Elevation map may cost up to $125m

Data from such mapping can be used in a computer model to predict floods, experts say
Feng Zengkun Straits Times 14 Jan 12;

A MAP that can show the height of land across Singapore and help fight the flooding problem could cost up to $125 million.

Users of the map, accurate to within 10cm, would have a better idea of where rainwater would flow during storms. The data, plus information on land use and rainfall patterns, would be a good fit for a flood-prediction computer model which does not now exist.

Private companies said the mapping could be done by planes or ground teams but added that the Government had to take on the work, given its scale.

Three private land surveyors put the cost of the undertaking at between $100 million and $125 million, based on smaller-scale work done for the construction industry. But this figure may be much lower if more advance tools are used.

On Tuesday, a 12-man expert panel set up to solve Singapore's flooding woes said a computer model would be essential for the country to plan long-term solutions.

The panel, appointed by the Government in June last year after a spate of floods in the past two years, gave its findings after a six-month study.

It advised the authorities to invest in a digital map of land heights, known as a digital elevation map, and said it would have to be accurate to within 10cm to be useful.

National water agency PUB's flood-prediction models - in place since the 1990s and upgraded over time - now forecast water movement only within drains and canals, and what degree of rainfall intensity they can handle. It said it is studying the panel's recommendations and will respond at a later date.

Professor Chan Eng Soon, head of the panel and dean of the engineering faculty at the National University of Singapore, said the current models are not enough. 'You must understand the entire catchment to know how water is going to flow,' he said on Tuesday.

The panel noted that higher resolution data needed for the improved computer model can be obtained using available technology.

The National Environment Agency has rainfall data going back to the 1860s. The Urban Redevelopment Authority records land-use data while the Singapore Armed Forces Mapping Unit produces our national topography maps.

The unit declined to comment on its maps' accuracy, how it gets data and how often the maps are revised but the private firms said it is likely that aerial methods - which are relatively cheap and fast - are used to do the work.

They added that the methods may include estimating land heights through aerial photos or a technique known as light detection and ranging (Lidar).

However, Dr Liew Soo Chin, head of research at the Centre for Remote Imaging, Sensing and Processing, said these aerial methods may not meet the panel's requirements.

Aerial Lidar involves fitting an aircraft with a global positioning system (GPS) and laser scanners. The scanners emit laser pulses which hit the ground and reflect back to the scanners. The distance to the ground is calculated based on the pulse's travelling time, and the data is combined with the GPS to produce a digital map of land heights.

No local companies use this method. Foreign firms have claimed error margins of 15cm or less using aerial Lidar but they said this technique suffers in urban and densely forested areas, where tall buildings and trees may distort the pulse reflections.

Mr Tan Eng Fei, owner of land surveying firm Wang and EF Tan Associates, said ground surveys - which involve moving tools across the land - can be much more accurate at just a 5mm deviation, but are also more expensive and time-consuming.

Based on foreign companies' prices, an aerial Lidar survey of Singapore would cost between $150,000 and $300,000, compared with ground surveys which local companies said could cost more than $100 million. A ground-based Lidar approach, which uses Lidar-equipped vehicles, may fall between the two extremes.

A Lidar plane can cover all of Singapore within two days, whereas a typical ground-survey team of three people would need three days to cover 10,000sqm, or a sliver of the country's total land area of 712 sq km.

The private firms said a national ground survey would have to be undertaken by the Government since the project would exceed their capabilities, and they are unlikely to be able to enter areas where access is restricted because of security concerns.

But surveyor Tan Choo How said a ground survey may not be entirely feasible. 'Some parts of Singapore are very thickly forested and inaccessible to ground teams. It may be better to fly Lidar planes very low instead to improve their accuracy. This may not be below 10cm but exceeding that by a little should not make a big difference to computer models,' he said.

Other countries to have embarked on such projects include Australia, where the state of Queensland was devastated by floods last year. The state's authorities will complete elevation maps of some coastal areas - accurate to 10cm - by the end of the year using a mix of approaches.