airborne lidar the technology slides adapted from a talk given by mike renslow - spencer b. gross,...
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TRANSCRIPT
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Airborne LIDARAirborne LIDARThe TechnologyThe Technology
Slides adapted from a talk given by
Mike Renslow - Spencer B. Gross, Inc.
Frank L.ScarpaceProfessor
Environmental Remote Sensing CenterCivil and Environmental Engineering
University of Wisconsin-Madison
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PRESENTATION OUTLINE
• Background of LIDAR
• Brief Technical Description
• Quality Control/Assurance Procedures
• LIDAR Data Characteristics
• Processing LIDAR Data (2 Steps)
• Data Set Characteristics
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BACKGROUND
• LIDAR (Light Detection And Ranging)– 30 Year Old Technology– Became Cost Effective Very Recently
• System Components– Laser Scanner, ABGPS, IMU, Precise Clock
(Multiple Planes of Reference)
– Robust Computer Support– Requires Calibration (Bore Sighting)
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BACKGROUND• Capacity to Capture Multiple Return Values /
Pulse– Derive many, many X, Y, Z Values
– Positional Data and Intensity Data• Multiple Configurations Possible
– Remarkably Large Data Files
• Accuracy– Standard Deviation 15-20 cm
– Vertical RMSE at 20 cm on Discrete ‘Hard Hit’ Points
– Horizontal Accuracy at 2X the ‘Footprint” Size
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Oscillating Mirror Scan Pattern
Rotating Mirror Scan Pattern
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LIDAR has Multiple Return
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POINT CLOUD OF ALL LIDAR POINTS IN DOUGLAS FIR FOREST
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LIDAR &Terrain Interaction
• For example; a calm still lake, will only reflect energy back within a few degrees of the nadir beam of the laser.
• A “wavy” lake on the other hand, will reflect energy back from wider incident angles.
• Diffuse surfaces (ground or tree) reflect energy back omnidirectionaly.
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LIDAR Intensity Collection
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Laser Intensity Raster - Detail
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TIN surface of Raw LIDAR Data
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‘Raw’ FIRST Return LIDAR Data
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Raw LAST Return LIDAR Data
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• Automatic programs begin the noise and vegetation/surface feature removal process
• These remove approximately 80% of vegetation (depending on the land cover and terrain characteristics)
• This part typically uses about 20% of the vegetation removal time budget
Automatic Vegetation Removal
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Trend Surface AnalysisGreen Points = Vegetation Brown Points = Trend Surface
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Before
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...after
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• Final vegetation and feature removal requires manual intervention.
• Custom selection routines are used in 3D and GIS Software to analyze the data and identify target points.
• Accurate interpretation of the LIDAR data requires supporting imagery.
• Removal of the remaining 20% of the vegetation and features will account for about 80% of the time budget
Manual Editing
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...after
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…final
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LIDAR vs. Traditional Mapping1”=100’ Scale Terrain Mapping Example
Compiled Mass Points are more widely spaced: 60 feet vs. 12 feet.
Compiled DTMs use breaklines; LIDAR usually does not (breaklines can be added from photogrammetric techniques).
Compiler can place points; LIDAR is indiscriminate.
Compiler must be able to SEE THE GROUND, LIDAR is self-illuminating & ‘looks’ down into the vegetation.
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Typical Wooded Area Example
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Detail with LIDAR Ground Points
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Processed TIN Surface
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• Contours are a cartographic construct used to visualize topography.
• Contours produced directly from the LIDAR TIN are usually not aesthetically pleasing.
• LIDAR data can be converted into a DEM Grid at the nominal post spacing which retains fidelity to the original data and which appropriately smoothes the contours.
DEM and Contour Generation
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Contours Generated from the DSM
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Contours Generated from the DEM
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Conclusions
• LIDAR is a powerful new technology for determining terrain elevations.
• There are still questions as to the horizontal accuracy.
• Appears to be a good companion technology to the existing photogrammetric methods of measuring terrain.