Oldow TLS-Basics Calculations

Introduction to Terrestrial Laser

Scanning (TLS)

• Similar to Sonar and Radar but uses Light (Light

Detection and Ranging)

• Initial of LiDAR use began in the 1960’s in studies of

atmospheric composition, surveying, law enforcement,

etc.

• Transmits a pulse of light and records the returned pulse

of light – records time, divides by two, and multiplies by

the speed of light for distance

• Able to record thousands of points a second recording

target position (X,Y,Z), intensity, and color (RBG)

• Capable of relative positioning at mm to cm accuracy

LASER SCANNERSLASER SCANNERSLASER SCANNERSLASER SCANNERS

• Beam deflection mechanism provides elevation and azimuth of the transmitted pulse

• Return-beam detection device records return time and provides range calculation from two-way travel time

• Energy of the return pulse (intensity) and the color (RBG) is • Energy of the return pulse (intensity) and the color (RBG) is recorded

• Full waveform now recorded on some TLS instruments

Time-of-Flight Measurement

Transmitter

Receiver

Range = travel time x speed of light/2

Record (azimuth, zenith, range, intensity)

Greaves, SPAR 2004.

BENEFITS OF LASER SCANNERS

• Imaging system provides an unprecedented density of geospatial information through a dense set of three-dimensional vectors to target points relative to the scanner location (point cloud)

• Scanner controlled by laptop computer that is also used • Scanner controlled by laptop computer that is also used for data acquisition and initial processing

• Combination with GPS allows fully geospatially referenced data set and opens potential for direct measurement of change (time series measurements)

3D POINT CLOUD

• Cartesian transformation of the laser pulse data (trans-formation of the range and reflectance images as well as the calculated XYZ coordinates) in scanner centered reference frame

• 3D point cloud of discrete locations derived from superimposing range and reflectance image for each laser superimposing range and reflectance image for each laser pulse

• 3D point clouds are the basis for subsequent analysis and used to create CAD or GIS models

REFLECTANCE IMAGE

• Looks like a black and white photograph of scan coverage

• Individual measured points are defined as reflectance values– highly reflecting (light) points are displayed in a – highly reflecting (light) points are displayed in a

light grey pixel– highly absorbing

(dark) points are displayed as a dark grey pixel– lack of a return is depicted as a black pixel

Laptop

Field Equipment

Terrestrial

Laser

Scanner

(LPM 800)

Controls to

align all the

scanning

data

Field Equipment

Topcon Topcon Topcon Topcon

Totalstation Totalstation Totalstation Totalstation

Imagine Imagine Imagine Imagine

SystemSystemSystemSystem

Camera

RTK GPS

Tripod

Integrating geometry with textureIntegrating geometry with texture by by position position

controlcontrol

GPSGPS Imaging Total StationImaging Total Station

Camera

Nikon D200

Total StationTotal Station

TOPCON HIPER LITE+ - RTK GPS SYSTEM

TOPCON IS

TOPCON Total Station

Nikon D200

OR

Examples of controls

Scanner Parameters

• Beam Divergence

• Angular Step

• Range Distance

• Field of View• Field of View

• Points Per Second

• Size and Weight

Scanner Parameters

• Beam Divergence

Df = (Divergence * d) + Di

Scanner Parameters

Scanner Parameters

• Angular Step

Spacing = d(m)*TAN(step)

Scanner Parameters

Scanner Parameters

• Range DistanceTarget Reflectance can change single scan range by hundreds of meters

Laser and CCD characteristics impact maximum and minimum range

distances from <5 meters to >2000 (6000) meters

• Field of View• Field of ViewRotational Base allows 360 degree rotation (azimuth)

Rotating mirror and gear drive allows ~90 degree vertical coverage

• Points Per Second Scan Time

• Size and WeightField Logistics

Beam Stepping Distance

• Beam stepping angle is specified in either degrees/minutes/seconds, in decimal degrees or in gons. There are 400 gons in a circle, just as there are 360 degrees in a circle.

• Unfortunately, the specs units are not radians (2π radians in a circle). If they were radians, a very rapid approximation of the stepping distance in meters can be made mentally. For small angles,

Stepping Distance = (angle in radians) * distance

e.g. Stepping Angle = 0.00005 radians (.05 mRadians)e.g. Stepping Angle = 0.00005 radians (.05 mRadians)

Stepping Distance (@800m) = 0.00005* 800 = 4 cm

1 gon = 0.9 deg 1 deg = 1.111 gon

1 deg = 0.01745 radians

• Minimum specs for stepping tend to be 0.0012 => 0.004 deg

0.002 deg = 0.035 mRadians = 3.5 cm at 1000 meters

Beam Divergence

• Beam Divergence

– Optech Ilris 0.00974 deg (0.17 mRadian)

– Riegl LMS 620i 0.004 deg (0.07 mRadian)

– Riegl LPM 321 0.046 deg (0.8 mRadian)

• Beam diameter at exit ranges from a few millimeters to centimeters

• Spot diameter at distance

diameter = beam at exit + divergence (radians) * distance

Riegl 620 = 2 mm + (0.00007 radians * 500 m) = 3.7 cm

Riegl LPM = 1 cm + (0.0008 radians * 500 m) = 41 cm

Laser Return Signal

Beam size at laser = 1 cm2

Beam divergence = 0.8 mrad

Beam intensity at laser = 1 cm-2

Distance to outcrop = 500 m

Reflectivity = 33%( )

( )

( )26.033.0*0008.0*100

**Re

cm 0008.01257/1

/

cm 1257*2/40

*2/

cm 40 cm 50000*0008.0

*

2

2-

22

2

=

==

=

==

=

==

=

==

=

π

π

π

distance**2tcrop/ReturnAtOuernsityAtLasReturnInte

tyReflectivitOutcropIntensityAObjectArearopturnAtOutc

OutcropBeamAreaAtmIntensityInitialBeatOutcropIntensityA

rBeamDiamteOutcropBeamAreaAt

distancedivergenceperAtOutcroBeamDiamet

( )

001660.00000000

cm 66.1)50000**2/(26.0 2112

=

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−π

π distance**2tcrop/ReturnAtOuernsityAtLasReturnInte

The return signal at the laser is substantially lower than the signal that is

emitted by the laser.

The example above assumes that the laser beam is 1 cm2 when it leaves the

laser and that the window to the receiver has an aperture area of 1 cm2 and

that the feature being imaged is 100 cm2.

Laser beam

with 3 milliradian div. Target

Diffuse reflection for reflectorless laser

rangefinders

diffuse reflection

Laser range-finder

receiver aperture

Not to scale

Material Description

Winter Snow and Ice

Vegetation (The Average Value of Many Types)

Soil

Silt

Sand

DR.

0.85

0.50

0.05 - 0.35

0.20 - 0.40

0.10 - 0.35

904 nm diffuse fractional reflections

of common materialOther lasers have different responses when operating at different wavelengths

SandGypsumClayDirtShale, CoralConcrete, AsphaltCoal Tar PitchPlywood, UnpaintedBrick, RedBark

0.10 - 0.35 0.55 - 0.700.40 - 0.500.30 0.45 0.10 0.05 0.50 0.250.20 - 0.25

Range Measurement versus IntensityCD Reflectors Mounted on a Wall

Note angle of points from wall pointing toward scanner

Range Error versus Intensity

LIDAR emits a short pulse of light and measures the time for the return

signal to reach the detector. Light travels at about 0.33 m / ns in air.

Distance = ½ * time of flight * velocity of light. Enough returned energy

must be received at the LIDAR detector to trigger the timing circuitry. If

the signal is very strong, the detector threshold will be reached faster than

if the signal is very weak. LIDAR detectors must compensate for this

effect in order to provide accurate measurement of distance.

LASER SCANNER ACCURACY

• Boehler, Vincent and Marbs, 2003.

• Tested scanners for accuracy

• Application was for cultural heritage

applications (we will revisit for natural applications (we will revisit for natural

surfaces)

• Manufacturer specifications not good

representation for real-world applications


• Angular accuracy

– Angles from combination of deflection of rotating

mirrors and rotation about a mechanical axis

– Provides azimuthal position– Provides azimuthal position

• Range accuracy

– Time of flight or phase comparison between

outgoing and returning signal provides range

– Noise-fuzz of points on a flat surface


• Resolution

– Ability to detect an object in point cloud

– Two specs contribute

• Smallest increment of angle between successive points • Smallest increment of angle between successive points

(can manually set)

• Size of laser spot (beam dispersion)

• Edge effects

– When a spot hits the edge of a target and receives

2 positions and/or 2 reflectivity values (material)


• Surface reflectivity

– Distance, atmospheric, incidence angle

– Albedo (ability to reflect)

• White strong, black weak• White strong, black weak

• Depends on spectra of the laser (green, red, near IR)

• Inclined surfaces of high reflectance (i.e., water ) can

create travel time anomalies (mutlipathing)

– Typically contribute accuracy-range errors larger

than manufacture specifications

Environmental Conditions

• Temperature (important to operate within

specification range)

• Atmosphere

– changes propagation speed slightly– changes propagation speed slightly

– dust, mist, raindrops, fog - a major problem

• Interfering radiation

– Sunlight strong relative to signal

• Influence or prevent (don’t shoot into sun)

Survey Control

• Surface referencing (using recognizable

physiographic features)

• Targets (reflectors and/or prisms)

• Geo-referencing (Total Station and GPS • Geo-referencing (Total Station and GPS

positioning)

• Multiple scan registration requires tight spatial

control

Calibration

• Repeatability

– Need to document multiple measurements of

known geometry

– Compare with allowable variance– Compare with allowable variance

• Quality Control

– Multiple measurements of known geometry with

multiple scanner positions

Resolution

• Measurement accuracy is governed by

instrument resolution

• Resolution is the smallest distance that can be

measured without ambiguitymeasured without ambiguity

• For laser scanning, this is the spacing of the

point cloud array

• Varies linearly with distance from the scanner

Resolution

Range

Measurement Accuracy

• The ability to generate physical dimensions

and location of an object

– Specified with a tolerance, e.g. +/- 6 mm

(and a confidence interval)(and a confidence interval)

– Not a laser scanner specification but a work

product specification

Resolution and Measurement Accuracy

• Absolute measurement accuracy can’t be

better than 2x instrument resolution


• Absolute measurement accuracy can’t be

better than 2x instrument resolution


• Modeling may help, caution required


• Overlapping dot problem (edge effect)

Resolution test

Measuring noise in range direction. Riegl Z420 is

comparable to Z360

Action Sequence in the Field

• First, establish the scan locations and ensure that they completely cover the target

area.

• Second, establish the location for the controls

• Third, review naming and number conventions to be used

• Make sure that the site name in the software and the folder and site abbreviation

in the camera set is correctly set (can be done night before)

• Set up controls and locate them with GPS (time series measurement reduce errors)

• Set up first scan site and decide on camera sites (if applicable)

• Scan controls before scanning the outcrop• Scan controls before scanning the outcrop

• The photo team with the Topcon IS needs to be working in parallel with the scan

team. One can get ahead of the other, but the jobs need to proceed in parallel. It

takes a lot of time.

• Review the progress with one another

• Double check the work

• Save all work to an archive file that is not used as a work file

• Review the data in the field if possible

• Start model construction as soon as possible in order to correct errors or fill in

unintentional holes in the data

LiDAR Site Selection

(multiple locations, selection of point density versus time)

• It is necessary to scan an outcrop from at least two oblique directions to minimize occluded parts of the outcrop. Three scans are good (left/center/right), and additional reverse directions are optimal.

• Point density is inversely dependent upon distance to the outcrop. If the distance has a wide range of values, the time to scan the outcrop can be optimized by selecting a finer outcrop. If the distance has a wide range of values, the time to scan the outcrop can be optimized by selecting a finer angular resolution for the more distant parts of the outcrop compared to the closer parts of the outcrop.

– Scan time is inversely dependent upon the square of the scan angluar resolution. Increasing the scan step angle by 2X reduces the scan time by 4X.

– Partition the outcrop scans to maintain a nearly uniform linear stepping distance at the outcrop surface.

Scan Positions

overhang

Choose scan positions to minimize occluded (shadowed or hidden)

geometries. Scanner blue will not image beneath the overhang or

the right side of the overhang. Scanner red will image underneath the

overhang and will image the right side of the overhang.

Moab Utah-Google Earth Screen Capture

Multiple Scan Positions

Moab Utah

Scan Partition as a Function of Range

Scan Partition as a Function of Angle of Incidence

Scan Partitioning Avoids Unnecessary Scan Time

Scan Partitioning

Scan of the “Pyramid” at Slaughter Canyon, Carlsbad Caverns National

Monument, New Mexico

Scanner was on a 200m high hill.

Scan ranges were 50m to 800m

Scan Partitioning

Scanning of the total outcrop at the scan step angle needed for the longest

scan would have dramatically increased the scan time.

Scanning the outcrop in a single scan which covered the entire outcrop would

Result in a large amount of empty data.

Placement and Survey of the Controls

• Use of scanned control reflectors improves the accuracy of the model and allows straight forward alignment of the individual scans

• Alignment of two scans requires an absolute minimum of three control points. It is best to have five or more available to accommodate errors.

• If multiple scan sites are used, it is not necessary to have all control reflectors visible from all of the scan sites. However, it is necessary that each scan site be able to see at least three reflectors that have been correlated with other scan sites

• The control reflectors should cover a wide area (preferably surrounding • The control reflectors should cover a wide area (preferably surrounding the image area), do not place reflectors in a linear fashion or group them in a tightly.

• The spacing of the reflectors optimally approximates or exceeds the distances in the scan region. However, this may not be practical.

• It is not necessary to have reflectors on the outcrop and/or within the image area, although it is desirable to do so if practical and is aesthetically acceptable (for photorealistic analysis).

Placement and Survey of the Controls

Scan Reflectors before Scanning Outcrop

• It is prudent to scan the reflectors before scanning the outcrop.

– If you do not have the controls with the scan data, you may not be able to use the scans

– If something happens to disorient the scanner or there is a power or software crash during the subsequent scans, the work up to that point can still be used

– For double protection, rescan at least some of the reflectors after – For double protection, rescan at least some of the reflectors after completing the outcrop scan. If the scanner has lost alignment, the final reflector scan will identify the problem.

• When using the LPM with the telescopic sight, the scan window must be larger than expected. There is parallax between the scanner and the telescope. This is a much larger problem at close range than at long range.

Collecting Field Data

Scan Pos 1

GPS Control

GPS

Photo Control


Scan Pos 1

GPS


Scan Pos 1

GPS


GPS

Scan Pos 2


GPS

Scan Pos 2


GPS

Scan Pos 2


Photos

Photos

Photos

Geospatial Referencing: GPSSummary of the approximate accuracy of GPS positioning versus methods. (Modified from

Featherstone, 1995)

• Accurate measurement of reference network

baselines with Total Station (mm)

• Time series measurement of individual reference

High-Resolution Geospatial Referencing: GPS

and Total Station

• Time series measurement of individual reference

reflectors/prisms with continuous GPS (cm)

• Simultaneous GPS solution of all reference sites and

network adjustment using TS baselines to provide

sub-cm results

Mickey Hot Springs, SE OregonZOOM OF DOQ

Problem: map a flat terrain and generate a cm level terrain map not feasible with airborne methods

DETAILED

AREA

470M

ZOOM of DOQ

Riegl Z360 mapping fairly flat surface

Actual examples of scans at MHS with RGB

channel so points are colored (not external

camera)

Scans 2, 3 and 4 are of detailed areas

Scans in southern area

Rotation of initial scan.

Note vegetation

Another perspective. Note shadows with

no points.

Perpendicular perspective

Example of merged scans

(reflectance image)

Color Version

Merged surface fit

Documents

Oldow TLS-Basics Calculations