GEOG 319 Passive Remote Sensing

UVIC Fall 2025.

Passive Remote Sensing

Measures reflected or emitted radiation, unlike ctive sensors which send out signals and measure backscatter. Uses the Sun’s light to sense objects.

Spectral Resolution

The number nd dimension of specific wavelength intervals in the electromagnetic spectrum to which a remote sensing instrument is sensitive. Defined by the number of bands and bandwidth.

Hyperspectral

Continuous bands, where each pixel has a complete spectrum.

Multispectral

Discrete bands, where each pixel has a discretely sampled spectrum.

Spatial Resolution

A measure of the smallest separation between two objects that can be solved by the sensor. Determined by IFOV and altitude.

Temporal Resolution

How often the sensor records imagery of a particular area. Depends on orbit, swath, sensor configuration, satellite constellation.

Pixel Mixing

When the reflectance of different surfaces is averaged in one pixel.

Radiometric Resolution

Sensitivity of a remote sensing detector to differences in signal strength as it records the energy reflected or emitted from the terrain. Defines the number of discriminable signal levels.

Nadir

Facing directly down 90 degrees.

Refraction

The bending of light as it passes from one medium to another. Caused by a change in speed of the electromagnetic wave between medium of different densities.

Scattering

Radiation scatters depending on the wavelength of light.

Mie Scattering

Prticles are similar in size to the wavelength, thus influencing longer wavelengths (red) in the lower atmosphere. Causes wildfire smoke sun, dust, etc.

Rayleigh Scattering

Occurs in the upper atmosphere, when particles are smaller than the wavelength of light. Gives the atmosphere its blue colour.

Non-selective Scattering

Particles are much larger than the wavelength. Occurs in the lower atmosphere. Scatters all light, examples include water droplets, fog, clouds.

Reflection

When wavelengths of energy are reflected in quasi-predictable directions.

Specular Reflection

Also called glint. All radiation is reflected at the same angle opposite direction.

Lambertian Scattering

Radiation is reflected equally in all directions.

Near-perfect Diffuse Reflector

Radiation is reflected in preferential directions.

Absorption

Energy is absorbed and converted into other forms of energy.

Why are different types of light absorbed?

Infrared light is absorbed due to the vibrational state of molecules. Visible and UV light is absorbed due to changes in the electronic state of an atom.

Atmospheric Components that absorb EM radiation

O3, O2, CO2, H2O, N2O, Aerosols

Atmospheric Windows

Parts of the electromagnetic spectrum that transmit radiant energy effectively (are not absorbed by Earth’s atmosphere) and can therefore be detected by sensors.

Radiance

Radiant flux per unit of solid angle leaving an extended source in a given direction per unit of projected area (W / m2sr)

When do we need radiometric/atmospheric correction?

Multi-temporal data, biophysical parameters are being estimated, classification

Irradiance

Incident radiant flux coming at 180 degrees

Path Radiance

The at sensor radiance component that does not originate from the target.

Reflected radiance from atmospheric scattering + adjacency effect/contaminated ground reflectance (reflected from neighbour objects).

Why is the Photosynthetically Active Radiation range 400-700 nm?

Photons at shorter wavelengths (UV) too energetic and damage cells, and are mainly absorbed by atmospheric ozone. Photons at much longer wavelengths do not carry enough energy to fuel photosynthesis.

Factors affecting light absorption by vegetation

• Pigment composition

• leaf structure

• water content

• age

• nutrient stress

• healthiness

• background

What is the blue shift of the red edge?

The sharp increase from red to NIR reflectance in healthy leaves - with pigmentation changes, this edge is shifted towards the blue range.

Why do healthy mature leaves absorb blue and red and reflect NIR?

• chlorophyll a > photosynthesis absorbs blue and red very efficiently

• high reflectance in NIR because it denatures proteins

What part of the plant structure reflects or transmits NIR?

Spongy mesophyll

What part of the electromagnetic spectrum is affected by plant turgidity?

Middle infrared, increase in turgidity = low MWIR reflectance

How does Mountain Pine beetle affect plants?

Mountain pine beetle infestations alter tree physiology, causing chlorophyll loss, needle drying, and water stress—all detectable in spectral data. Affected trees show increased red and SWIR reflectance (from pigment and moisture loss) and decreased NIR reflectance (from structural damage). Key indicators include drops in NDVI and shifts in the red edge (~0.7 µm), allowing detection of infestation stages via sensors like Landsat and Sentinel-2.

LAI

Leaf Area Index - total one-sided area of leaf tissue per unit ground surface.

NDVI

The Normalized Difference Vegetation Index (NDVI) measures plant health using sunlight reflected in the red (0.6–0.7 µm) and near-infrared (0.7–1.3 µm) bands captured by passive sensors. Healthy vegetation absorbs red light for photosynthesis and strongly reflects NIR, giving high NDVI values (≈0.6–1). Stressed or sparse vegetation reflects more red and less NIR, producing low NDVI values (≈0–0.3), making NDVI a key indicator of vegetation vigor and canopy density.

Total Radiance for Water Remote Sensing

Lt = Lp + Ls + Lv + Lb

Lt = at sensor radiance

Lp = path radiance

Ls = water surface radiance

Lv = volumetric subsurface radiance

Lb = bottom radiance

What is Lv?

Subsurface volumetric radiance. Also called water column radiance, related to water colour. The radiance that penetrates the air-water interface, interacts with the organic/inorganic constituents in the water, and then exits the water column without encountering the bottom.

What does Lv depend on?

Light penetration (irradiance) and water optical constitents (attenuation = absorption and scattering)

Euphotic Zone

The sunlit, uppermost part of the body of water. Where photosynthesis can occur.

Water Optical Constituents for Coastal and Inland waters

Water itself, colour-dissolved organic material (CDOM), phytoplankton/pigments, inorganic material. These all affect absorption and scattering of light.

Oceanic Water Optical Constituents

Water and phytoplankton. These all affect absorption and scattering of light. Phytoplankton cannot thrive as there is a lack of nutrients (little river input, open ocean) so usually clear.

Water Absorption Properties

Water is a very strong absorber of light, and therefore reflects very small amounts of light. Strongly absorbs UV and IR light. Lowest absorption and scattering centered around blue visible light.

What is the difficulty in relying on blue signal for water mapping?

Blue light penetrates water best, but is also the most influenced by the atmosphere due to rayleigh scattering. Also has low overall reflectance, and up to 90% of the signal measured may come from the atmosphere not the ocean. Atmospheric correction mandatory.

Sources of CDOM

1. Decomposed phytoplankton (autochthonous)

2. Decomposed terrestrial organic matter (allochthonous)

All this decomposition introduces dissolved organic matter (DOM) into oceanic, near-shore, and inland water bodies.

How to measure CDOM?

1. Collect sample

2. Filter with a membrane

3. Measure filtrate using spectrophotometer

How does CDOM affect water?

1. impacts the absorption of light in the water column (visible light)

2. changes the colour of water to a tea-like appearance

How does inorganic suspended material affect water remote sensing?

Light scattering by suspended minerals is a function of quantity (concentration), size, and nature of the particles. As the concentration of particulates increase so does scattering.

Phytoplankton Pigments

Chlorophyll a, carotenoids (beta-carotene), and biliproteins (phycocyanin, phycoerythrin)

Fluorescence

The portion of the light absorbed by phytoplankton cells that is emitted at a longer wavelength. Water with a high concentration of phytoplankton exhibits fluorescence at 690 nm.

Cyanobacteria Effect on Water Remote Sensing

Contain pigments like chlorophyll-a and phycocyanin that change water’s spectral reflectance.

They increase reflectance in the green (∼560 nm) and near-infrared (∼700 nm) bands, while absorbing more in the blue and red regions.

Remote sensors use these spectral changes to detect and monitor algal blooms, estimate biomass, and assess water quality (e.g., turbidity, trophic state).

What is Lb?

The radiance that reaches the bottom of the waterbody, is reflected from it, and propagates back through the water column, and then exits the water column. Useful if information about the bottom is needed (e.g. depth, algae)

Optically Shallow

Enough light reaches the bottom to influence remote sensing data, allowing for depth and habitat mapping, and it is wavelength dependent.

Optically Deep

Bottom that is too far or too turbid and therefore not enough light reaches the bottom, making the seafloor invisible to sensors

UAV vs Satellite Tradeoffs

Spatial scale: cm resolution, but only at site level

Temporal scale: multiple flights a day vs few days/weeks - need to be taken in-person

What is Lp?

The portion of the radiance recorded by a remote sensing instrument resulting from the downwelling solar and sky radiation that never actually reaches the water surface. Atmospheric noise (path radiance).

What is Ls?

Sunglint - the radiance from downwelling solar and sky radiation that reaches the air-water interface and is reflected from the water surface. Contains valuable spectral information about the near-surface characteristics of the water body. If solar zenith angle and sensor viewing angle are almost identical then we may get a purely specular reflection from the surface of the water body.

Why does water appear blue in true colour composites?

Water appears blue because pure water absorbs longer wavelengths (red, orange, yellow, and infrared) much more strongly than shorter wavelengths.

Blue light (~450 nm) is absorbed the least and is scattered back out of the water column, making it the dominant wavelength detected by remote sensors.

This effect is most visible in deep, clear (Case 1) waters with few suspended or dissolved materials.

In contrast, suspended sediments or organic matter (CDOM) absorb blue light and increase reflectance in green or brown wavelengths, causing greenish or brownish water tones instead of blue.

What is the best wavelength for discriminating land from pure water?

Near-infrared and middle-infrared between 740 - 2500 nm. At these wavelengths water bodies appear black because they absorb all of the incident radiant flux, while land reflects signifianct NIR and MIR energy because of vegetation and bare soil and so appear bright in contrast.

Describe four characteristics of a spectral curve influenced by algae in water.

1. strong chlorophyll a absorption of blue light

2. strong chlorophyll a absorption of red light

3. reflectance maximum around 550 nm (green peak) caused by relatively lower absorption of green light by algae

4. prominent reflectance peak around 690 to 700 nm caused by an interaction of algal-cell scattering and a minimum combined effect of pigment and wter absorption

What makes it possibe to obtain relatively accurate water surface temperature measurements through remote sensing?

The emissivity of water is close to 1. The remote sensor radiant temperature measurement is therefore approximately equal to the true kinetic temperature assuming atmospheric effects are accounted for.

Describe the factors that cause the reflectance of maple leaves to vary between the visible and infrared wavelengths as they transition from summer to fall.

In summer, healthy maple leaves contain abundant chlorophyll a and b, which strongly absorb blue (∼450 nm) and red (∼670 nm) light for photosynthesis. Visible reflectance is low in those regions but higher in the green, giving leaves their green appearance. The spongy mesophyll layer, rich in air spaces, efficiently scatters near-infrared (NIR, 700–1300 nm) radiation, producing high NIR reflectance.

As autumn approaches, chlorophyll breaks down due to reduced sunlight and cooler temperatures. Carotenoids and anthocyanins become relatively more prominent, altering visible-band reflectance: absorption in blue and green decreases, while red reflectance increases, yielding yellow to red hues.

Leaf structure deteriorates—cells collapse, turgor pressure drops, and air-cell interfaces in the spongy mesophyll diminish. These changes reduce internal scattering and therefore lower NIR reflectance. Water content also declines, slightly increasing absorption in the short-wave infrared (SWIR) region.

Collectively, these processes shift the “red edge” (the steep rise between red and NIR reflectance) toward shorter wavelengths—a “blue shift”—because of reduced chlorophyll absorption and weakened mesophyll scattering. The spectrum thus transitions from low red / high NIR in summer to higher red / lower NIR in fall.

What are the challenges with using NDVI to compare across regions?

Soil colour, moisture content, atmospheric conditions, presence of dead material in the canopy itself, and NDVI saturation all change regionally and seasonally.

NOAA GOES

Geostationary Operational Environmental Satellite collects thermal infrared data at a spatial resolution of 8 km for weather prediction. Images obtained every 30 minutes night and day.

Radiant Temperature

the concentration of the amount of radiant flux exiting an object.

Basis of Thermal Remote Sensing

For most real world objects except glass and metal, there is a high positive correlation between the true kinetic temperature of an object and the amount of radiant flux from the object. We can utilize radiometers placed some distance from an object to measure its radiant temperature which correlates to its kinetic temperature.

Case 1 Water

Open ocean environments. Mostly clear water; main optical constituent is phytoplankton.

Case 2 Water

Coastal or inland waters. Contains multiple independent constituents: phytoplankton, suspended sediments, CDOM.

Wien’s Displacement Law

Describes the relationship between the true temperature of a blackbody (T) in degrees Kelvin and its peak spectral exitance (dominant wavelength).
lambda max = k/T

k = 2898 micrometers Kelvin

Use to determine the dominant wavelength of any object by substituting its temperature into this equation.

Stefan-Boltzann Law

The total spectral radiant exitance measured in Watts/m2 leaving a blackbody.

Mb = oT⁴

Why is knowing an object’s dominant wavelength important to thermal infrared remote sensing?

The dominant wavelength provides valuable information regarding the part of the thermal infrared spectrum in which we might want to sense the object. For example, if we are looking at 800K forest fires that have a dominant wavelength of 3.62 micrometers, then the most appropriate remote sensing system might be a 3-5 micrometer thermal infrared detector.

Emissivity

The ratio between the actual radiance emitted by a real world selective radiating body and a blackbody at the same thermodynamic temperature. All selectively radiating bodies have emissivities ranging from 0 to 1 that fluctuate depending on the wavelengths of energy being considered.

Why is it important to know about emissivity when conducting a thermal infrared remote sensing investigation?

The reason is that two objects lying next to one another on the ground could have the same true kinetic temperature but have different apparent temperatures when sensed by a thermal radiometer simply because their emissivities are different.

Factors that influence the emissivity of an object

• colour: darker coloured objects are better absorbers and emitters

• chemical composition

• surface roughness (greater surface roughness relative to size of incident wavelength = greater surface area and potential for absorption and reemission)

• moisture content

• compaction

• field of view

• wavelength

• viewing angle

1 = reflectance + emissivity

Relationship that describes why objects appear as they do on thermal infrared imagery. Because the terrain theoretically does not lose any incident energy to transmittance, all energy leaving the object must be accounted for by this relationship.

Why do metal objects appear cold in thermal infrared imagery?

Because metal has a very low emissivity since it reflects most of the incident energy and absorbs very little.

The radiant temperature of an object is related to its true kinetic temperature and emissivity via the following relationship:

T_rad = ε^1/4 T_kin

Thermal Capacity

The ability of a material to absorb heat energy. Measured by the quantity of heat required to raise the temperature of one gram of the material by 1 celsius.

Why does the temperature of a lake not vary much between night and day?

Because water has the largest heat capacity of any common substance (1.0), it takes a lot of energy to raise or lower its temperature. During the day, the lake absorbs heat without warming quickly, and at night it releases heat slowly. This gradual heating and cooling keep the lake’s temperature relatively stable over a 24-hour cycle.

Thermal Conductivity

A measure of the rate that a substance transfers heat through it. It is measured as the number of calories that will pass through 1 cm3 of material in 1 second when two opposite faces are maintained at 1 celsius difference in temperature.

Thermal Inertia

The thermal response of a material to temperature changes.

Diurnal Temperature Cycle

The 24 hour cycle of the sun. Beginning at sunrise, the Earth begins intercepting mainly short wave energy from the Sun. From dawn to dusk, the terrain intercepts the incoming short wave energy and reflects much of it back to the atmosphere, where we can use optical remote sensors to measure the reflected energy. After sunrise and near sunset thermal crossovers occur where some materials like soil, rock, water have the exact same radiant temperature so thermal remote sensing cannot be performed.

Why is NIR especially effective for identifying floating kelp?

Because kelp is a plant, it has a strong NIR reflectance peak similar to terrestrial vegetation, while water absorbs it—creating clear contrast. False-colour NIR imagery highlights kelp and other vegetation in bright red tones, aiding canopy detection.

Up to what depth can kelp be detected or grow, and why does this matter for remote sensing?

Kelp typically grows only to ~30 m depth. Beyond this, light availability drops and submerged kelp becomes undetectable in optical satellite imagery.

What problem does spectral unmixing solve in kelp mapping?

Spectral unmixing helps separate mixed pixels—common in medium-resolution imagery—where kelp, water, and other materials occupy the same pixel. It improves accuracy when canopy is patchy or partially submerged.

What factors influence the accuracy of kelp detection from space?

Species differences (maturity timing), tides (low tide preferred), waves, algae blooms and water debris, sun glint, coastal shadows, bed type, water clarity, temporal resolution, and spatial resolution

What roles can remote sensing play in the sensing of soils?

• can play a limited role in the identification, inventory, and mapping of surficial soils not covered with dense vegetation

• can provide information about rocks/minerals that are not completely covered by dense vegetation, or via geobotany

• can be used to extract geologic information including, lithology, structure, drainage patterns, and geomorphology (landforms)

Soil reflectance over exposed soil (no vegetation) is a result of…

Lt = Lp + Ls + Lv

soil texture, soil moisture, organic matter content, mineral composition

Soil Texture

percentage of sand, silt, and clay in the soil, defined by particle size.

differences in reflectance patterns for silt and sand

as the grain size increases (sand), the probability of photon absorption increases due to the larger internal path of the grain. smaller grain size (clay and silt) show more surface scattering than absorption.

Effect of particle size and moisture content

The amount of moisture held in a soil layer is a function of the particle size. The smaller the particle, the greater its ability to hold more moisture for longer periods. Clay = high retention (small grain size and closely packed), Sand = drains rapidly (large grain size and lots of air pockets)

How does Lv change with soil moisture content?

Dry soil, Lv = volume reflectance, light penetrates into the particle and can be absorbed

Wet soil, Lv = light absorption by water in the interstitial space

What effect will increased moisture have on soil reflectance?

Increased soil moisture reduces reflectance across most wavelengths. Water absorbs light—especially in the NIR—so wetter soils appear darker and have a flatter spectral curve with lower overall reflectance.

Organic matter

The presence of organic material causes overall reduction in soil reflectance due to light absorption. Organic matter absorbs more towards the blue/green region of the spectrum

Biological Crusts

A layer of living organisms covering the soil (moss, lichen, fungi, bacteria). Resulting reflectance is then a mix of soil and biological spectral responses.

How does the presence of iron oxide affect the spectral curve of soil reflectance?

Iron oxides increase reflectance in the red and shorten visible wavelengths, giving soils a reddish hue. They create a characteristic spectral peak in the red region and a steeper rise from blue → red. Reflectance decreases again toward NIR.

Why is hyperspectral data (e.g., AVIRIS) effective for mineral detection

Multispectral satellites provide fewer bands than hyperspectral sensors but still capture key wavelengths (e.g., SWIR) useful for identifying minerals. Examples:

• ASTER (Terra): highlights rock/soil differences (though SWIR stopped working after 2008).

• WorldView-3: SWIR bands detect specific minerals (e.g., alunite absorption around 2145 nm).

How can vegetation help reveal soil or mineral characteristics?

Vegetation can show spectral changes in response to soil chemistry or toxic metals—such as altered green reflectance, NIR changes, or red-edge shifts (680–800 nm). Geobotany uses these plant responses, species changes, or vegetation stress patterns to infer underlying lithology or ore deposits, even when minerals themselves are not directly visible.

What does specral reflectance look like for dry simple soils?

Increasing reflectance with increasing wavelength, especially in the visible, near and middle infrared portions of the spectrum.

Why does remote sensing data of exposed soil surfaces obtained after a major precipitation event such as a thunderstorm appear noticeably darker?

Because the water in the surficial soil absorbs much of the incident radiant energy, especially in the visible and NIR portions of the spectrum, resulting in less radiance exiting towards the system.