Lecture 3 Vegetation and interaction with the earths surface Flashcards
What can happen when Electromagnetic radiation interacts with the Earth’s surface or an object?
- Reflected * Absorbed * Transmitted
when does * Reflected * Absorbed * Transmitted vary
with the spectral and structural properties of the object it interacts with
What is spectral reflectance
defined as the reflectance within a specified wavelength range (e.g. 0.3-0.4(micro m)
What does spectral reflectance depend on?
- Material type (e.g. vegetation, soil, water, metal)
- Nature of the surface Specular Smooth (no interaction) – very bright
Diffuse reflectance Rough surface – many directions of reflectance - Spectral wavelength
- Other factors (time, slope, condition etc)
How does time of day impact spectral reflectance
sun angle varies through the day as does sunlight that reaches the surface
How would dryness of the soil affect spectral reflectance?
dry soils are brighter – increase reflectance, wet soils would cause more absorption.
how does water reflectance change (think of it like a graph?)
reflectance stops at NIR as it is absorbed, most reflectance occurs in the visible part of the spectrum
How can different land types be established
Each object has a unique spectral property (reflection, absorption and transmission) which produce a unique spectral signature. Therefore as they reflect light differently
How would a forest change reflection
Structure of vegetation influence amount of energy reflected by the sensor. some areas are dark in shadow or bright in sunlight. the density of canopy.
photosynthesis equation
6co2 + 6H20 =+SUNLIGHT -> C6H1206 + 6CO2
Explain the importance of chlorophyll and wavelegths
Chloroplasts contain chlorophyll A and B which absorb energy int he blue and red wavelengths. When the leaf is healthy this energy is reflected in the green wavelengths.
What happens to leaves in Autumn
Chlorophyll in the leaf is decreasing, absorbs less blue and red and reflects more energy in the red wavelengths.
What happens to NIR and leaves
The cell structure - air spaces, NIR scattered around internal structure and measured by the sensor. Occurs at the upper half of the leaf and more importantly between cell and air spaces * High reflectance. Some is transmitted through the leaf but high reflectance. Leaf Maturity * Amount of airspace increases with level of maturity – reflectance increases with age.
SWIR AND leaves
SWIR – 1.2 microns onwards – controlled by amount of moisture in the leaf – as the leaves age the amount of chemicals and moisture in the leaf decrease reducing reflectance.
what is the red edge
boundary between the NIR and red wavelengths – often used to estimate chlorophyll content in leaves.
what is REP
Red edge position (REP) is the point of maximum slope of the curve in red/near infrared region.
what happens to the red edge in autumn
Autumn – position of red edge shifts further to the left – higher reflectance in red part of the spectrum.
SWIR and water relationship
Higher the moisture – lower the reflectance Amount of reflectance is inversely proportional to water content.
Three major water absorption bands * 1.4 m * 1.9 m * 2.7 m Atmosphere and Materials on the surface contain water Two minor water absorption bands * 0.96 m * 1.1 m.
What other factors effect spectral reflectance of vegetation?
- Soil background (i.e. beneath the canopy) – brightness eg sand higher reflectance.
- Solar (illumination) and sensor (viewing) geometry – most satellites take measurements at same time of day, if taken at an agle shadow can influence this – changing brightness.
- Vegetation 3D structure (height, size & shape of leaves, shape of the crown etc) mountainous region – one side is in shadow the other is in shadow.
- Phenology (lifecycle of plants (or animals)). Summer – higher reflectance as leaves at their healthiest.
Temporal characteristics of vegetation growth * Depends on:
- plant available water: rainfall/irrigation * temperature * vegetation type (evergreen vs. deciduous)
What can happen to radiation when it interacts with a surface?
Transmitted, reflected, or absorbed
Name 4 factors that influence the magnitude of spectral reflectance from a surface?
- type of material (vegetation, soil, concrete, metal) reflect differently which allows us to identify these materials.
- Nature of the surface (e.g. surface 3D structure) – specular (smooth) – for example on still water – lots of reflectance in one direction Diffuse (rough)
- Spectral wavelength range
- Other factors – time of day, the slope, mountainous areas some ins hade some in light, material can change, raining vs dry soil.
