Unit 6 Flashcards
What is LIDAR?
ACTIVE optical remote sensing for terrain mapping
What is a major benefit of LIDAR?
it works reasonably well under dense vegetation cover (only terrain mapping tech that can do this)
How is laser light unique?
is a beam of a pure single wavelength, that is highly focused (does not diverge), and is coherent (light waves in phase with one antoher)
How is laser light created?
E- are excited in a specific material by an electric current or other EM energy, once they return to their ground state, they emit energy with a specific wavelength
How does a pulse of LIDAR work?
- laser generates a pulse
- pulse reflected off object and returned to sensor
- time from start to return of pulse and intensity is measured
- time is converted to distance
- the distance to target and the position + orientation fo the aircraft is used to determine elevation and location
why is LIDAR a ranging device?
it is used to measure the distance of an object from a sensor, not just to image the object
What is the difference between discrete return and full waveform LIDAR?
DR a number of returns are recorded for each laser pulse
FW a continuous profile of returned energy intensity is recorded for each pulse
what are the benefits of DR?
it is cheaper, less data to store, and therefor easier to process
what are the downsides for FW?
can be challenging to store and process the data
What is a new replacement to FW and why?
Single-photon LIDAR, the detectors are more sensitive and require less power
What are the pros and cons of SPL?
Pro: higher sensitivity gives a larger field of view and coverage
Con: only efficient in green wavelength whereas NIR is better for vegetation mapping
What is not a good storage method for LIDAR and why?
ASCII limits the amount of information we can store and only gives us the point (x,y,z) coordinates
What is a better file storage options for LIDAR and why?
LAS files are better because they give additional information
-scan angle, intensity, GPS time, point classification etc.
How is scanning LIDAR used?
- topographic surveys mainly digital elevation models (DEM)
- forest canopy and vertical structure mapping/modelling
why is scanning lidar beneficial for DEMs?
each point is individually georeferenced therefore no orthorecitfiying is necessary
What is a ground-based LIDAR method?
Terrestrial laser scanning (TLS)
How does TLS work and what are its applications?
creates vertical terrestrial profiles/cross-sections along a line of site
scans along an azimuth axis = horizontal angular step
ex. forest inventory, land surveys, and self driving cars
Compare airborne laser scanning and spaceborne laser scanning DEMs
ALS are much finer since they are closer to the earth’s surface but do not cover the same geographical area
What is the primary use of ALS?
Digital elevation models (DEMs) and Canopy Height Models (CHMs)
what is a benefit and limit of TLS
benefit = not limitations from top of the canopy limit = less point density as you move away from the scanner due to angular step = less information
what is the consequence of a wider beam for ALS?
lower pulse density and therefore lower resolution
how is the scan angle related to resolution?
The scan angle determines the total field of view
What are some spaceborne LIDAR missions?
Orbital = GLAS (single beam), Atlas (single-photon), GEDI (full waveform)
Also for scanning Mars (MOLA), Mercury (MLA) and the moon (LOLA)
