airbornelasermapping.com

A reference source on an emerging technology

The "Skinny"

Airborne laser mapping is a cheap, fast, accurate way of creating 3D digital pictures of the earth's surface or objects on it. Currently it is mainly used for mapping or survey applications.

An Overview ..... The Technology ..... The Applications ..... The Future .....

OVERVIEW - AN EMERGING SURVEY TECHNOLOGY

Airborne laser mapping is an emerging technology in the field of remote sensing that is capable of rapidly generating high-density, geo-referenced digital elevation data with an accuracy equivalent to traditional land surveys but significantly faster than traditional airborne surveys.

Airborne laser mapping offers lower field operation costs and post-processing costs compared to traditional survey methods. Point for point, the cost to produce the data is significantly less than other forms of traditional topographic data collection making it an attractive technology for a variety of survey applications and data end-users requiring low cost, high-density, high accuracy geo-referenced digital elevation data.

Airborne laser mapping use a combination of three mature technologies; rugged compact laser rangefinders (LIDAR), highly accurate inertial reference systems (INS) and the global positioning satellite system (GPS). By integrating these subsystems in to a single instrument mounted in a small aeroplane or helicopter, it is possible to rapidly produce accurate digital topographic maps of the terrain beneath the flight path of the aircraft.

The absolute accuracy of the elevation data is 15 cm; relative accuracy can be less than 5 cm. Absolute accuracy of the XY data is dependent on operating parameters such as flight altitude but is usually 10's of cm to 1 m.

The elevation data is generated at 1000s of points per second, resulting in elevation point densities far greater than traditional ground survey methods. One hour of data collection can result in over 10,000,000 individually geo-referenced elevation points. With these high sampling rates, it is possible to rapidly complete a large topographic survey and still generate DTMs with a grid spacing of 1 m or less.

The technology allows for extremely rapid rates of topographic data collection. With current commercial systems it is possible to survey one thousand square kilometres in less than 12 hours and have the geo-referenced DTM data available within 24 hours of the flight. A 500 kilometre linear corridor, such as a section of coastline or a transmission line corridor, can be surveyed in the course of a morning, with results available the next day.

Airborne laser mapping instruments are active sensor systems, as opposed to passive imagery such as cameras. Consequently, they offer advantages and unique capabilities when compared to traditional photogrammetry. For example, airborne laser mapping systems can penetrate forest canopy to map the floor beneath the treetops, accurately map the sag of electrical power lines between transmission towers or provide accurate elevation data in areas of low relief and contrast such as beaches.

Airborne laser mapping is a non-intrusive method of obtaining detailed and accurate elevation information. It can be used in situations where ground access is limited, prohibited or risky to field crews.

Commercial airborne laser mapping systems are now available from several instrument manufacturers while various survey companies have designed and built custom systems. Similar to aerial cameras, the instruments can be installed in small single or twin-engine planes or helicopters. Since the instruments are less sensitive to environmental conditions such as weather, sun angle or leaf on/off conditions, the envelope for survey operations is increased. In addition, airborne laser mapping can be conducted at night with no degradation in performance.

A number of service providers are operating these instruments around the world, either for dedicated survey needs or for hire on a project basis. Some organisations are starting to survey areas on speculation and then offering the laser-generated data sets for resale similar to the satellite data market.

THE TECHNOLOGY

While the core technologies for airborne laser mapping have been in development for the past 25 years, the commercial market for these instruments has only developed significantly within the last five years. This commercial development has been driven by the availability of rugged, low-cost solutions for each of the core subsystems and the growing demand for cheap, accurate, timely, digital elevation data.

In operation, a pulsed laser rangefinder mounted in the aircraft accurately measures the distance to the ground by recording the time it takes a laser pulse to reflect back to the aircraft from the ground or from objects such as buildings, trees or power lines. Since the speed of light is known, the elapsed time is converted to an accurate distance or slant range. Some instruments record multiple returns from a single laser pulse to capture a vertical profile along the slant range. A scanning or rotating mirror is used to provide coverage across the path of the aircraft with swath widths dependent on scan angle and operating altitude. Simultaneously the INS subsystem records the roll, pitch and heading of the aircraft to determine its orientation in space, while the GPS subsystem provides the precise location of the aircraft through a differential kinematic solution. During post-processing the INS orientation and GPS position solutions are combined with the laser slant ranges to calculate accurate XYZ co-ordinates for each laser return.

The technology does not provide a real-time solution; it requires additional post-processing after the field operations and data collection are completed to generate the final XYZ data points. Post-processing is based on proprietary software developed by each instrument manufacturer but has significantly faster turn-around times than conventional survey techniques, on the order of 10's of hours compared to 10's days for traditional methods.

In addition to directly generating digital XYZ data points, post-processing software modules for the automatic analysis and classification of various features are being developed. Software modules already exists for such activities as vegetation classification and removal while other modules are being developed for automatic feature extraction, building recognition or automatic power wire detection and modelling.

APPLICATIONS

Depending on the application, airborne laser mapping technology is either a complementary or a competitive technology when compared to existing survey methods. For many survey applications airborne laser technology is currently deployed in conjunction with other more traditional sensors including standard aerial cameras, digital cameras, multispectral scanners or thermal imagers. However, in certain applications, such as forestry or coastal engineering, it offers capabilities not achievable with any other technology.

The most active application areas are:

1. DTM Generation for a Variety of GIS/Mapping Related Products.

Airborne laser mapping is a rapid, cost-effective source of high-accuracy, high-density elevation data for many traditional topographic mapping applications. The technology allows large area topographic surveys to be completed significantly faster and at a reduced cost compared to traditional survey methods.

2. Forestry.

The use of airborne laser mapping in the forestry industry was one of the first commercial areas investigated. Accurate information on the terrain and topography beneath the tree canopy is extremely important to both the forestry industry and natural resource managers. Accurate information on tree heights and densities is also critical information that is difficult to obtain using conventional techniques. Airborne laser technology, unlike radar or satellite imaging, can simultaneously map the ground beneath the tree canopy as well as the tree heights. Post-processing of the data allows the individual laser returns to be analysed and classified as vegetation or ground returns allowing DTMs of the bare ground to be generated or accurate representative tree heights to be calculated. Consequently, airborne laser mapping is an extremely effective technique for forestry companies when compared to photogrammetry or extensive ground surveys.

3. Coastal Engineering.

This is another area where airborne laser technology offers state-of-the-art type performance with significant advantages over other survey techniques. Since traditional photogrammetry is difficult to use in areas of limited contrast, such as beaches and coastal zones, an active sensing technique such as airborne laser mapping offers the ability to complete surveys that would be too costly to contemplate using other methods. In addition, highly dynamic environments such as coastal zones often require constant updating of baseline survey data. Airborne laser mapping offers a cost-effective method to do this on a routine basis. It is also used for mapping and monitoring of shore belts, dunes, dikes and coastal forests.

4. Corridor or Right-of-Way Mapping.

Airborne laser mapping allows rapid, cost-effective, accurate mapping of linear corridors such as power utility right-of-ways, gas pipelines, or highways. A major market is mapping power line corridors to allow for proper modelling of conductor catenary curves, sag, ground clearance, encroachment and accurate determination of tower locations. For example data acquired through airborne laser surveys can be combined with simultaneous measurements of air and conductor temperature and load currents to establish admissible increases in load-carrying capacity of power lines.

5. Construction.

Timely and accurate digital, geo-referenced elevation data is useful in a variety of construction and engineering activities. Examples include highway corridors, open-pit mines or daily surveying of large construction sites.

6. Flood Plain Mapping

Accurate and updated models of flood plains are critical both for disaster planning and recovery and flood insurance purposes. Airborne laser mapping offers a cost-effective method of acquiring the topographic data required as input for various flood plain modelling programs.

7. Urban Modelling.

Accurate digital models of urban environments are required for a variety of applications including telecommunications, wireless communications, law enforcement and disaster planning. An active remote sensing system such as a laser offers the ability to accurately map urban environments without shadowing.

8. Disaster Response and Damage Assessment.

Major natural disasters such as hurricanes or earthquakes stress an emergency response organisation's abilities to plan and respond. Airborne laser mapping allows timely, accurate survey data to be rapidly incorporated directly in to on going disaster management efforts and allows rapid post-disaster damage assessments. It is particularly useful in areas prone to major topographic changes during natural disasters; areas such as beaches, river estuaries or flood plains.

9. Wetlands and Other Restricted Access Areas

Many environmentally sensitive areas such as wetlands offer limited ground access and due to vegetation cover are difficult to asses with traditional photogrammetry. Airborne laser mapping offers the capability to survey these areas. The technology can also be deployed to survey toxic waste sites or industrial waste dumps.

Since airborne laser mapping is a relatively new technology, applications are still being identified and developed as end-users start to work with the data. There are on going efforts to identify areas where this technology allows value-added products to be generated or where it offers significant cost reductions over traditional survey methods.

FUTURE DEVELOPMENTS

In addition to the above commercial applications, numerous research efforts are under way to investigate other areas where airborne laser mapping may offer significant advantages. Additional data processing capabilities such as automatic feature extraction are being developed. Improvements to the sensor designs, added capabilities such as simultaneous intensity capture or integrated digital cameras and increased reliability/decreased operating costs are all under consideration.