N.J.P.L.S. An Introduction to LiDAR Concepts and Applications
Presentation Outline LIDAR Data Capture Advantages of Lidar Technology Basics Intensity and Multiple Returns Lidar Accuracy
Airborne Laser Scanning ALS/LiDAR is an active remote sensing technology that measures distance with reflected laser light. LiDAR: Light Detection and Ranging or Laser Imaging Detection and Ranging) 1 st developed in 1960 by Hughes Aircraft inc. Modern computers and DGPS make it practical. Typically used in very accurate mapping of topography. New technologies and applications are currently being developed.
LiDAR Overview What LiDAR is... What LiDAR is not...... Light Detection And Ranging... highly accurate topographic data... Active Sensing System - Uses its own energy source, not reflected natural or naturally emitted radiation... Ranging of the reflecting object based on time difference between emission and reflection. (Multiple Returns per pulse of light)... Direct acquisition of terrain information... photography... A substitute for photography... All weather... Able to see through trees - if sunlight can reach a spot on the ground so can the laser; if sunlight can t reach the ground due to various vegetation, than neither can the laser Examples of LiDAR uses...... generate digital elevation models and contours for use in: engineering design and design reviews (irrigation design, pivots, pipelines, surge valves and livestock pipelines) conservation planning (HEL Determinations, practice design, landscape visualization) floodplain mapping hydrologic modeling Soil Survey Dam safety assessments (slope, aspect) Surface Feature Extraction (trees, shrubs, roads, buildings) Vegetation Mapping (Height, Density) Landform Extraction (ridges, benches, valleys)
ELEMENTS OF GIS: DATA TYPES Vector Data (Points, Lines, Polygons) Esri shapefile AutoCAD.dwg files Microstation.dgn files TIN (Triangular Irregular Network) Geodatabase Raster Data TIFF JPEG MrSID DEM (Digital Elevation Model) Lidar Light Detection And Ranging
LiDAR: How it works Each time the laser is pulsed: Laser generates an optical pulse Pulse is reflected off an object and returns to the system receiver High-speed counter measures the time of flight from the start pulse to the return pulse Time measurement is converted to a distance (the distance to the target and the position of the airplane is then used to determine the elevation and location) Multiple returns can be measured for each pulse Up to 200,000+ pulses/second Everything that can be seen from the aircraft is measured
Advantages of LiDAR Technology Provides a highly accurate means of elevation model collection for 1 or 2 contours Acquisition can take place day or night shadows that are problematic in mountainous areas are not an issue with LiDAR Unlike photography, acquisition can take place below cloud cover cloud shadows no issue Very cost effective for larger projects Does not provide break lines, nor is it imagery
Why is This Technology Exciting Time to Collect 1 Million Points Conventional Surveying: 15.5 years Photogrammetry: 1.5 years Lidar: 6.7 seconds @ 150 khz Costs can be significantly less for the right projects
Aircraft Requirements Flying heights from 3,000 to 6,000 feet Speeds ranging from 90 to 130 knots Ability to carry equipment, personnel, and full fuel load
What it is not Photography We can shade the elevation and intensity data to create imagery Doesn t capture breaklines Doesn t capture planimetric features Advances in software may allow automatic feature extraction soon
Shading by Elevation
Urban LiDAR Layout
Why Remington & Vernick use Engineers a Laser? and Affiliates LiDAR takes advantage of two of the unique properties of laser light: 1. The laser is monochromatic. It is one specific wavelength of light. The wavelength of light is determined by the lasing material used. Advantage: We know how specific wavelengths interact with the atmosphere and with materials. 2. The light is very directional. A laser has a very narrow beam which remains concentrated over long distances. A flashlight (or Radar) on the other hand, releases energy in many directions, and the energy is weakened by diffusion. Advantage: The beam maintains its strength over long distances. 3mrad divergence = 3 m at 1 km and 15m at 5 km.
Issues with LiDAR Data LIDAR is indiscriminate it places elevation points on everything. This includes cars, houses, trees, etc. LIDAR only places mass points, or random xyz points. It does NOT pick up breaklines, or lines of abrupt change in the ground elevation LIDAR is NOT imagery. LIDAR data can be shaded, however, to offer a relief image
Bare Earth Model Significant editing must be employed to create a Bare Earth Model which models the natural ground Some automated procedures may be used. Imagery backdrop may be necessary In some cases, traditional photogrammetry may be necessary to add breaklines
Discrete Return Data: Millions of X,Y,Z points Area is approximately: 1 X 0.75mi. includes ~ 440,000 returns
Applications for Discrete-Return LIDAR Digital Elevation Model (DEM) 4m DEM of PREF Subset
From Point Clouds to 3-D Surface Models Points are used to create 3D surface models for applications. Triangular Irregular Networks (TIN)s are used to classify the points and to develop Digital Elevation Models (DEM)s. Points must be classified before use: bare earth points hit the ground ; other point categories include tree canopy and buildings. Correct identification of bare earth is critical for any lidar mapping application.
LIDAR Mapping of Forest Structure
Intensity and Multiple Returns Most units today have the ability to measure multiple returns and the intensity of the returned signal for each This enables specialized applications using the LIDAR data
Aerial Photo
Intensity Plot
LIDAR Surfacing
Lidar Returns and Cloud Cover
LiDAR Applications LIDAR has significant fixed cost but can be very cost effective for large projects Appropriate for a wide range of projects including forestry, corridor studies, obstruction mapping, flood studies, city/county mapping, and transportation projects Required accuracy must be carefully evaluated
Value of Noise in the Data Comair Jet Interstate
River 470 Tall
LiDAR Accuracy Quality of the hardware and software Knowledge of the planners, operators, office staff Flying height Scan angle (also important for vegetation penetration) GPS configuration (PDOP and Number of SVs) Distance from base station to aerial platform Laser power Laser rep rate Laser rep rate
LiDAR Accuracy Accuracy of elevation in range of 6 to 30 centimeters (0.20 to 0.98 feet) Accuracy of XY position in range of 10 to 46 centimeters (0.33 to 1.51 feet) Accuracy depends on pulse rate, flying height, GPS configuration, location of ground stations, and position of the scanner with respect to nadir r rep rate
GIS Resources in New Jersey Using available GIS resource data can conserve costs The New Jersey Department of Environmental Protection offers aerial photography, LiDAR and GIS datasets The data is available at no cost GIS data can also be obtained at the County, Agency level Meadowlands DVRPC USGS A data sharing agreement is generally needed
Raster Remington & Vernick Processing Engineers and Affiliates Contour Generation at set Intervals
USGS DEM Lidar DEM
Advanced Remington & Vernick Engineers Analytics and Affiliates Watershed Analysis - Drainage Basin Calculation for Discharge Points
Where Do We Go From Here? Improve Accuracy Software Processing Automated Feature Classification - Building Footprints - Roof Types - Pervious/Impervious -Vegetation Data Fusion
WHAT IS METADATA? Metadata describes Content, quality, condition and other characteristics about data Who, what, where, why and how of a data set Metadata includes information you need to Determine which data exist for a geographic location Determine if a set meets your need Acquire the set you identified Process and use the set Metadata answers questions Who created and maintains the data? Why were the data created? What is the content and structure of the data? When collected? When published? Where s the geographic location? How were the data produced?
LiDAR Summary Simply another tool in our toolbox It is not right for every project, but it can provide substantial cost savings for the right project Large-scale, high-accuracy projects still require conventional mapping solution LIDAR and the software we use in processing will continue to improve with time
QUESTIONS & ANSWERS Thank you!