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Local Operations in GIS

Local operations mean doing math or logic on each pixel one by one, using just the value in that pixel (not its neighbors).

Map Algebra

Just like we do math in school, we can do math on maps using raster data.
For example:

  • Add two maps together (e.g., rainfall map + temperature map).

  • Multiply or subtract values in the pixels.

Reclassification

Reclassification means changing the values in the raster based on some rules.

For example:

  • Pixels with values 1–10 → change to 1 (low)

  • Pixels with values 11–20 → change to 2 (medium)

  • Pixels with values 21–30 → change to 3 (high)

It helps us group data into categories (like low, medium, high).

Logical Operations

This is like asking Yes or No questions for each pixel.

Examples:

  • "Is the value greater than 50?"
    → If yes, mark it as 1
    → If no, mark it as 0

This helps in finding areas that match certain conditions.

Arithmetic Operations

These are basic math operations done on raster maps:

  • Add (+): Combine two rasters

  • Subtract (−): Find differences

  • Multiply (×): Scale or compare values

  • Divide (÷): Normalize or balance data

Example:

Imagine we have two raster maps:

  • Map A: shows rainfall in mm

  • Map B: shows vegetation

We can:

  • Add the maps to see overall water impact

  • Reclassify rainfall into low, medium, high

  • Use logical operation to find "Where rainfall > 100mm?"

  • Use arithmetic to calculate average rainfall


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