bedrock now provide the basis of the network, and as the island’s area has grown these benchmarks have been progressively extended by means of so-called datum piles drilled through the land-fill material down to bedrock.

The work of surveying and mapping the seabed began with a detailed geological survey and analysis of the ocean floor by Britain’s Geological Survey. The first stage consisted of studies of seismic reflections. These were followed by the setting up of a grid of 100m × 100m units in the sea. Analysis of the seismic profiles, of a total length of 335km, and of data gathered from 236 bore holes and 404 CPTs (cone-penetration tests) then provided a detailed picture of the seabed surrounding Chek Lap Kok. The main earthmoving works on and around Chek Lap Kok have now been completed.

This land-fill operation in the South China Sea was completed within the time and cost budgets, despite a period of unexpectedly heavy monsoon rains. The detailed sea-floor profiles available to Ian Ayson and his colleagues enabled them to monitor the work of reclamation continually from their three survey boats equipped with digital GPS, echosounder, heave compensator, computer, and navigation software they developed. To determine their position, the dredgers brought over to Hong Kong for this work from Belgium and the Netherlands, used DGPS. Jointly with the survey boats from the subcontractors, they provided a continual flow of data about the areas of seabed they covered, and on the logistically most efficient distribution of the volume of fill material. These data were also available for computing the amount of work done by subcontractors and the volume of material they had moved.

A special technique was used to survey the 13km long seawall. Close to shore, hand-held lines were used to determine depth and a floating taut-wire to reproduce the cross-section every 10m over the water. Echosounders and DGPS recorded dredging and seawall construction where such work was further than 75m from the shore. This combination made it possible to move speedily along the seawall and document it at one-metre intervals for each 10m cross-section. Above water, most of the work was done with Leica total stations. In many places the seawall trench drops steeply or forms a profile to a depth of 80m to protect the island from the impact of waves, erosion, and flooding.

Ian Ayson reports that "the combination of photogrammetry with orthophoto techniques, digital terrain models, and the land-information system proved itself as the best overall survey and documentation method, able to provide a quick, comprehensive overview of the entire site. Once every four weeks from the time that work on the project began, the Hong Kong Government Survey & Mapping Office, on behalf of the Airport Authority, has been flying its King Air equipped with a Wild RC10A aerial-camera system over Chek Lap Kok.

At a height of 1000 metres above ground, it takes four strips of 230mm × 230mm exposures, 36 in all, with 60% longitudinal and 30% lateral overlap. Ayson explains: "At our offices in the Central Plaza tower we process and rectify these photographs with a computerized photogrammetry system and digital scanner. We use standard 3D photogrammetry methods on a computer screen to produce a digital terrain model of the site. From these data we can prepare a contoured topographic plan and calculate volumes. The second step is to fit the photographs together to make up an orthophoto map.

In this process we transform the aerial photographs into a true-to-scale photo-map, with a positional accuracy of ±100mm and a height accuracy of ±200mm. That’s enough to give us an overall picture and permit an evaluation of progress. Of course, these tolerances are far too large for subsidence data. As far as possible, we combine aerial-triangulation points and control points with our fixed reference points for terrestrial surveys. We combine this true-to-scale photo-map with contours from our digital terrain model and other relevant survey, design, and map data from the various databases. Our own software and special routines enable us to produce special plans and enlargements of photo-maps with overlays of data for our engineers, designers, planners, and contractors."