lidar expected accuracy presentation

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Expected Accuracies of LiDAR Acquisition

September 16, 2009GIS in the Rockies Presented By: Matt Aschbrenner

© 2007, The Sanborn Map Company, Inc. Privileged and confidential information.Distribution or copying prohibited unless approved in writing. 2

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 PlanningWhat 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 NightSafety considerations.

• Leaf on or Leaf offApplication dependent.

• Weather

• Smoke

• Summer, Spring, Fall, or WinterMost 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 theGeomagnetic activity from theSun 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 VsCheck 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 GEMINIVertical accuracy achieved: 5.8 -12 cm Horizontal accuracy achieved: 14 -27 cmAchieved 3.7 centimeters once

•After MPia, ALS-60 and GEMINIVertical accuracy achieved: 3 – 12 cmHorizontal 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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