Unit 6

Question 1

What is LIDAR?

Answer 1

ACTIVE optical remote sensing for terrain mapping

Question 2

What is a major benefit of LIDAR?

Answer 2

it works reasonably well under dense vegetation cover (only terrain mapping tech that can do this)

Question 3

How is laser light unique?

Answer 3

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)

Question 4

How is laser light created?

Answer 4

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

Question 5

How does a pulse of LIDAR work?

Answer 5

• 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

Question 6

why is LIDAR a ranging device?

Answer 6

it is used to measure the distance of an object from a sensor, not just to image the object

Question 7

What is the difference between discrete return and full waveform LIDAR?

Answer 7

DR a number of returns are recorded for each laser pulse

FW a continuous profile of returned energy intensity is recorded for each pulse

Question 8

what are the benefits of DR?

Answer 8

it is cheaper, less data to store, and therefor easier to process

Question 9

what are the downsides for FW?

Answer 9

can be challenging to store and process the data

Question 10

What is a new replacement to FW and why?

Answer 10

Single-photon LIDAR, the detectors are more sensitive and require less power

Question 11

What are the pros and cons of SPL?

Answer 11

Pro: higher sensitivity gives a larger field of view and coverage
Con: only efficient in green wavelength whereas NIR is better for vegetation mapping

Question 12

What is not a good storage method for LIDAR and why?

Answer 12

ASCII limits the amount of information we can store and only gives us the point (x,y,z) coordinates

Question 13

What is a better file storage options for LIDAR and why?

Answer 13

LAS files are better because they give additional information
-scan angle, intensity, GPS time, point classification etc.

Question 14

How is scanning LIDAR used?

Answer 14

• topographic surveys mainly digital elevation models (DEM)

- forest canopy and vertical structure mapping/modelling

Question 15

why is scanning lidar beneficial for DEMs?

Answer 15

each point is individually georeferenced therefore no orthorecitfiying is necessary

Question 16

What is a ground-based LIDAR method?

Answer 16

Terrestrial laser scanning (TLS)

Question 17

How does TLS work and what are its applications?

Answer 17

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

Question 18

Compare airborne laser scanning and spaceborne laser scanning DEMs

Answer 18

ALS are much finer since they are closer to the earth’s surface but do not cover the same geographical area

Question 19

What is the primary use of ALS?

Answer 19

Digital elevation models (DEMs) and Canopy Height Models (CHMs)

Question 20

what is a benefit and limit of TLS

Answer 20

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

Question 21

what is the consequence of a wider beam for ALS?

Answer 21

lower pulse density and therefore lower resolution

Question 22

how is the scan angle related to resolution?

Answer 22

The scan angle determines the total field of view

Question 23

What are some spaceborne LIDAR missions?

Answer 23

Orbital = GLAS (single beam), Atlas (single-photon), GEDI (full waveform)
Also for scanning Mars (MOLA), Mercury (MLA) and the moon (LOLA)