David Jonas
LiDAR Specialist, AAMHatch,Australia
[email protected]
Light Detection and Ranging (LiDAR) is a broad-acre mapping tool which has been widely adopted throughout North America and Europe, but what contribution can it make in Asia ?
The Asian region offers a completely different landscape in terms of weather, vegetation, funding, aviation support, security concerns and engineering practices. Asia’s economic growth will require significant amounts of spatial data acquisition to support the infrastructure development and land-use management required, but can the LiDAR business and technical models employed elsewhere simply be transported to the Asian region ?
AERIAL SURVEY LANDSCAPE
LiDAR is a proven survey technique. It does not do everything, but it is a valuable tool to have in the surveyor’s tool box. However, any business and any project must operate in the local landscape available to it. As a rule, LiDAR providers in the US and Europe have access to a substantial General Aviation industry, detailed survey infrastructure, and a mature LiDAR consumer. Much of this infrastructure does not exist in Asia, and must be created by the LiDAR provider, often on a project- by-project basis.
LiDAR’s viability will be reviewed through six factors: weather, vegetation, funding, aviation support, security concerns and engineering practices.
1. WEATHER
Asian weather is characterised by low cloud base, significant wet seasons, and atmospheric haze.
The low and persistent cloud base, particularly in tropical regions, is the single most dominant feature when planning, pricing and conducting a LiDAR survey. The cloud base precludes large parts of the region from benefiting from some of the recent sensor improvements. Modern LiDAR sensors can now fly at over 3000m and achieve vertical accuracies and point spacings obtained from 800m when the technology was first commercialised. These increased flying heights offer 3X efficiency savings in data collection. However it is simply unwise to plan and cost a survey based on high altitudes if the chances of getting sky-clear conditions are remote.
Hence a Low Cloud Base prevents tropical projects getting the benefits from the improvements in LiDAR quality from higher altitude.
Of course, cloud base is not a fixed parameter when designing a LiDAR survey in Asia. The various monsoons or wet seasons will change the prevailing weather conditions during the year. Hence the timing of the survey can be a strong determinant in project costing in Asian regions. A survey requiring eight hours data capture may take the survey aircraft 4 days in the flying-season, but 4 weeks in the off-season.
There will possibly be areas within the project which cannot be captured by LiDAR unless significant standby times (and fees) are allocated. Experience shows that there is often a mountain range which is “always” covered in cloud, and the survey aircraft has to work around this area to capture the balance of the project. LiDAR surveys in tropical Asia need to recognise this and decide whether a reliable terrain model over the majority of the area provides value to the overall project. The risk of standby charges can be reduced if the survey is planned for the optimal time of the year.
The third atmospheric factor prevalent in Asia is haze. The good news here is that, generally speaking, the top-end LiDAR sensors are strong enough to penetrate through most haze conditions. The only notable exception experienced by the author is when the survey area is in the vicinity of billowing smoke, which the laser is not able to penetrate. What is more a concern with heavy haze is that the airport may be closed, precluding the survey aircraft from taking off. Asia’s long periods of persistent cloud mean that it will generally take longer to define a certain sized area when compared to other parts of the world, but LiDAR still remains the most cost-efficient survey technique capable of achieving 0.15m accuracies and definition of the terrain under vegetation.
2. VEGETATION
The comment is often made: “LiDAR is okay in Europe, but we have dense Asian jungles so it won’t work here” … the comment is certainly valid but the conclusion is not. Parts of Asia are covered with dense tropical canopies, thick undergrowth, or both that do detract from quality of a LiDAR survey.
The LiDAR’s perspective is essentially vertical, so a true assessment of the likely LiDAR vegetation penetration is to walk into the jungle and look directly up. One will see that there is significantly more sky visible looking straight up, than at even 15° off-zenith. Not surprisingly, not many people look straight up when walking in forests … only aerial surveyors, perhaps !
The most dense vegetation the author has been involved with was at Lihir Island, PNG (ref: www.aamhatch. com.au/resources/pdf/publications/ne ws/ScanHoriz082004.pdf). In this primary tropical forest, a walk through the site was quite dark. Looking straight up revealed very few gaps in the overhead canopy. Analysing the LiDAR data afterwards showed that only 7% of the emitted LiDAR measurements reached the ground. Given the aircraft speed, this was not going to give the required terrain definition, so the site was flown three times.
Dense vegetation does affect the quality of the terrain model acquired by LiDAR. The fact remains though that, in densely vegetated areas, LiDAR remains the most efficient survey technology available. The fact that Asia has more dense vegetation than elsewhere actually strengths the case for its use in this region.
3. FUNDING AND MAXIMISING PROJECT OUTCOMES
LiDAR has had to change the funding models for survey data in Asia on two fronts. Firstly, Asian projects are used to paying lump sum fees for their survey work. Whilst that can be accommodated, it is not a reflection of true costs and so can lead to inequities. The time (and so cost) of an aerial survey in Asia can be so dependant on weather, that Rates Based or Lump Sum plus variation can provide more equitable funding. This represents a deviation from accepted practices, and many Asian Project Managers are uncomfortable with this approach for the survey component, even though it is norm for many other aspects of same engineering project. Secondly and simply, LiDAR generally costs more than many applications used for survey data for the project. Before LiDAR,
- infrastructure corridors were planned on sketchy existing topo maps,
- flood studies conducted on occasional river cross-sections,
- infrastructure planned on sparse field work,
- forestry managers used topo maps for slope analysis,
- electricity companies sited transmission towers with visual helicopter inspection,
- telco cell networks designed on approximate building footprints,
- landslips managed with discrete monitoring stations, and
- earthworks designed on a few field spot heights.
In each case, the survey represented a tiny percentage of the overall project budget. Now along comes LiDAR to provide significantly better information on which the engineers can base their work. By accurately knowing the true shape of the ground under the trees, and the detailed height of the buildings and trees:
- infrastructure corridor routes can be optimised at design stage;
- flood studies can better find the river breaks and overland flow characteristics;
- infrastructure designed knowing there will be no “on site” surprises;
- forestry inventory showing exact slope maps and tree heights of their estates;
- electricity distribution towers are sited knowing ground shape and tree clearances;
- telco cell networks design with the exact size and shape of every building;
- landslips mitigation plans can use a complete drainage plan and slope map; and
- earthworks volumes significantly improved.
In each case, better quality engineering or management decisions are made as they are based on better initial information. This improved spatial data can cost more, but any investment in LiDAR is easily recouped by the project with just one less suburb flooded, a 1% reduction in earthworks required, one less tower placed, or 5% reduction in the risk profile knowing that designs are based on reliable site data.
Increased accountability for project outcomes demands a more proactive understanding and long-term mitigation of terrain-controlled influences, whether for road design, flood and landslide hazard mitigation, or coastal risk profile. Project Managers in Asia are coming to terms with the fact that better initial survey data may cost more up front, but improved decisions downstream will recoup this investment for the project many times over.
