Exam prep Flashcards
What is remote sensing?
The science of obtaining info about a feature without being in contact with it… by measuring electromagnetic energy
What are the 4 types of remote sensing equipment?
- Optical
- Thermal
- Radar - synthetic Aperture radar (SAR)
- Laser scanning - Light Detecting and Ranging (LiDAR)
Explain the magnetic spectrum
Radio waves (longest, 10 ^ 8)
Microwaves
Visible light
Infrared
Ultraviolet
X-ray
Gamma rays (shortest 10 ^ -18)
What is the amplitude of a wave?
The distance from a wave’s midpoint to its crest or trough
Describe wavelength
The distance between crest. Measured in meters
Explain the wave frequency
The number of crests passing a given point in 1 second
Name for 4 most common Earth Observation sensors
Landsat
MODIS
Sentinel-2
Planet
Define Landsat
- Has data from 1970s to present day
- Free to access
- Spatial resolution of 30 m
Define MODIS
- Launched in 02 and currrently being taken out of service
- Can use different spatial resolutions (50m ,500m, 1000m)
- Has daily global coverage
Define Sentinel-2
- European complement to Landsat
- 2 satellites work together 2A and 2B
- Spatial resolution of 10 and 20 m
What is Spatial Resolution?
Is the amount of area an image pixel represents
What is Temporal resolution?
How often measurements are made
What is Spectral Resolution?
- The number of wavelengths being measured
- The area of the electromagnetic spectrum being measured
What is Radiometric Resolution?
A measure of how well a sensor can detect and differentiate between different levels of radiation emitted or reflected
What applications for optical sensing?
- Land Cover Mapping
- Land Cover Change
Problems with Optical sensing?
- Cloud coverage
- Interference with the earth’s atmosphere
What is Thermal Sensing?
- Similar to optical, but measures temperature
- Has a lower spatial resolution as there is less energy to measure
Where on the electromagnetic spectrum is Thermal sensing?
From Microwaves to Infrared
What is the purpose of Thermal remote sensing?
To measure
Evapotranspiration
Heat hot spots in cities
Volcanics - areas which are hotter
Glacial mapping - areas which are colder
What is Radar?
- An active remote-sensing instrument
- It can see through clouds and work in the dark
Where is radar on the electromagnetic spectrum?
Between Microwaves and Infrared
Name Applications of Radar Data for Earth Observation?
- Mapping water (flood events)
- Mapping oil spills
- Mapping sea ice extent
- Mapping land cover change
What is LiDAR?
stands for “Light Detection and Ranging”
It creates and measures a 3D image of the landscape
What can LiDAR be used for?
- To create Digital surface models
- Create Digital terrain models
- To map tree canopy cover (forest density)
- To model hydrological events
Why are Tropical Wetlands important?
Wetlands represent 1/3rd of global methane emissions
Methane emissions will double from wetlands due to climate change
methane has 80% more warming power than C02
Name one tropical Wetland
The Sudd, In South Sudan
What is the Spectral curve?
The spectral curve is a graph that compares Wavelength nm (X axis) and reflection % (Y axis).
What equation is used to work out the NDVI (Normalised Difference vegetation Index)?
NDVI = NIR - R / NIR + R
NIR - Near Infrared
R Red
Vegetation range is -1 to 1, values > 0.5 tend to be green vegetation
What 2 Landsat bands are best for measuring water?
Green and SWIR as they will have the maximum difference over water
SWIR (Short Wave Infrared)
What equation is used to work out the mNDWI (Modified Normalised Difference Water Index)?
mNDWI = G - SWIR1 / G + SWIR1
SWIR - Short Wave Infrared
used for mapping water turbidity (water quality)
What equation is used to work out the NDBI (Normalised Difference Built-up Index)?
NDBI = SWIR1 - NIR / SWIR1 + NIR
Designed for urban mapping, it is also useful for mapping bare soil and sand
What 2 Landsat bands are best for identifying soil?
Near Infrared and Shortwave
used to avoid the ‘noisy’ visible part of the spectrum
Define land cover
The physical material covering the surface
Define land use
When the land is modified or managed by humans for benefit
example; creating a settlement from a woodland
What are the 3 types of classifiers?
Unsupervised
Supervised
Rule-based
What is an unsupervised classification?
Has little to no input for the user
Great for data exploration
automatically groups image pixels based on spectral similarity
What is supervised classification?
When you manually class your pixel groups
manually produce training data that represents different classes
using pc algorithm to align/classify unknown pixels
What is training data and why is it important?
It is a set of manually chosen points to determine classes, in which the pc algorithm will class similar pixels to
it is a laborious process
You may need to split some classes, for example forest - deciduous and conifer forest would be differently classed
rf, dt etc
What are the 7 different classifier methods?
Parallelpiped
Minimum Distance
Mahalanobis
Gaussian Maximum
Decision tree
Random forests
Rule-based
what is the parallelepiped classification?
The unkown pixels are classified according to the range in which it lies
It is a simple method of classification
if they do not fit any training data sets, they have left unclassified
What is the minimum Distance classifier?
All pixels will be closest to one or other of class, means there are no unclassified pixels
can lead to falsely unclassified pixels
Define the Mahalanobis distance classifier
Similar to the minimum distance classifier, except it has class covariance
This allows for class variability
Define the Gaussian Maximum Likelihood classifier
It assumes the training data is correct, therefore unkown pixels can be assigned to classes based on the probability or likelihood of tgem belonging to a particular class
What is a decison tree?
A form of classification that creates a series of rules to separate different parts of the feature space
it is easy to understand and see the results
What is a random forest classification?
It uses multiple decision trees
One of the most popular methods for remote sensing
randomly subsets the training data and data variables
What is a Rule based classification?
When you manually define a series of rules to classify the pixels
It can be repeated automatically
Requires thresholds
very basic, but very effective
can be time-consuming and complex
Difference between using pixels or objects/segments for classification
Objects contain a lot of info
more efficient - few data points
produces better results but you lose spatial precision
Why are forests important?
About 1/3rd of the world is covered by forest, 4-5 billion hectares of forests
Accounts for 50% of plant productivity, and 45% of the carbon stored is in forests
1/3rd population relies on forests and forest products
Tropical rainforests are home to the most species per acre
What are the 4 types of forest?
Temperate Deciduous
Boreal Evergreen (confier)
Savanna
Tropical Rainforest
What are the 6 forest types?
Montane forest
Lowland forest
Mangrove forest
Closed woodland
Open woodland
Thicket
What is a Montane forest, and where can it be found?
A forest found on mountain slopes
Typically found in high-elevation areas, eg Aples or the Rockies (between 500-4000m above sea level)
What is a Lowland forest, and where can it be found?
A forest that thrives on flat, generally found below 1000m
Home to a wide range of animals, it has great biodiversity
Found in the tropical belt, warm climate, lots of rainfall
What is a mangrove forest, and where can it be found?
Found between the salt marsh and intertidal zones
Like a silt-rich environments
can be found in salt or fresh water conditions
It can be a good natural way of coastal protection and stores carbon
What is the difference between open and closed woodland?
Open woodlands have a tree coverage of less than 30%
Closed woodlands have a tree coverage of at least 40%
What is a thicket, and where can it be found?
A dense ggrowth of small trees or shrubs
Can be found anywhere, e.g., corners of fields, embankments
What is an error matrix, and why is it important for remote sensing?
An error matrix is the clearest way to show errors
Uses true positive/ negatives and false positives/negatives to work out the error basis
Why are mangroves important?
They provide coastal protection
Carbon-rich storage
Food production - fish, honey etc
A wide biodiversity, fish and insects/birds call it their homes
Why are mangroves under threat?
Coastal development
Logging - mangrove wood is highly demanded
Marine pollution
Climate change - rise in sea level and temp
What is Biomass?
the dry weight of plant material
Measured in Mg ha-1 (Milligrams per hectare)
Why is biomass of interest?
It stores a lot of carbon from plant matter
If left alone, it won’t release CO2, as it is stored
How to estimate Above Ground Biomass?
By measuring the trunk diameter, all the branch thicknesses and recording the foliage
- small branches - < 7 cm diameter
- larger/coarse branches - > 7 cm diameter
- Stemwood - top of tree trunk, < 7 cm diameter
- and the tree trunk from the stump upwards
Ways of sampling biomass?
Using destructive methods like weighing before and after it has been dried
Terrestrial laser Scanning: non-destructive, allows for more trees to be measured at a single time, takes detailed 3D images of the trees
The importance of glaciers
They contain 70% of the total freshwater
Glaciers are very sensitive to climate change, and are key indicators of change
They have high albedo, meaning their recession contributes to higher surface temps
As ice melts, the sea levels continue to rise, impacting 2 billion ppl within 100km of the sea
Why is remote sensing used to monitor glaciers?
They are difficult to get to
To monitor change in ice area
To record a change in the ice volume by looking at ice thickness and temperatures
The glacial dynamics - fast-flowing ice?
The surface characteristics, change in albedo
To spot potential hazards, eg avalanche or glacial lake flooding
