Topic 5: LiDAR Flashcards
what does LiDAR stand for
Light detection and Ranging
what is an INS
Inertial navigation system
- measures acceleration and gyroscope of device.
LiDAR Data
X,y,z
- intensity
what is LITE
LiDAR in space technology experiment
first detailed view of vertical structure of cloud and aerosol form surface through middle atmsophere
how is data typically collected in lidar
sensors are flown on aerial platforms
what are some applications of LiDAR
DTM
Geology
Forestry
Oceanography
Glacier mapping
what is a Laser
focus coherent beams of light energy with very little divergence.
how does LiDAR use lasers
sends pulses of light to a target and the reflected light which returns is measured
single return
if all the incidence energy is reflected from a solid object
multiple returns
when some light passes through canopy and hits multiple objects resulting in multiple returns. Similar to Volumetric
what is PRF
pulse repetition frequency
has been increasing with advancements in technology
increased PRF results in greater accuracy
what is a DTM
Digital terrain model shows elevation of the bare earth
what is a DEM
digital elevation models are used to create DTM and DSM, database containing elevation points
what is a DSM
digital surface model showing the elevation of all features on a landscape. contains trees, buildings, and other structures above ground
methods of creating DEM’s
- in-situ surverying (expensive)
- interferometric SAR (errors with veg)
Photogrammetry (less accurate than LiDAR)
-LiDAR
what are the two main wavelengths of LiDAR
Infared
blue-green
Blue-green wavelength uses
bathymetric
NIR wavelength uses in LiDAR
terrestrial topographic mapping
discrete return LiDAR system
records X,Y,Z and intensity
intensity from amplitude of returned signal
how does multiple returns work
A signal can have multiple returns if it interacting with a complex surface (tree canopy) part of the single will be returned, resulting in lower intensity in each returns than if it was all one return.
discrete return LiDAR system
records multiple returns into descrete levels
intenisty of pulse range through multiple returns
what is one downside of multiple returns
increased the size of datasets significantly
Full Waveform LiDAR
records the entire waveform of the return laser pulse, very high precision
results in extremely high data volumes
two types of LiDAR sensors
1.Profiling
2. imaging
they differ in how swath data is collected
Profiling Lidar
Nadir facing. an echo profile along the flight path of sensor recorded
Imaging LiDAR
directing pulses over a swath beneath the platform traveling
- extended echo profile
- mos common in areal LiDAR systems
are spaceborne LiDARS Profiling or Imaging?
Profiling, like
swath width, vs footprint
swath with is the width of the footprint. footprint usually oval.
how do height and swath width relate
greater heigh, greater swath width. opposite true
how is footprint measured
diameter
how does height and footprint relate
greater height, greater footprint. OT. based on divergence
rough footprint size of airborne Lidar
0.2m - 0.9m
rough footprint size of spaceborne LiDAR
13m
swath pattern is determined by…
scanning mechanism
- profiling
- imaging
also a function of point density
what are the three main data processing techniques for LiDAR
-filtering
-classification
- interpretation
what is LiDAR FIltering
removing unwanted data, removal of partial returns in order to keep first and last returns
what does a first return represent
DSM
what does a Last return represent
DTM
what the difference between first and last return represent
Height Model (eg. CHM)
what is a filtering algorthrim
something that is used to separate point clouds
what is a CHM
canopy Height model (DSM-DTM)
what is the goal of classsification
-find specific structure after filtering
-appling class labels
LiDAR classification can be based on..
elevation, slope or intensity
ways to enhance LiDAR imagery
combine it with Optical to improve interpretation
interpolation
used to fill in between areas of no data for DTM
what is the goal of interpolation
to correct a smooth, accurate continuous dataset from descrete points
what are the two common methods for Interpolation
- inverse distance weighting
- Spline
- other methods are based on nearest Neighbour
Inverse Distance Weighting (IDW)
interpolation method
weights are assigned to to known values within a neighbourhood
- useful for locationally dependant vairables
-trends not accounted for,
Can create Duck Egg pattern
Spline
an interpolation method
- piece-wise
- smooth cuves are locally fitted to set a data points using polynomial functions
- quick DEM creation
- small scale features retained
- creates an uncharcteristically smooth surface
LAS file format
standard for storing point cloud data
benefits of LAS file format
available software
datasets easily transferable
eliminates need for proprietary software
what are three wavelength dependent applications of LiDAR
- atmospheric applications
- bathymetry
- vegetation surface
differential measurement of LiDAR
different LiDAR wavelengths are measured to note differneces
what is Differential Absorption LiDAR (DIAL)
where intensity of returns at different wavelengths are used to Infer Atmospheric Properties.
NASA’s Airborne topographic mapper is used for
measuring thinning ice sheets
what is 3DEP
3D Elevation Profile
- generates LiDAR PC and DEMs
what is Terrestrial Laser scanner
ground based version of a Airborne LiDAR
- short ranged scanning
- photorealistic 3D mapping
