Lidar 101: An Introduction to Lidar Technology, Data, and Applications National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center Coastal Geospatial Services Division Coastal Remote Sensing Program November 2012 For More Information: Keil Schmid, NOAA Coastal Services Center [email protected], (843) 202-2620 Authors and Contributors: Jamie Carter, Keil Schmid, Kirk Waters, Lindy Betzhold, Brian Hadley, Rebecca Mataosky, and Jennifer Halleran, NOAA Coastal Services Center Suggested Citation: National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center. 2012. “Lidar 101: An Introduction to Lidar Technology, Data, and Applications.” Revised. Charleston, SC: NOAA Coastal Services Center. NOAA Coastal Services Center 2234 S. Hobson Ave. Charleston, SC 29405 (843) 740-1200 www.csc.noaa.gov Regional Offices: NOAA Pacific Services Center, NOAA Gulf Coast Services Center, and Offices in the Great Lakes, Mid-Atlantic, Northeast, and West Coast Table of Contents 1. Introduction ............................................................................................................... 1 2. What Is Lidar? ............................................................................................................ 2 Overview ............................................................................................................................. 2 What Is Lidar? ..................................................................................................................... 3 Lidar Platforms .................................................................................................................... 4 Basic Terminology ............................................................................................................... 5 Basic Principles and Techniques ......................................................................................... 7 Applications – A Quick Overview ........................................................................................ 9 History ............................................................................................................................... 11 Summary ........................................................................................................................... 11 3. Data Produced by Lidar Sensors ................................................................................ 13 Overview ........................................................................................................................... 13 Improvements over Previous Data ................................................................................... 13 Vertical Accuracy ......................................................................................................... 13 Horizontal Resolution ................................................................................................. 14 Temporal Resolution ................................................................................................... 16 Accuracy ............................................................................................................................ 16 Accuracy Assessment Techniques .............................................................................. 17 Descriptive Terms ....................................................................................................... 17 Data Types ......................................................................................................................... 18 Points .......................................................................................................................... 18 Digital Elevation Models (DEMs) ................................................................................ 20 Contours ...................................................................................................................... 23 Summary ........................................................................................................................... 24 4. Use of Lidar Data ...................................................................................................... 25 Overview ........................................................................................................................... 25 Obtaining Lidar .................................................................................................................. 25 U.S. Interagency Elevation Inventory ......................................................................... 26 Digital Coast (NOAA Coastal Services Center) ............................................................ 27 Loading Data into a GIS ..................................................................................................... 27 Lidar Data and ArcGIS ................................................................................................. 28 Metadata ........................................................................................................................... 39 Summary ........................................................................................................................... 39 5. Data Customization and Specification ....................................................................... 41 Overview ........................................................................................................................... 41 Data Attributes .................................................................................................................. 41 Return Numbers .......................................................................................................... 43 Classification ............................................................................................................... 45 Breaklines .......................................................................................................................... 47 Accuracy Specification and Tests ...................................................................................... 48 Qualitative Review of Lidar Data ...................................................................................... 50 Data Attribute Specification in Digital Coast .................................................................... 53 Summary ........................................................................................................................... 55 6. Examples of Coastal Lidar Applications ..................................................................... 56 Overview ........................................................................................................................... 56 Shoreline Mapping ............................................................................................................ 56 Inundation Mapping ......................................................................................................... 61 Wetland Habitat Delineation ............................................................................................ 64 Summary ........................................................................................................................... 69 Table of Abbreviations ..................................................................................................... 70 Works Cited ..................................................................................................................... 71 1. Introduction Light detection and ranging (lidar) mapping is an accepted method of generating precise and directly georeferenced spatial information about the shape and surface characteristics of the Earth. Recent advancements in lidar mapping systems and their enabling technologies allow scientists and mapping professionals to examine natural and built environments across a wide range of scales with greater accuracy, precision, and flexibility than ever before. Several national reports issued over the past five years highlight the value and critical need of lidar data. The National Enhanced Elevation Assessment (NEEA) surveyed over 200 federal, state, local, tribal, and nongovernmental organizations to better understand how they use enhanced elevation data, such as lidar data. The over 400 resulting functional activities were grouped into 27 predefined business uses for summary and benefit-cost analysis (NDEP, 2012). Several of these activities will be described in more detail in the applications section of this document. There are many considerations and trade-offs that must be understood in order to make sound decisions about the procurement, processing, and application of lidar data. This document provides introductory and overview information, as well as in-depth technical information, to support decision-making in all phases of lidar projects. While the information presented here is not comprehensive, it covers aspects of the technology that are the most common subjects of discussion within the coastal management community. 1 2. What Is Lidar? Overview Lidar has become an established method for collecting very dense and accurate elevation data across landscapes, shallow-water areas, and project sites. This active remote sensing technique is similar to radar but uses laser light pulses instead of radio waves. Lidar is typically “flown” or collected from planes where it can rapidly collect points over large areas (Figure 2-1). Lidar is also collected from ground-based stationary and mobile platforms. These collection techniques are popular within the surveying and engineering communities because they are capable of producing extremely high accuracies and point densities, thus permitting the development of precise, realistic, three-dimensional representations of railroads, roadways, bridges, buildings, breakwaters, and other shoreline structures. Collection of elevation data using lidar has several advantages over most other techniques. Chief among them are higher resolutions, centimeter accuracies, and ground detection in forested terrain. This section will address 1) the basics of lidar, 2) the terminology, and 3) some examples of how the data are routinely used. Figure 2-1. Schematic diagram of airborne lidar performing line scanning resulting in parallel lines of measured points (other scan patterns exist, but this one is fairly common) 2 What Is Lidar? Lidar, which is commonly spelled LiDAR and also known as LADAR or laser altimetry, is an acronym for light detection and ranging. It refers to a remote sensing technology that emits intense, focused beams of light and measures the time it takes for the reflections to be detected by the sensor. This information is used to compute ranges, or distances, to objects. In this manner, lidar is analogous to radar (radio detecting and ranging), except that it is based on discrete pulses of laser light. The three-dimensional coordinates (e.g., x,y,z or latitude, longitude, and elevation) of the target objects are computed from 1) the time difference between the laser pulse being emitted and returned, 2) the angle at which the pulse was “fired,” and 3) the absolute location of the sensor on or above the surface of the Earth. There are two classes of remote sensing technologies that are differentiated by the source of energy used to detect a target: passive systems and active systems. Passive systems detect radiation that is generated by an external source of energy, such as the sun, while active systems generate and direct energy toward a target and subsequently detect the radiation. Lidar systems are active systems because they emit pulses of light (i.e. the laser beams) and detect the reflected light. This characteristic allows lidar data to be collected at night when the air is usually clearer and the sky contains less air traffic than in the daytime. In fact, most lidar data are collected at night. Unlike radar, lidar cannot penetrate clouds, rain, or dense haze and must be flown during fair weather. Lidar instruments can rapidly measure the Earth’s surface, at sampling rates greater than 150 kilohertz (i.e., 150,000 pulses per second). The resulting product is a densely spaced network of highly accurate georeferenced elevation points (Figure 2-2)—often called a point cloud—that can be used to generate three-dimensional representations of the Earth’s surface and its features. Many lidar systems operate in the near-infrared region of the electromagnetic spectrum, although some sensors also operate in the green band to penetrate water and detect bottom features. These bathymetric lidar systems can be used in areas with relatively clear water to measure seafloor elevations. Typically, lidar-derived elevations have absolute accuracies of about 6 to 12 inches (15 to 30 centimeters) for older data and 4 to 8 inches (10 to 20 centimeters) for more recent data; relative accuracies (e.g., heights of roofs, hills, banks, and dunes) are even better. The description of accuracy is an important aspect of lidar and will be covered in detail in the following sections. 3 Figure 2-2. Lidar point and surface products The ability to “see under trees” is a recurring goal when acquiring elevation data using remote sensing data collected from above the Earth’s surface (e.g., airplanes or satellites). Most of the larger scale elevation data sets have been generated using remote sensing technologies that cannot penetrate vegetation. Lidar is no exception; however, there are typically enough individual “points” that, even if only a small percentage of them reach the ground through the trees, there are usually enough to provide adequate coverage in forested areas. In effect, lidar is able to see through holes in the canopy or vegetation. Dense forests or areas with complete coverage (as in a rain forest), however, often have few “openings” and so have poor ground representation (i.e., all the points fall on trees and mid-canopy vegetation). A rule of thumb is that if you can look up and see the sky through the trees, then that location can be measured with lidar. For this reason, collecting lidar in “leaf off” conditions is advantageous for measuring ground features in heavily forested areas. Lidar Platforms Airborne topographic lidar systems are the most common lidar systems used for generating digital elevation models for large areas. The combination of an airborne platform and a scanning lidar sensor is an effective and efficient technique for collecting elevation data across tens to thousands of square miles. For smaller areas, or where higher density is needed, lidar 4 sensors can also be deployed on helicopters and ground-based (or water-based) stationary and mobile platforms. Lidar was first developed as a fixed-position ground-based instrument for studies of atmospheric composition, structure, clouds, and aerosols and remains a powerful tool for climate observations around the world. NOAA and other research organizations operate these instruments to enhance our understanding of climate change. Lidar sensors are also mounted on fixed-position tripods and are used to scan specific targets such as bridges, buildings, and beaches. Tripod-based lidar systems produce point data with centimeter accuracy and are often used for localized terrain-mapping applications that require frequent surveys. Modern navigation and positioning systems enable the use of water-based and land-based mobile platforms to collect lidar data. These systems are commonly mounted on sport-utility and all-terrain vehicles and may have sensor-to-target ranges greater than a kilometer. Data collected from these platforms are highly accurate and are used extensively to map discrete areas, including railroads, roadways, airports, buildings, utility corridors, harbors, and shorelines. Figure 2-3. Mobile lidar collected from a vehicle (left) and a boat (right) (images courtesy of Sanborn and Fugro) Airplanes and helicopters are the most common and cost-effective platforms for acquiring lidar data over broad, continuous areas. Airborne lidar data are obtained by mounting a system inside an aircraft and flying over targeted areas. Most airborne platforms can cover about 50 square kilometers per hour and still produce data that meet or exceed the requirements of applications that demand high-accuracy data. Airborne platforms are also ideal for collecting bathymetric data in relatively clear, shallow water. Combined topographic and bathymetric lidar systems on airborne platforms are used to map shoreline and nearshore areas. Basic Terminology A discussion of lidar often includes technical terms that describe the level of accuracy (a very important aspect of lidar data), data collection, and the ensuing processing steps. • LAS – abbreviation for laser file format; the LAS file format is a public file format for the interchange of 3-dimensional point cloud data between data users. Although developed primarily for the exchange of lidar point cloud data, this format supports the exchange 5 of any 3-dimensional x,y,z tuplet. LAS is a binary file format that maintains information specific to the lidar nature of the data while not being overly complex. • RMSE – abbreviation for root mean square error; a measure of the accuracy of the data similar to the measure of standard deviation if there is no bias in the data. • Accuracy , Fundamental Vertical Accuracy (FVA) – a measure of the accuracy of the z data in open areas at a high level of confidence (95%); calculated from the RMSE using the formula RMSE x 1.96 = FVA. • Classification – data that have been processed to define the type of object that the pulses have reflected off; can be as simple as unclassified (i.e., object not defined) to buildings and high vegetation. The most common is to classify the data set for points that are considered “bare earth” and those that are not (unclassified). • Return Number (First/Last Returns) – many lidar systems are capable of capturing the first, second, third, and ultimately the “last” return from a single laser pulse. The return number can be used to help determine what the reflected pulse is from (e.g., ground, tree, understory). • Point Spacing – how close the laser points are to each other, analogous to the pixel size of an aerial image; also called “posting density” or “nominal point spacing.” The point spacing determines the resolution of derived gridded products. • Pulse Rate – the number of discrete laser “shots” per second that the lidar instrument is firing. Systems used in 2012 were capable of up to 300,000 pulses per second. More commonly, the data are captured at approximately 50,000 to 150,000 pulses per second. • Intensity Data – when the laser return is recorded, the strength of the return is also recorded. The values represent how well the object reflected the wavelength of light used by the laser system (e.g., 1,064 nanometer for most commercial topography sensors in the U.S.). These data resemble a black and white photo but cannot be interpreted in exactly the same manner. • RTK GPS (Real Time Kinematic GPS) – satellite navigation that uses the carrier phase (a waveform) that transmits (carries) the Global Positioning System (GPS) signal instead of the GPS signal itself. The actual GPS signal has a frequency of about 1 megahertz, whereas the carrier wave has a frequency of 1500 megahertz, so a difference in signal arrival time is more precise. The carrier phase is more difficult to use (i.e., the equipment is more costly); however, once it has been resolved, it produces a more accurate position in relation to the higher frequency. • DEM, or Digital Elevation Model – a surface created from elevation point data to represent the topography. Often a DEM is more easily used in a geographic information system (GIS) or computer-aided design (CAD) application than the raw point data it is constructed from. 6