Laser Scan Patterns Elliptical Pattern Rotating Optical Pattern Sinusoidal Pattern Saw Tooth Pattern Used by the AHAB Used by Riegl / TopoSys Used by Leica Used by OptechDragonEye and TopEye• Advantages and disadvantages with each scan pattern (ex. data uniformity, power consumption, duplicate points, accuracy along edge, field of view, etc.)• Some LiDAR data will look different, based on the sensor
Raw LiDAR LiDAR is collected in a proprietary format, based on the sensor’s manufacturer. This data is typically referred to as “raw” (unprocessed) LiDAR point cloud data. Sensormanufacturers have their own post-processing software that combines raw scan data with GPS (position) data and IMU (orientation) data to produce a georeferenced LiDAR file (LAS). Atthis point, the point cloud data is “dumb” – no data classifications have been assigned; typically organized by individual flight lines
Post-Processing Coverage Check Identifies data voids and verifies that LiDAR dataset covers the entire project extent Generate LAS files from hardware vendor’s post-processing software (i.e. merge GPS, IMU and LiDAR sensor inputs based on time) Validate & adjust relative accuracy of adjacent flight lines Adjust flight line data for roll bias and/or other data collection issues Shift entire LiDAR point cloud to match ground control points
LAS File Format The LAS file format is an open, public file format for the interchange of 3D point cloud data between users (as defined by ASPRS) Developed by ASPRS in conjunction with LiDAR vendors and industry members of the ASPRS Standards Committee Binary format (smaller); high performance (faster) http://www.asprs.org/society/committees/standards/LiDAR_ exchange_format.html
Breaklines Definition: Linear vector features that describe an abrupt change in the elevation of the terrain which might affect contours, hydrology and other engineering models Natural breaklines (hard): Ridge lines Toe of hill Edge of water body (ex. pond, lake) or stream Soft (man-made) breaklines: Roads Retaining Walls Dams
Project Specifications LiDAR - Ground Sample Distance (GSD) Average distance between LiDAR points on the ground Can also be expressed in ‘points per square meter’ (PPSM) Example: One (1) meter GSD to support generation of 2’ contours LiDAR - Vertical Accuracy Absolute accuracy of LiDAR points to known ground surface Example 1: ± One (1) foot vertical accuracy at 95% confidence Example 2: Root Mean Squared Error (RMSEZ) = 0.60 foot = 7.2 inches Orthophotography (pixel resolution) Example: One (1) foot orthophotos (typically georectified using LiDAR-derived surface model)
Point Density vs. Point Spacing Point Density = 1 / Point Spacing2 1 meter Point Spacing 1 meter 1 meter 1 meter Point Density = 1 point / sq. meter 2 meter Point Spacing 0.5 meter Point Spacing 0.5 meter 2 meters1 meter 2 meters 0.5 meter Point Density = 4 points / sq. meter Point Density = 0.25 points / sq. meter
Keys to a Successful Project Understand your mapping requirements and the purpose for completing a LiDAR project. Utilize a qualification-based selection process to select your LiDAR consultant. Stayaway from low price bid projects! Price-based selection causes some firms to cut corners (ex. offshore labor) to lower project cost. Hire a photogrammetric firm that owns a LiDAR sensor. Request a quality control plan.
Keys to a Successful Project Dedicate the appropriate number of internal resources to the project. Know exactly how the quality control is going to be performed by the consultant and internally. Understand the differences in LiDAR technology. The age of the sensor determines capabilities; pulse rate, roll compensation, field of view are unique to each system. Determinewhich accuracy specification is going to be adhered to (i.e. ASPRS, NDEP, etc.)
Keys to a Successful Project Hybrid accuracy standards should only be used as long as there is very detailed metadata and documentation that clearly explain the accuracy results. Do not exclude the ground truth surveying from a project. Request a LiDAR flight plan in the Request For Qualifications that clearly demonstrates the consultant’s understanding of the data acquisition issues.
Contact Information Bruce Adey, GISP LiDAR/Photogrammetry Discipline Lead E-mail: firstname.lastname@example.org Direct: (303) 353-3949 Mark A. Stucky, GISP MARS® Technical Support Specialist Senior GIS Analyst E-mail: email@example.com Direct: (303) 353-3933 Thank You!
Online LiDAR Resources USGS-NGP LiDAR Base Specification Version 1.0 http://pubs.usgs.gov/tm/11b4/TM11-B4.pdf FEMA Guidelines and Specifications for Flood Hazard Mapping Partners http://www.fema.gov/plan/prevent/fhm/gs_main.shtm ASPRS LAS Specification http://www.asprs.org/society/committees/standards/lidar_exchange_f ormat.html USGS Center for LiDAR Information Coordination and Knowledge (CLICK) http://lidar.cr.usgs.gov/
Online LiDAR Resources International LiDAR Mapping Forum (ILMF) http://www.lidarmap.org SPAR Point Group http://www.sparpointgroup.com/ LiDAR News http://lidarnews.com/ National LIDAR Dataset (USA) http://en.wikipedia.org/wiki/National_LIDAR_Dataset_-_USA USGS National Elevation Dataset (NED) http://ned.usgs.gov/
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