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Remote Sensing of Active-Fire and Post-Fire Effects
Presentation 1-2
Why Use Remote Sensing?
Good Day!
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1.2.1 Field Sampling Limitations
1. Intrusive Sampling
‘The act of taking the measurement can change the properties of the thing you are measuring.’
c.f., The Heisenberg Uncertainty Principal
A limitation of field sampling includes the simple premise that by taking a measurement you change the property of that which you are trying to measure.
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1.2.1 Field Sampling Limitations
1. Intrusive Sampling Examples
Touching water to measure surface tension - Producing ripples that change the result
A classical example of intrusive sampling included trying to measure the surface tension of a pond. Touching the surface of the water will change the result of the property you are trying to measure.
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1.2.1 Field Sampling Limitations 1. Intrusive Sampling Examples
Clipping leaves prior to analysis
- leaf properties change due to water stress Measuring fuel properties for modeling - clipping or trampling changes O2flow
Many intrusive sampling examples exist when considering fuels or fire management scenarios.
For example, clipping tree leaves in order to measure nutrient contents or the degree of water stress or
Clipping biomass to measure fuel load in an area prior to a prescribed fire.
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1.2.1 Field Sampling Limitations
1. Intrusive Sampling Examples
Casting a shadow while taking spectral measurements - the reflectance is lowered by the shadow
Measuring temp of a hot liquid with a thermometer - the colder thermometer will cool the liquid
Casting a shadow will reduce the reflectance (or albedo) of a surface.
Using a cool thermometer to measure the temperature of a hot liquid will cause a lower than actual temperature to be recorded.
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1.2.2 Assessment of Forest Stands
Line Plot Cruises:
• Systematic layout of sample locations
• 0.1 acre plots
• Measure every tree in plot
• Other locations then spatially interpolated
A further limitation of field sampling arises when considering the assessment of forest stands.
For forest inventory purposes, it is logistically impossible to measure every tree in an entire forest.
A common way around this is to conduct line plot cruises where, a systematic layout of fixed radius plots are created.
These are then visited by the field crews and each tree in the plot is measured.
Typical measurements include, tree height, species, dbh, crown base height, etc.
The trees at the other locations in the forest are then spatially interpolated using methods such a kriging. Control surfaces for the kriging might include potential vegetation type or elevation.
7 Double Sampling:
• Systematic layout of sample locations
• 0.1 acre plots
• Count trees in every location
• Measure trees at every 2nd, 3rd, 4th, or 5thlocation
• Other locations then spatially interpolated
1.2.2 Assessment of Forest Stands
In larger forests, even line plot cruises may involve the sampling of too many plots.
This is often overcome by using doubling sampling.
In double sampling the trees are counted at every plot but the trees are only measured every few plots.
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1.2.2 Assessment of Forest Stands
• Too many trees counted per location
• Too many trees measured
• Tree measurements are typically clustered
• Only a small sample of the entire population is measured
• High cost to train field crews and conduct measurements
• Long time to sample large areas – difficult to sample remote or wilderness areas
Solution: Use Remotely Sensed Imagery!
Remote Sensing data measures the whole population at once
The example of forest inventory sampling highlights several limitations of using only field-based sampling.
In summary, it can be to costly to employ enough personnel to sample enough trees in a very large forest.
Furthermore, if additional measurements are needed (such as measurement of surface fuels) then this process will take longer and can become even more costly.
Lastly, if samples are required across large or within remote areas, access to plots may become an issue, especially in wilderness areas.
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Consider the Measurement of Fire Behavior:
1.2.3 Sampling Fires
Let us now consider the case of field sampling as part of wildland fire use fires or for rapid-response activities.
As always safety comes before the need to acquire field measurements.
Therefore, motivation clearly exists to develop remote sensing methods that can accurately characterize the fire behavior or immediate post-fire effects at a safe distance. Such methods have the potential to help us monitor the real-time behavior or spread of the fire, or at least to improve the performance of fire models.
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NASA 2000
1.2.3 Sampling Fires
A further consideration is that fires can be very large and occur in remote and isolated areas, making field measurements of the entire fire logistically impossible.
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1.2.4 Limitations of Remote Sensing
Many people think that Remote Sensing has the ability to provide all the information required to perform large-scale research projects:
i.e. Remote Sensing = No Fieldwork This is NOT true
Each Remote Sensing method provides only a layer of information. It may allow you to do Less fieldwork, but will never fully replace it.
Remote Sensing Cannot Measure Everything – (e.g., grass species with Landsat data)
You can only reliably detect objects that are greater in size to the smallest pixel size of your imagery.
