Sanborn LiDAR Accuracy Presentation

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Expected Accuracies of LiDAR Acquisition
September 16, 2009
GIS in the Rockies
Presented By: Matt Aschbrenner

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Presentation OverviewPresentation Overview
• How Do We Collect the Best Data?
• Planning
• Network Survey
• Check Point Survey
• Collection and Flying
• Field Check and Approximate
Processing
• LiDAR Data Accuracy
• Q & A Session

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How Do We Acquire the Best Data?
• Network and Check Point Surveys are practical tools in helping
to reach or surpass expected accuracies.
• Specific factors of local are necessary when planning the most
efficient schedule of collection.
• Timely acquisition can be achieved by understanding weather
patterns, terrain, and mobilization issues of a job site.
• On location procedures and processes analyze the quality of
Raw Data and make certain expected accuracies are
achieved.
In LiDAR acquisition, many variables warrant consideration to
ensure accurate and high quality LiDAR data.

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Planning - CoveragePlanning - Coverage
• With the desired Point Spacing established, a Flight Plan and
Flight Lines are the next step to achieving the correct “density” of
points in a collection area.
• The two most important factors in planning Flight Lines are
overlap and LiDAR sensor settings.

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PLANNING - OVERLAPPLANNING - OVERLAP
• 50% Overlap is the preferred amount to ensure that there will be
no “gaps” in between two Flight Lines. This amount is optimal in
areas of varying terrain.
• This percentage could be higher in mountainous areas.
• Overlap is generally not lower than 30%.
• Below is an example of 40%, In blue is the Flight Line Footprint
and in red is the Overlap.

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Planning – LiDAR Sensor Settings
Sensor Settings determine point spacing and coverage.
Sensor Settings:
• FOV (Field of View)
• AGL Altitude
• Pulse Rate
• Scan Rate
• Airspeed
• Laser Current (%)

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Planning – LiDAR Sensor Settings

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• Plan based on
– Workable blocks of data
– Delivery tiles
– Baseline requirements
– Flightline distance
limitations
– Control locations
– Accuracy
– Application
– Topography
LiDAR Project Planning
What you should know?
Planned Flight Lines

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Network Survey

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Network Survey
• Network Surveys are acquired to correct for error in the
GEOID model.

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Network Survey –– Establishing Control
• Establish control for entire mapping program prior to collection using a
minimum of two HARN Class Horizontal and two Vertical First Order
Class II; NGS established Points.
• Perform Fully Constrained Network Adjustment.
• Apply necessary velocity adjustments to reflect
Epoch required.
• Adjustment supports a mapping operation
not a Survey.
• Provide Adjustment to all LiDAR providers
involved in the program prior to processing.

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Check Point Survey

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Check Point Survey
• One of the best tools to validate
LiDAR accuracy is a
Check Point Survey.
• A Check Point is a ground
reference point that can be
checked against LiDAR Data,
Vertically and Horizontally.
• The Survey is collected by logging
a baseline between two GPS Base
Stations. One Base Station is set
up at a random location in the
project area while the other Base
Station is set up at a known
Network point.

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Check Point Survey
Check Points are set up in various locations dependent on
vegetation types and generally dispersed throughout the project area.

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Collection and FlyingCollection and Flying

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Collection and Flying ONSITE PROJECT PLANNING
• Day or Night
Safety considerations.
• Leaf on or Leaf off
Application dependent.
• Weather
• Smoke
• Summer, Spring, Fall, or Winter
Most collects are done in the spring and fall.
Summer collects take place for special
applications such as Forestry Winter collects
based on geographic location.

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Collection and Flying -- COLLECTION PRACTICES
• PDOP and KP-index forecasts.
• Static initialization of GPS and IMU.
• Procedural S-Turns prior to
collection and calibration.
• Calibration for every mission.
• Line lengths no greater than 20
minutes.
• Fly over remote Base Station prior
to flying lines.

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Collection and Flying–– PDOP & KP PLANNINGPDOP & KP PLANNING
PDOP and number of satellites. KP-index is a measure of the
Geomagnetic activity from the
Sun on the earth’s atmosphere.
• Need to forecast PDOP and KP-index to ensure accurate data.
• PDOP- Positional Dilution Of Precision

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Collection and Flying–– MISSION COLLECTIONMISSION COLLECTION
Calibration

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Separate Sensor Collection Vs
Check Point Survey at runway calibration site
Calibration after every installation
• Required to make sure the system is operating
correctly.
Calibration every three months
• Plane vibration can effect system.
Calibration every mission
• Provides necessary information incase of on
foreseen occurrences.
• Fly 2 lines perpendicular to runway at start.
• Fly 1 line perpendicular and one parallel to runway
at end.
• Ensure ability to correct for Roll, Pitch, scan angle
and other potential bias’.
Collection and Flying –– CALIBRATIONCALIBRATION
Four Runway Calibration Scans

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Collection and Flying– PILOT DISPLAY– PILOT DISPLAY

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Collection and Flying
OTHER PROJECT SITE CONSIDERATIONS
Restricted Flying Areas:
• Certain areas may have flight restrictions.
EX: Military, Large Airports, Urban development.
Terrain:
• Extra overlap may be necessary in locations with
sharp elevation changes to ensure complete
coverage over mountain peaks and valleys.
Prevailing weather and wind patterns:
• Weather may determine the best time of day to collect.
• Flightlines may need to be reoriented due to wind patterns.

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Field Check – APPROXIMATE PROCESSING
The goal of the Field Check is to establish the
quality of pre-processed raw data
The three specific components are as follows:
• GPS Processing
• IMU Processing
• Laser Point Processing

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Field Check–– GPS PROCESSING
• Greatest source of Error in LiDAR
system.
• Static initialization at start.
• Static session at end.
• PDOP less than 3.2.
• Achieve under 10cm combined solution
for field check.
• KP index under 4.
• Processing is easier.

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Field Check–– IMU PROCESSING
IMU – Inertial Measurement Unit
Measures the attitude (roll, Pitch and Yaw) of the aircraft 200 times a second.
• This is the second largest source of error in a LiDAR system.
• Process checked using the lever arm offsets.
• If under 2cm for X,Y and Z the data is good.

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Field Check–– LASER POINTS
There are two methods of checking the quality and coverage of the
Laser Points:
• The first method is derived by sub-sampling the points and displaying
them over a project boundary. The Trajectory of these points is computed
using a SBET (Smoothed, Best Estimated Trajectory).

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Field Check–– LASER POINTS, CONT.
The next Method of checking laser point data is the footprint coverage
analysis. In this process, the edge of scan for each flight line is computed
and used to create Footprint, Overlap, and Gap shapefiles.
Green - Footprint Purple - Overlap
Orange – Gap (unfinished project area)

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LiDAR Data Accuracy
•Vertical accuracy required usually 9.24 -18 cm
•Horizontal accuracy required usually 50 cm – 1.0m
•Before MPia, ALS-60, and GEMINI
Vertical accuracy achieved: 5.8 -12 cm
Horizontal accuracy achieved: 14 -27 cm
Achieved 3.7 centimeters once
•After MPia, ALS-60 and GEMINI
Vertical accuracy achieved: 3 – 12 cm
Horizontal accuracy achieved: 10 – 27 cm
•Data meets accuracy specification

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LiDAR Data Accuracy – ACCURACY REPORTING– ACCURACY REPORTING
• NSSDA Standard
• NMAS Specification
• FEMA Specification
• ASPRS Specification
• RMSE

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Q & A SESSIONQ & A SESSION
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