Introduction to Probability
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✅ 1. What is Probability?
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Definition: Probability is the numerical measure of the likelihood that a particular event will happen.
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Relevance to GIS: Helps in risk mapping, spatial modeling, disaster prediction, land-use forecasting, and uncertainty analysis in geospatial data.
๐ง 2. Key Concepts and Terminology
| Term | Description | GIS Example |
|---|---|---|
| Experiment | A process that generates data | Mapping landslide zones in a valley |
| Sample Space (S) | All possible outcomes | {High risk, Medium risk, Low risk} zones |
| Event (E) | Specific outcome or a group of outcomes | E: “Zone is high risk” |
๐ข 3. Basic Probability Formula
P(E)=Total number of outcomesNumber of favorable outcomes๐ Example:
You analyze 100 buildings in a flood-prone area. 25 buildings are in high-risk zones.
➕ 4. Additive Rule of Probability
Formula:
If A and B are mutually exclusive events,
๐ Example:
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P(A): Probability a village lies in an earthquake-prone area = 0.3
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P(B): Probability a village lies in a landslide-prone area = 0.2
Assuming no overlap (mutually exclusive),
There’s a 50% chance a village lies in either hazard-prone area.
✖️ 5. Multiplicative Rule of Probability
Formula:
If A and B are independent events,
๐ Example:
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P(A): Probability of heavy rainfall in July = 0.6
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P(B): Probability of soil erosion in same region = 0.5
So, there’s a 30% chance both occur together.
๐ 6. Conditional Probability
Formula:
Probability of A occurring given that B has occurred.
๐ Example:
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P(A ∩ B): Probability that a house is damaged and in a flood zone = 0.15
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P(B): Probability that a house is in a flood zone = 0.3
There’s a 50% chance a house is damaged if it’s in a flood zone.
๐ 7. Applications of Probability in GIS
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Risk assessment (e.g., flood, landslide, earthquake)
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Modeling uncertainty in spatial data
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Disaster preparedness mapping
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Decision-making under uncertainty
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Probability surfaces for habitat suitability or resource potential
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