We are accustomed to seeing the surveyor with his tools by the side of the road performing his measurements. Many professions regularly rely on aerial photographs, satellite imagery or topographic maps to provide the spatial information required for their projects. However, these sources of spatial information are either too expensive or do not provide sufficiently accurate, detailed digital elevation data which is crucial to many applications.

The challenge to provide a reliable source of accurate, 3-D digital elevation data at a fraction of traditional mapping costs has been met by LIDAR (LIght Detection And Ranging). LIDAR is able to obtain Digital Terrain Models(DTM) with accuracies of better than 6 inches in a much shorter time frame, and considerably less cost, than a similar product using any other technology.

How it works

A laser produces a light pulse projected to a mirror and is reflected out the bottom of a plane or helicopter. The light pulse travels down until it reaches an object, the pulse is then reflected back to the system. Since the speed of light is a constant, a standard mathematical equation can be used to determine the distance the light traveled in the recorded time interval. This measurement tells you the height, or Z data point, of an object. In addition to knowing the height of the object, global positioning systems are used to verify longitude and latitude providing the X and Y data points of a three-dimensional data set. Finally, an inertial measurement unit on board the plane or helicopter provides the digital information regarding pitch, yaw and roll.

The laser can obtain as many as 3,000,000 3-D points per minute with ground point density as close as one-to-two feet and produce one foot contours as a final mapping data output. Digital terrain models produced using this technology have been used for detailed planning and engineering projects in myriad applications, saving tens of thousands of dollars compared to traditional techniques.

Applications

Petroleum, Pipeline and Utility Surveys:

The construction of pipelines, the mapping of broad area exploration sites, route locations and geophysical exploration surveys require a precise knowledge of the topography. Accurate terrain models from airborne LIDAR can quickly assess the feasibility of construction in certain areas, the condition of towers, vegetation growth and intrusions into the right of way corridor can be checked.

LIDAR data has become an integral component in the planning and design process when conducting bare-earth assessments, risk-management studies and right-of-way surveys. The data is able to show details such as tree height and distance from the canopy to the ground, accurately map linear corridors, modeling of power line sag, ground clearance, encroachment and determination for tower locations as well as three-dimensional building outlines and footprints.

Transportation Surveys

Transportation engineers need to examine existing roads and highways, interchanges and access corridors, water and railroads in the development stage of highway planning and construction. LIDAR data is able to provide the most economical and accurate x, y and z coordinates available for highway and transportation projects.

Airport designers and engineers need data that provides accurate and detailed terrain information in the developing and planning stages of airport construction and modification. Engineers are now employing LIDAR data to conduct studies involving noise abatement, Line of Site (LOS), obstruction mapping and landing patterns for optimal performance and minimal impact.

Flood and Shoreline Erosion Surveys

LIDAR data can accurately reflect beach topography much more efficiently than traditional methods such as photogrammetry, which can be difficult to use in areas of limited contrast. The technology also is used to map and study shore belts, coastal areas, dunes and dikes. Coastal managers use LIDAR data to study the effects of severe storms and hurricanes and in turn enable them to better prepare for those types of situations.

FEMA flood maps, watershed management, hydrological modeling and flood and storm-surge inundation forecast are accomplished with the help of LIDAR data used by the U.S. Corps of Engineers. The information is used to create effective emergency response plans and show low elevation areas that would be hit the hardest, streets that would be cut off and what alternate routes would be available during flood events.

Forestry, Environment and Mining Surveys

Forestry and resource managers require precise area-wide measurements that include tree stand density, tree height, canopy cover, road or corridor access, inventory management, commercial harvest estimates of standing timber volumes and ground elevations. LIDAR is an accurate and economical means to gather these forest measurements which can also be applied to establish roughness coefficients for determining the resistance of water flow during flood events.

On the environmental front, LIDAR data is being reviewed to maintain inventory on national priority list sites, aid in watershed management and ecosystem analysis and the creation of airshed dispersion models. Surface mining use of LIDAR data is becoming vital for site planning and volumetric runoff management analysis, mine modeling, permitting, reclamation, overburden calculations, and volumetric calculations.

Conclusion

LIDAR is an ideal means of gathering elevation data because the high density of accurate data surveyed, better than 6 inches, is gathered universally over the area flown. The laser collects as many as 3,000,000 3-D points per minute with ground point density as close as one-to-two feet and produce one foot contours as a final mapping output. By comparison, field surveying usually results in the capture of transects at specific intervals across the study area. Also, photogrammetry tends to generalize the contour intervals and not provide data that lies in between.

The reality is that Lidar can catch numerous small undulations and obscure elements often missed by other surveying methods and is generally considered to be much faster and less expensive for comparable mapping products.