Download - Reality Check: Processing LiDAR Data
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Reality Check: Processing LiDAR Data
A story of data, more data and some more data
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Red River of the North
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Red River of the North
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Red River of the North
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Red River of the North
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Introduction and BackgroundFEMA Grant to DNR in 2006
Create a high resolution Digital Elevation Model for Floodplain Mapping in the Red River Valley
Partnered withClay CountyMn/DOTNorman CountyWhite Earth ReservationWild Rice Watershed District
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Background
Red River Watershed17,700 Square Miles (Minnesota only)
LiDAR Collect Area (red outline)
3663 Square Miles
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BackgroundData Specifications
Vertical Accuracy 12 cm RMSE
• Capabable of supporting 1.2’ contour generation
15 cm RMSE• Capable of supporting 1.7’
contour generation
Horizontal Accuracy 0.5 meters RMSE
1 meter nominal point spacing
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BackgroundData to be delivered
All LiDAR PointsBare Earth PointsBare Earth GridBuilding PointsVegetation PointsIntensity ImageryEdge of water break lines
Used to enforce flat water areas….
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Delivery Formats…LAS – ASPRS LiDAR Exchange Format
Binary, Open standard, not vendor specificStores a variety of point information
Number of returns Return Number Intensity X,Y, Z values Scan Direction Classification Scan Angle Rank GPS Time
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Delivery FormatsASCII Comma-delimeted
Very generic formatDigested by most any softwareLimited amount of point information available
x, y, z
Large because no compression
Raster Grid1 meter resolution created from bare earth pointsGeneric and portable binary formatInteger format, centimeter Z-Values
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Working with LiDAR DataLiDAR datasets tend to be very large
LAS Format All Returns – 7 million points, 100 mb / square mile Bare Earth – 3 million points, 45 mb / square mile
ASCII Format All Points – 4 million points, 75 mb / square mile Bare Earth – 3 million points, 73 mb / square mile
Grid Format 2.8 mb / square mile in integer format 11.2 mb / square mile in floating point format
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Data volume = high density
1.02 meters
90 meters
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Now for some math….1 sq mile = 296mb3663 sq miles = 1,084,248 mb
Just over 1 gb worth of data for the project area!
Implications?Lot’s of horsepower, er hertz-powerLot’s of disk spaceLot’s of staff-time to copy, handle and process dataLot’s of network capability for Web based delivery
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AccuracyLiDAR has the capacity to collect very high accuracy data
Depends on the flight height and the mission parametersIncreasing accuracy = Increasing Costs!
Limitation is the positional accuracy of the airborne GPS system
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AccuracyValidated the project data deliverables for a pilot project area of 100 square milesTwenty-four control points captured by Mn/DOT survey crewsAccuracy proved to be very good.Two sites were removed from the assessment
One was in a swamp wet in spring, dry during survey
One was just outside the study area
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Accuracy Assessment
ding, ding ding ding
13.4 cm * 1.96 = 26 cm at the 95% confidence level
Assumptions: Normal Distribution Average of zero
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LiDAR Derived ProductsWhat Data Do People Use?
Primary use products Contours Raster Digital Elevation Model
Most users don’t bother with the raw LiDAR data Not a lot of tools available but this list is growing ArcGIS extensions are now available to read LAS format
LiDAR data Derived products from raw LiDAR is a growing research field
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Creating ContoursContours are typically created from a raster DEMContours don’t always look visually appealing
Jagged lines that may wrap on themselves
Smoothing the DEM can help3x3 averaging filter works wellMakes the interpolation routine work harder
Commonly called Neighborhood analysis
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Creating Contours….
400’
1’ contours on shaded relief
Non-filtered DEM Filtered DEM
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Contours…..
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Contours…..
Difference surface of Original – Smoothed DEMMaximum Difference 0.9 feet
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VisualizationLidar data can be visualized a number of ways
Shaded Relief images can reveal very subtle relief Especially with high detail data Helpful for data validation and looking for anomalies and
errors in the data
3-Dimensional viewingCross-sectionsContour generation
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Shaded relief imagery
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Intensity…..
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Terrain Modeling…..
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Lessons LearnedUse a Tiling Scheme of Your Choice
PLS Section boundaries work well
Validate the data using for a small pilot areaSaves you and the vendor time if there are problems
Do an independent accuracy assessment!You might be surprised at what you find
Smoothed DEMs generate smooth contours