n.j.p.l.s. an introduction to lidar concepts and...

N.J.P.L.S.

An Introduction to LiDAR

Concepts and Applications

Remington & Vernick Engineers and Affiliates Continuous Improvement Committee Seminar

Presentation Outline

• LIDAR Data Capture

– Advantages of Lidar

– Technology Basics

– Intensity and Multiple Returns

– Lidar Accuracy

Airborne Laser Scanning

• ALS/LiDAR is an active remote

sensing technology that measures

distance with reflected laser light.

LiDAR: Light Detection and Ranging or

Laser Imaging Detection and Ranging)

• 1st developed in 1960 by Hughes

Aircraft inc.

• Modern computers and DGPS make

it practical.

• Typically used in very accurate

mapping of topography.

• New technologies and applications

are currently being developed.

LiDAR OverviewWhat LiDAR is . . .

. . . Light Detection And Ranging

. . . highly accurate topographic data

. . . Active Sensing System

- Uses its own energy source, not

reflected natural or naturally emitted

radiation

. . . Ranging of the reflecting object based on

time difference between emission

and reflection. (Multiple Returns per

pulse of light)

. . . Direct acquisition of terrain information

Examples of LiDAR uses . . .

. . . generate digital elevation models and

contours for use in:

engineering design and design reviews

(irrigation design, pivots, pipelines,

surge valves and livestock pipelines)

conservation planning

(HEL Determinations, practice

design, landscape visualization)

floodplain mapping

hydrologic modeling

Soil Survey

(slope, aspect)

Dam safety assessments

Surface Feature Extraction

(trees, shrubs, roads, buildings)

Vegetation Mapping

(Height, Density)

Landform Extraction

(ridges, benches, valleys)

What LiDAR is not . . .

. . . photography

. . . A substitute for photography

. . . All weather

. . . Able to ‘see through’ trees

- if sunlight can reach a spot on the

ground so can the laser; if sunlight

can’t reach the ground due to vari-

ous vegetation, than neither can the

laser


ELEMENTS OF GIS: DATA TYPES

• Vector Data (Points, Lines, Polygons)

– Esri shapefile

– AutoCAD .dwg files

– Microstation .dgn files

– TIN (Triangular Irregular Network)

– Geodatabase

• Raster Data

– TIFF

– JPEG

– MrSID

– DEM (Digital Elevation Model)

– Lidar – “Light Detection And Ranging”

Remington & Vernick Engineers and Affiliates Continuous Improvement Committee SeminarLiDAR: How it works

Each time the laser is pulsed:

• Laser generates an optical pulse

• Pulse is reflected off an object and returns to the system

receiver

• High-speed counter measures the time of flight from the start

pulse to the return pulse

• Time measurement is converted to a distance (the distance to

the target and the position of the airplane is then used to

determine the elevation and location)

• Multiple returns can be measured for each pulse

Up to 200,000+ pulses/second

Everything that can be seen from the aircraft is measured


Advantages of LiDAR Technology

• Provides a highly accurate means of elevation model collection for 1’ or 2’ contours

• Acquisition can take place day or night… shadows that are problematic in mountainous areas are not an issue with LiDAR

• Unlike photography, acquisition can take place below cloud cover… cloud shadows no issue

• Very cost effective for larger projects

• Does not provide break lines, nor is it imagery


Why is This Technology Exciting

• Conventional Surveying: 15.5 years

• Photogrammetry: 1.5 years

• Lidar: 6.7 seconds @ 150 kHz

Costs can be significantly less for the right projects…

Time to Collect 1 Million Points

Remington & Vernick Engineers and Affiliates Continuous Improvement Committee SeminarAircraft Requirements

• Flying heights from 3,000 to 6,000 feet

• Speeds ranging from 90 to 130 knots

• Ability to carry equipment, personnel, and

full fuel load


What it is not…

• Photography

– We can shade the elevation and intensity data to create “imagery”

• Doesn’t capture breaklines

• Doesn’t capture planimetric features

– Advances in software may allow automatic feature extraction soon


Shading by Elevation

Urban LiDAR Layout

Remington & Vernick Engineers and Affiliates Continuous Improvement Committee SeminarWhy use a Laser?

• LiDAR takes advantage of two of the unique properties of

laser light:

• 1. The laser is monochromatic. It is one specific

wavelength of light. The wavelength of light is determined by

the lasing material used.

• Advantage: We know how specific wavelengths interact

with the atmosphere and with materials.

• 2. The light is very directional. A laser has a very narrow

beam which remains concentrated over long distances. A

flashlight (or Radar) on the other hand, releases energy in

many directions, and the energy is weakened by diffusion.

• Advantage: The beam maintains its strength over long

distances.

• 3mrad divergence = 3 m at 1 km and 15m at 5 km.


Issues with LiDAR Data

• LIDAR is indiscriminate… it places elevation points on

everything. This includes cars, houses, trees, etc.

• LIDAR only places mass points, or random xyz points. It does

NOT pick up breaklines, or lines of abrupt change in the ground

elevation

• LIDAR is NOT imagery. LIDAR data can be shaded, however, to

offer a relief image


Bare Earth Model

• Significant editing must be employed to create a “Bare Earth Model” which models the natural ground

• Some automated procedures may be used. Imagery backdrop may be necessary

• In some cases, traditional photogrammetry may be necessary to add breaklines

Discrete Return Data: Millions of X,Y,Z points

Area is approximately: 1 X 0.75mi.

includes ~ 440,000 returns

Applications for Discrete-Return LIDAR

Digital Elevation Model (DEM)

4m DEM of PREF Subset

From Point Clouds to 3-D Surface Models

• Points are used to create 3D

surface models for applications.

• Triangular Irregular Networks (TIN)s

are used to classify the points and to

develop Digital Elevation Models

(DEM)s.

• Points must be classified before

use: “bare earth” points hit the

“ground”; other point categories

include tree canopy and buildings.

• Correct identification of “bare earth”

is critical for any lidar mapping

application.

LIDAR Mapping of Forest Structure


Intensity and Multiple Returns

• Most units today have the ability to measure multiple

returns and the intensity of the returned signal for

each

• This enables specialized applications using the LIDAR

data

Aerial Photo

Intensity Plot

LIDAR Surfacing

Lidar Returns and Cloud Cover


LiDAR Applications

• LIDAR has significant fixed cost… but can be very cost effective

for large projects

• Appropriate for a wide range of projects including forestry,

corridor studies, obstruction mapping, flood studies, city/county

mapping, and transportation projects

• Required accuracy must be carefully evaluated

Value of “Noise” in the Data

Interstate

Comair Jet

River

470’ Tall

Remington & Vernick Engineers and Affiliates Continuous Improvement Committee SeminarLiDAR Accuracy

• Quality of the hardware and software

• Knowledge of the planners, operators, office staff

• Flying height

• Scan angle (also important for vegetation penetration)

• GPS configuration (PDOP and Number of SVs)

• Distance from base station to aerial platform

• Laser power

• Laser rep rate

• Laser rep rate

Remington & Vernick Engineers and Affiliates Continuous Improvement Committee SeminarLiDAR Accuracy

• Accuracy of elevation in range of 6 to 30 centimeters (0.20 to 0.98 feet)

• Accuracy of XY position in range of 10 to 46 centimeters (0.33 to 1.51 feet)

• Accuracy depends on pulse rate, flying height, GPS configuration, location of ground stations, and position of the scanner with respect to nadir

• r rep rate


GIS Resources in New Jersey

• Using available GIS resource data can conserve costs

– The New Jersey Department of Environmental Protection offers aerial

photography, LiDAR and GIS datasets

– The data is available at no cost

– GIS data can also be obtained at the County, Agency level

• Meadowlands

• DVRPC

• USGS

– A data sharing agreement is generally needed

Remington & Vernick Engineers and Affiliates Continuous Improvement Committee SeminarRaster Processing

• Contour Generation at set Intervals


USGS DEM Lidar DEM

Remington & Vernick Engineers and Affiliates Continuous Improvement Committee SeminarAdvanced Analytics

• Watershed Analysis- Drainage Basin Calculation for Discharge Points

Remington & Vernick Engineers and Affiliates Continuous Improvement Committee SeminarWhere Do We Go From Here?

• Improve Accuracy

•Software Processing

•Automated Feature Classification- Building Footprints- Roof Types- Pervious/Impervious-Vegetation

•Data Fusion


WHAT IS METADATA?

• Metadata describes…

– Content, quality, condition and other characteristics about data

– Who, what, where, why and how of a data set

• Metadata includes information you need to…

– Determine which data exist for a geographic location

– Determine if a set meets your need

– Acquire the set you identified

– Process and use the set

• Metadata answers questions…

– Who created and maintains the data?

– Why were the data created?

– What is the content and structure of the data?

– When collected? When published?

– Where’s the geographic location?

– How were the data produced?


LiDAR Summary

• Simply another tool in our toolbox

• It is not right for every project, but it can provide substantial cost savings for the right project

• Large-scale, high-accuracy projects still require conventional mapping solution

• LIDAR and the software we use in processing will continue to improve with time


QUESTIONS & ANSWERS

Thank you!

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