Raster Data Structure

Raster Data

Raster data is like a digital photo made up of small squares called cells or pixels. Each cell shows something about that spot — like how high it is (elevation), how hot it is (temperature), or what kind of land it is (forest, water, etc.).

Think of it like a graph paper where each box is colored to show what's there.

Key Points

What's in the cell?
Each cell stores information — for example, "water" or "forest."
Where is the cell?
The cell's location comes from its place in the grid (like row 3, column 5). We don't need to store its exact coordinates.

How Do We Decide a Cell's Value?

Sometimes, one cell covers more than one thing (like part forest and part water). To choose one value, we can:

Center Point: Use whatever feature is in the middle.
Most Area: Use the feature that takes up the most space in the cell.
Most Important: Use the most important feature (like a road or well), even if it's small.
Percentages: Record how much of each feature is in the cell.

How is Raster Data Stored?

Raster files can be very big, so we use smart ways to save space.

1. Direct Coding

Simple: Just store every cell in order, row by row.
Easy to understand, but uses a lot of space.

2. Compression Methods

a. Chain Code

Stores the outline of shapes by saving the direction of lines.
Good for drawing boundaries, but hard to edit.

b. Run-Length Encoding

Saves repeating values like "5 trees" instead of "tree, tree, tree, tree, tree."
Great for large areas with the same value.

c. Block Code

Groups nearby cells with the same value into square blocks.
Saves space by storing just one value per block.

d. Quadtree

Splits the map into 4 parts.
If a part has the same value (like all water), it stores it as one piece.
If not, it keeps dividing into smaller squares.
Very smart and space-saving, especially for maps with both simple and complex areas.

Warm regards.
..
Vineesh V
UGC Nodal Officer
Assistant Professor of Geography,
PG and Research Department of Geography,
Government College Chittur, Palakkad.

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