Raster Data & Digital Elevation Models (DEM)
Raster data represents the world as a **grid of cells (pixels)**, where each cell holds a value. It's the format used for satellite imagery, elevation data, temperature maps, and more. ---
📋 Prerequisites
- QGIS Desktop 3.22+ installed on your computer.
- Basic understanding of GIS data concepts.
Step-by-Step Instructions
Understand Raster Structure
A raster is defined by: - **Rows and Columns** – the grid dimensions - **Cell/Pixel Size** – the spatial resolution (e.g., 30m x 30m per cell) - **Cell Value** – the data stored (could be elevation, reflectance, temperature, or a classification code) - **Extent** – the geographic area the raster covers ---
What Is a Digital Elevation Model (DEM)?
A **DEM** is a raster where each pixel value represents the **elevation (height)** of the terrain at that location. ### Related Terms: - **DSM (Digital Surface Model):** Includes buildings, trees, and other surface objects - **DTM (Digital Terrain Model):** Represents only the bare ground, with objects removed **Common DEM sources:** SRTM (30m, free), ASTER GDEM (30m, free), Cartosat DEM, LiDAR-derived DEMs (very high resolution) ---
Download a Free DEM
1. Go to the **USGS EarthExplorer** website (earthexplorer.usgs.gov) 2. Search for your area of interest 3. Under Data Sets, select **Digital Elevation → SRTM** 4. Download the GeoTIFF file for your region ---
Load and View a DEM in QGIS
1. **Layer → Add Layer → Add Raster Layer** 2. Browse and select your DEM `.tif` file 3. Right-click the layer → **Properties → Symbology** 4. Change render type to **Singleband Pseudocolor** for a color elevation map ---
Common Raster Operations
| Operation | Purpose | |---|---| | **Clip** | Cut raster to your area of interest | | **Reproject** | Change the raster's CRS | | **Resample** | Change pixel size/resolution | | **Reclassify** | Convert values into categories (e.g., elevation bands) | | **Raster Calculator** | Perform math on pixel values (e.g., NDVI) | To clip a raster in QGIS: **Raster → Extraction → Clip Raster by Mask Layer** ---
🧠 Quick Recap & Practice Questions
## Quick Recap
- Raster = grid of pixels, each storing a value
- DEM = elevation raster; DSM includes surface objects, DTM is bare earth only
- Free DEM sources: SRTM, ASTER GDEM
- Key raster tools: Clip, Reproject, Resample, Reclassify, Raster Calculator
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## Practice Questions
1. What's the difference between a DSM and a DTM?
2. Name two free sources for downloading DEM data.
3. Which QGIS tool would you use to cut a large raster down to your study area?
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*Next Tutorial: Georeferencing Scanned Maps in QGIS*