Remote Sensing: Lecture 20-22

Question 1

What three bands are needed to display a multispectral image in natural colour?

Answer 1

Red, green and blue

Question 2

What are the characteristics of water, healthy vegetation, and unhealthy vegetation when observed in visible and near infrared wavelengths?

Answer 2

Visible:
* Water: Blue, green or brown
* Healthy veg: green
* Unhealthy veg: brown
IR
* Water: dark, absobs NIR
* Healthy veg: bright, reflects a lot of NIR
* Unhealthy veg: darker than healthy as less NIR is absorbed

Question 3

Which two bands are used to calculate the Normalised Difference Vegetation Index?

Answer 3

Red and infrared bands

Question 4

What are typical NDVI values for different cover types, such as water, soil, sparse/unhealthy vegetation, and dense/healthy vegetation?

Answer 4

NDVI always lies between -1 and 1.
* water: close to zero (absorbs most of the near-infrared light and reflects very little)
* soil: low values (low reflectance but more than water)
* unhealthy veg: low to moderate values (lower than healthy, higher than soil)
* healthy veg: high values - high reflectance

Question 5

Why is it difficult to fully automate the detection of illegal tree clearing using satellite imagery?

Answer 5

There is a lot of time put into preprocessing such as normalising effects (e.g. cloud or shadow). There has to be a manual checking phase on-site to correct things missed

Question 6

In an NDVI time series, what can cause a temporary drop in NDVI, and what can cause a persistent drop in NDVI?

Answer 6

A temporary drop may be due to cloud coverage whereas a longterm drop indicates that vegetation has been removed or became unhealthy

Question 7

What are the advantages and disadvantages of using satellite remote sensing to monitor water volume in lakes?

Answer 7

Advantages:
* Landsat has been orbiting for many years and has a lot of data that can be used (e.g. in sea level rise)
* Wide Coverage: Satellite remote sensing can cover large areas, allowing for the monitoring of multiple lakes simultaneously, which is particularly useful in remote or inaccessible regions
* Cost-Effective: Compared to ground-based monitoring, satellite remote sensing can be more cost-effective, especially for large-scale studies, as it reduces the need for extensive fieldwork

Disadvantages:
* Resolution Limitations: The spatial resolution of satellite images may not be sufficient to capture small lakes or detailed features, potentially leading to inaccuracies in volume estimates
* Cloud Cover Interference: Weather conditions, such as cloud cover, can obstruct satellite imagery, making it difficult to obtain clear images during critical monitoring periods
* Depth Variability: Satellite measurements typically assess surface area rather than volume directly, and the relationship between surface area and volume can vary significantly among lakes, complicating volume estimations