3D Point Clouds In Remote Sensing Flashcards
What is a point cloud
Point clouds are datasets that represent objects or space
What are the 2 main techniques for point cloud acquisition?
Active techniques: LiDAR, Range camera
Passive technique: Multi view stereo(MVS)
Structure from motion (SFM)
What is the difference between ACTiVE and passive techniques
In active technique, the sensor send signal to the object which reflects the signal back to the sensor
In passive, instead of single images, we use multiple images from different perspective point. Based on these different points observations,points cloud are created.
What is Lidar?
Light detection and Ranging
This is a remote sensing method that uses laser to measure elevation like ground forest and buildings.
Uses ultraviolet visible infrared source to send objects
2 ways to collect LiDAR data
1) Airborne
- Topographic LIDAR: NIR Laser (900-1064nm)
- Bathymetric LIDAR: Combination of Green and NIR wavelength
2) Terrestial:
- Static
- Mobile
What Are the main component of airborne LIDAR System?
1) LIDAR unit(in the airplane) : used laser to scan the earth from side to side
2) GPS Receiver: Tracks altitude and x,y,z Location of the airplane.
3) Inertial Measurement unit (IMU)
Tracks the tilt of the plane in the sky as it flies which is important for accurate elevation calculations
4) Computer: Records data as the LIDAR scans the Earth surface are
Difference between Natural light and laser light.
- Natural Light
- Emitted in random directions
- Variable wavelength
- variable amplitude
- No phase correspondence
- Laser Light
- emitted in a single direction
- constant wavelength
- Constant amplitude
- Total Phase correspondence
How do we calculate distance in LIDAR
Distance(P) = c X (t/2)
Where C is speed of light
T is duration
Different systems are used when it comes to airborne LIDAR.
1) Discrete Return LIDAR system: records individual points for the peaks in the waveform curve.
Discrete return identify peaks and record a point at each peak location in the waveform curve.
A fullwaveform LIDAR system records a Distribution of returned light energy.
They are more complex to process however they capture more info compared to LIDAR
Difference between Digital Surface model (DSM) and Digital Terrain Model (DTM)?
DSM (useful for landscape modeling, city modeling etc
DTM (required for flood, drainage model
Question
1) Digital Elevation Model
2) Vegetation Model
- Canopy height model
3) Other data models
- intensity image
- surface roughness
- water Flow map
Application of LiDAR
1) Agric: Biomass modeling
2) Forestry:carbon mapping
3) Urban:3D city modeling
4) Hydrology: Surface roughness mapping
5) Geology: digital modeling of crops
Explain passive techniques
In passive Technik, we don’t directly measure. We take pictures and convert to point cloud.
Stereo photogrammetry vs SFM
Stereophotogrammetry involves estimating the 3D coordinates of points on an object, employing measurements made in two or more photographic images taken from different positions. The image is calculated from a collection of points obtained along an x, y, and z coordinate system.
Difference between photogrammetry and SFM is that in photogrammetry, we use 2 images while in SFM we use several images showing the same location.
It has multiple perspective points
SFM Techniques includes
Multiple overlapping images
Step 1: identify and match 3 D locations
(Aligned images)
Step 2: Bundle adjustment (sparse point cloud)
Step 3: multi view stereo (3D Dense point Cloud)
Step 4:geo rectification ( dense point cloud with real coordinates
