Lidar

passive vs active remote sensing

passive - sensors that measure and record electromagnetic energy reflected or emitted by the surface

active - sensors that generate energy, emit it towards a target and measure the return value

what are examples of active remote sensing

lidar and radar

benefit of active remote sensing over passive

active does not require the sun to operate but passive does

what does LIDAR stand for

Light

Detection

And

Ranging

what does lidar use to measure the distance to the target

light in the form of a laser pulse

what is created by lidar

a 3D point cloud based on the time between laser emission and return

fixed position vs mobile lidar

fixed - camera set on a tripod that can rotate but does NOT move from the spot

mobile - camera set on a vehicle (boat, van , car…)

what is a pulse made of

burst of light energy

what is the return

reflected light energy recorded by the sensor

can a laser pulse have only a single return

NO - can have multiple returns for a single laser pulse

what is the critical parameter when measuring the distance of the reflection

TIME

what is a footprint referring to

the size of the laser sampling area co

contrast large vs small footprints

small - footprint between 10 - 90 cm in diameter from an airborne LIDAR sensor

large - footprint between 10 - 25 m in diameter from satellite platforms

what footprint has high resolution? low resolution?

high resolution = small footprint

low resolution = large footprint

what footprint has more detail? what footprint has a general detail?

more detail = small footprint

less detail = large footprint

what is waveform

the distribution of light energy returned to the LIDAR sensor

contrast the two types of waveform

full - the full distribution of returned energy to the sensor (continuous measuring return energy)

discrete - identification and recording of peaks in return energy

how many discrete returns can each pulse have

1 to 4

when are there more ground returns, leaves with trees vs without?

without trees = more ground return

what are the four LIDAR components

Differential GNSS

Inertial Measurement unit

scanner

computer processing system

what is differential Global Navigation satellite system (GNSS)

return locations are based on speed and timing of the laser pulse from emission to reception

what must be known of the LIDAR system for GNSS

precise position of LIDAR

what is Inertial Measurement unit (IMU)

electronic device that measures and reports the unit’s angular rate, orientation and acceleration

what must IMU be close to

the sensor origin

what does IMU detect changes in

roll, pitch, yaw

what are the three types of scanning mechanism

1. oscillating mirror

2. rotating polygon

   1. palmer scan

what is the most common scanning mechanism and the resulting ground pattern

oscillating mirror mechanism = sawtooth ground pattern

what can the oscillating mirror scanning mechanism scan up to

30 degrees

how much do flightlines overlap using oscillating mirror

about 30%

what are the four primary measurements integrated by a time stamp (point light returned)

1. range

2. scan angle

3. orientation

4. position

what are 4 sources of error for LIDAR

1. atmospheric interference

2. shifts/drifts in aircraft position

3. scan angle

   1. terrain

how can scan angle error be fixed

scan lines have to be cut

what causes terrain errors

complex terrain that returns don’t capture variation in the footprint

how can shift errors be fixed

using GCP points to adjust the point cloud

what is atmospheric interference

energy returned by particles in the atmosphere

what are data products of LIDAR

1. classified point clouds

2. DEM

3. DSM

   1. CHM

what are digital surface models

Uses rasters to measure ground elevation heights WITH the objects on the ground

what are canopy height models

DSM - DEM

what is the MOST common laser wavelength

1064 nm

what surfaces have high reflectance with 1064 nm wavelength? low?

high reflectance from snow, vegetation, dry soil and litter

low reflectance from water

what is a limit of using 1064 nm wavelengths

hazardous to the eyes so limited in when can be used

what is a result of using a laser with 1064 nm wavelengths

large background noise due to spectral irradiance from sun

what are the three main laser wavelengths

1064 nm (NIR)

532 nm (green)

1550 nm (SWIR)

what wavelength does green light have

532 nm

what wavelength is commonly combined with 1064 nm

green (532 nm)

when are green and NIR wavelengths combined

for bathymetry

contrast absorption and reflection of NIR and green light in water

NIR is reflected by the surface in small amounts or absorbed (not often absorbed = maps water surface)

green has a higher transmission in water and reflects the water body’s bottom (maps water bottom)

what is bathymetry

\measurement of the underwater topography of bodies of water

when are SWIR lasers used

civil and commercial LIDAR systems

what is safer on the eyes; SWIR or NIR

SWIR

does SWIR have high or low atmospheric transmission

SWIR

describe the effects of SWIR absorption in water

strong water absorption that reduces reflectance from objects like snow and vegetation

what does multispectral LIDAR combine

combination of structural and spectral measurements