Working with Rasters

Practical Guide: Working with Rasters in GIS

1. Introduction

Rasters are grid-based data structures used to represent spatial information, such as satellite imagery, elevation models, and land cover data. This guide will walk you through the essential steps for working with raster data in a GIS environment, using software like QGIS or ArcGIS.

2. Setting Up Your Environment

• Install GIS Software: Ensure you have QGIS or ArcGIS installed on your computer.
• Open the Application: Launch the GIS software of your choice.

3. Loading Raster Data

A. In QGIS

1. Add Raster Layer:

• Click on the Layer menu.
• Select Add Layer > Add Raster Layer.
• Browse to your raster file (e.g., .tif, .img) and click Open.

B. In ArcGIS

1. Add Data:

• Click on the Add Data button in the toolbar.
• Navigate to your raster file and select it to add it to the map.

4. Exploring Raster Data

• View Metadata: Right-click on the raster layer and select Properties to view metadata, including cell size, extent, and coordinate system.
• Identify Values: Use the Identify tool to click on different locations within the raster to see the pixel values.

5. Basic Raster Operations

A. Displaying Rasters

• Adjust the Symbology:
• QGIS: Right-click the layer, select Properties, and go to the Symbology tab to change color ramps or apply styles.
• ArcGIS: Use the Layer Properties to adjust the renderer and apply color schemes.

B. Raster Calculations

You can perform calculations using raster data.

Example: NDVI Calculation

1. Load Red and Near-Infrared (NIR) Bands: Load the necessary bands (e.g., Band 4 for Red and Band 5 for NIR).
2. Raster Calculator:

• QGIS: Use the Raster Calculator (found under Raster > Raster Calculator) to compute NDVI:
  (NIR - Red) / (NIR + Red)
• ArcGIS: Use the Raster Calculator from the Spatial Analyst toolbox with the same formula.

6. Reclassifying Rasters

Reclassification changes the values of a raster to a new set of values.

1. Reclassify Tool:

• QGIS: Use the Reclassify by Table tool found under Raster > Miscellaneous.
• ArcGIS: Use the Reclassify tool in the Spatial Analyst toolbox.

1. Define Reclassification Rules: Specify the original values and their new classifications.

7. Raster Analysis

You can perform various analyses on raster data, such as terrain analysis or suitability modeling.

A. Terrain Analysis

• Slope and Aspect:
• QGIS: Use the Terrain Analysis tools under the Raster menu.
• ArcGIS: Use the Slope and Aspect tools in the Spatial Analyst toolbox.

B. Suitability Analysis

1. Combine Multiple Rasters: Use weighted overlay analysis to combine multiple criteria layers.
2. Use Raster Calculator: Apply weights to different rasters and sum them up to create a suitability map.

8. Exporting Raster Data

After processing, you may want to save your raster data.

1. Export Raster:

• QGIS: Right-click the raster layer and select Export > Save As. Choose the format and specify the output location.
• ArcGIS: Use Data > Export Data to save the raster to a new file.

9. Conclusion

This practical guide has introduced you to the fundamental processes for working with raster data in GIS. By understanding how to load, analyze, and manipulate raster datasets, you can enhance your spatial analysis capabilities.