In remote sensing, a composite image is a visual representation created by combining multiple spectral bands of a single image. This process allows for a richer visualization and analysis of the Earth's surface features compared to viewing individual bands separately.
Specifically, as highlighted by reference materials:
- Colour Composite Images: The remote sensing images, which are displayed in three primary colours (red, green and blue) is known as Colour Composite Images. It means associating each spectral band to a primary colour results in a Colour Composite Image.
Essentially, you take different spectral bands captured by a satellite or aerial sensor (like visible red, infrared, etc.) and assign each one to one of the primary color channels on a display: Red (R), Green (G), or Blue (B).
Why Create Composite Images?
Remote sensing sensors capture data across various portions of the electromagnetic spectrum. Different features on the Earth's surface reflect or emit energy differently in these various bands. By combining these bands into a single color image, we can:
- Visualize invisible features: Some features are most distinct in non-visible bands (like infrared). Composites allow us to see these features using colors we can perceive.
- Enhance specific features: By assigning bands strategically, we can make certain elements like vegetation, water bodies, or urban areas stand out.
- Aid interpretation: Different band combinations (recipes) provide unique views, helping analysts identify and map various land covers and conditions.
Types of Colour Composites
Colour composites are broadly categorized based on how the bands are assigned to the RGB display channels:
- True Color Composite (TCC): This is created by assigning the red spectral band to the Red display channel, the green spectral band to the Green display channel, and the blue spectral band to the Blue display channel. This results in an image that closely resembles what the human eye would see from space.
- Example: R=Red band, G=Green band, B=Blue band (e.g., Landsat 8 Bands 4, 3, 2 assigned to R, G, B respectively).
- False Color Composite (FCC): In an FCC, the assignment of spectral bands to the RGB display channels is different from their natural color appearance. This is often done to highlight specific features that are more visible in non-visible light.
- Example: R=Near-Infrared band, G=Red band, B=Green band (e.g., Landsat 8 Bands 5, 4, 3 assigned to R, G, B respectively). In this common FCC, healthy vegetation appears bright red because it strongly reflects near-infrared light.
- Example: R=Shortwave Infrared (SWIR) band, G=Near-Infrared band, B=Red band (e.g., Landsat 8 Bands 6, 5, 4 assigned to R, G, B respectively). This combination is useful for differentiating vegetation types and soil moisture.
How it Works
Imagine you have satellite data for a location in three bands: Visible Red, Visible Green, and Near-Infrared.
| Spectral Band | How assigned for FCC (Vegetation Focus) | Display Channel | What you see |
|---|---|---|---|
| Visible Red | Assigned to Green Channel | Green | Denser red means less green assigned |
| Visible Green | Assigned to Blue Channel | Blue | Denser green means less blue assigned |
| Near-Infrared | Assigned to Red Channel | Red | Denser infrared means more red assigned |
When you combine these, areas with high Near-Infrared reflection (like healthy vegetation) will appear bright red in the final image because the Near-Infrared band was assigned to the Red display channel.
Composites are essential tools for remote sensing analysis, enabling a wide range of applications from environmental monitoring and urban planning to disaster assessment and agriculture.
