Google Earth, a ubiquitous tool for exploring our planet, offers more than just satellite imagery. Buried within its interface lies a treasure trove of topographic data, enabling you to extract elevation information for a wide range of applications, from landscape design to scientific research. This comprehensive guide will walk you through the process of obtaining topography from Google Earth, covering various methods and essential considerations.
Why Extract Topography from Google Earth?
Before diving into the ‘how,’ let’s consider the ‘why.’ Topographic data, essentially the elevation of the Earth’s surface, is crucial for numerous disciplines:
- Landscape Architecture and Design: Understanding terrain is fundamental for site planning, grading, and drainage design.
- Civil Engineering: Topographic surveys are essential for road construction, infrastructure development, and flood modeling.
- Environmental Science: Analyzing terrain aids in watershed delineation, habitat mapping, and erosion studies.
- Geology and Geophysics: Elevation data is used for geological mapping, fault analysis, and landslide hazard assessment.
- GIS (Geographic Information Systems): Topography forms a base layer for various GIS analyses and spatial modeling.
- Recreation and Outdoor Activities: Creating custom maps for hiking, biking, and other outdoor pursuits.
While professional surveying and LiDAR (Light Detection and Ranging) provide the most accurate topographic data, Google Earth offers a readily accessible and often sufficient alternative, especially for preliminary assessments and smaller-scale projects. It’s important to understand the limitations regarding accuracy, which we’ll discuss later.
Methods for Extracting Topography from Google Earth
Several methods exist for extracting elevation data from Google Earth. We’ll cover three primary techniques:
- Using the Elevation Profile Tool (Basic): This method is straightforward and provides a basic elevation profile along a path you draw on the map.
- Exporting KML/KMZ Files to GIS Software (Intermediate): This technique involves creating paths or polygons in Google Earth and exporting them as KML/KMZ files. These files can then be imported into GIS software (e.g., QGIS, ArcGIS) for more advanced analysis and elevation extraction.
- Using Third-Party Google Earth Pro Plugins (Advanced): This method utilizes specialized plugins designed to extract elevation data directly from Google Earth Pro into a CSV or other usable format.
Method 1: Using the Elevation Profile Tool (Basic)
This method is ideal for quickly visualizing the elevation change along a specific route or line. It’s best suited for simple projects where high accuracy is not paramount.
Step-by-Step Instructions:
- Open Google Earth Pro: Ensure you are using Google Earth Pro. It is now free and offers more advanced features than the standard Google Earth web version. If you don’t have it installed, download it from the Google Earth website.
- Navigate to Your Area of Interest: Use the search bar or zoom and pan to locate the area for which you want to extract topography.
- Create a Path:
- Click on the ‘Add Path’ button in the toolbar (it looks like a line).
- Carefully draw a path along the route or area you’re interested in. Click to add vertices (points) to the path. Consider the density of vertices. More vertices will result in a more accurate elevation profile.
- Double-click to finish the path.
- Name and Save the Path: A dialog box will appear. Give the path a descriptive name and click ‘OK’. The path will now be saved under ‘My Places’ in the left-hand panel.
- View the Elevation Profile:
- Right-click on the path you just created in the ‘My Places’ panel.
- Select ‘Show Elevation Profile’.
- Analyze the Elevation Profile: A graph will appear at the bottom of the Google Earth Pro window, showing the elevation along the path. The x-axis represents the distance along the path, and the y-axis represents the elevation.
- Extract Elevation Data: While the elevation profile tool doesn’t directly export data, you can hover your mouse cursor along the graph to see the elevation at specific points along the path. You can manually record this data in a spreadsheet or text file. Be patient as this is a manual process.
- Considerations:
- Accuracy: The accuracy of this method depends on the resolution of the underlying elevation data used by Google Earth. It’s generally sufficient for visualization but may not be suitable for precise engineering calculations.
- Limitations: You cannot directly export the elevation data using this method. Manual data entry is required.
Method 2: Exporting KML/KMZ Files to GIS Software (Intermediate)
This method provides a more robust way to extract elevation data and allows for further analysis in GIS software. KML (Keyhole Markup Language) and KMZ (Keyhole Markup Language Zipped) are file formats used to store geographic data in Google Earth.
Step-by-Step Instructions:
- Open Google Earth Pro: Again, ensure you’re using Google Earth Pro.
- Navigate to Your Area of Interest: Locate the area for which you need elevation data.
- Create Paths or Polygons:
- For Paths: Use the ‘Add Path’ tool as described in Method 1 to draw lines along which you want to extract elevation. These are useful for extracting elevation along a trail, road, or stream.
- For Polygons: Click the ‘Add Polygon’ button in the toolbar (it looks like a shape with multiple sides). Draw a polygon encompassing the area of interest. Polygons are useful for extracting elevation within a defined region. Click to add vertices, and double-click to finish the polygon.
- Name and Save the Features: Give each path or polygon a descriptive name and click ‘OK’ to save them to ‘My Places’.
- Export as KML/KMZ:
- Right-click on the folder in ‘My Places’ containing the paths or polygons you created. Alternatively, you can right-click on individual paths or polygons.
- Select ‘Save Place As…’.
- Choose either ‘KML’ or ‘KMZ’ as the file format. KMZ files are compressed versions of KML files and are generally preferred for easier sharing.
- Choose a location to save the file and click ‘Save’.
- Import into GIS Software (e.g., QGIS):
- Download and Install QGIS: If you don’t already have it, download and install the free and open-source QGIS software from https://www.qgis.org/en/site/.
- Open QGIS: Launch QGIS.
- Add the KML/KMZ File:
- Go to ‘Layer’ -> ‘Add Layer’ -> ‘Add Vector Layer…’.
- Browse to the location where you saved the KML/KMZ file.
- Select the file and click ‘Open’.
- Extract Elevation Data in QGIS:
- The method for extracting elevation depends on if you created paths or polygons.
- For Paths:
- You need to obtain elevation data for points along the path. First you need to convert the path into points.
- Go to Vector -> Geometry Tools -> Extract Vertices.
- Select the path layer as the Input Layer.
- Run the tool. This will create a new layer with the vertices as point features.
- Now you need to sample the raster elevation data using these points. You will need a raster Digital Elevation Model (DEM) that covers the area of interest. If you don’t have one, you can download one from various sources (USGS Earth Explorer, Open Topography, etc.).
- Go to Raster -> Analysis -> Sample raster values.
- Select the point layer (vertices) as the layer containing the point coordinates.
- Select the raster DEM as the Raster layer(s) to sample.
- Add a prefix to the new field that will contain the elevation data (e.g. “elevation_”).
- Run the tool.
- Open the attribute table of the resulting layer. You will now have a new field containing the elevation data for each point along the path. You can export this attribute table to a CSV file for further analysis.
- For Polygons:
- You need to obtain elevation data for points within the polygon. There are multiple ways to do this. One common method is to create a grid of points within the polygon and then sample the elevation data at those points.
- Go to Vector -> Research Tools -> Regular points.
- Select the polygon layer as the Input layer.
- Define the Spacing. This determines the density of the points. A smaller spacing will result in more points and a more accurate representation of the elevation.
- Run the tool. This will create a new layer with a grid of points within the polygon.
- Now you need to sample the raster elevation data using these points. You will need a raster Digital Elevation Model (DEM) that covers the area of interest. If you don’t have one, you can download one from various sources (USGS Earth Explorer, Open Topography, etc.).
- Go to Raster -> Analysis -> Sample raster values.
- Select the point layer (grid points) as the layer containing the point coordinates.
- Select the raster DEM as the Raster layer(s) to sample.
- Add a prefix to the new field that will contain the elevation data (e.g. “elevation_”).
- Run the tool.
- Open the attribute table of the resulting layer. You will now have a new field containing the elevation data for each point within the polygon. You can export this attribute table to a CSV file for further analysis. You can then calculate statistics (mean, standard deviation, etc.) of the elevation data within the polygon.
- Further Analysis: QGIS allows for extensive analysis of the extracted elevation data, including creating contour maps, calculating slope and aspect, and generating 3D visualizations.
Method 3: Using Third-Party Google Earth Pro Plugins (Advanced)
Several third-party plugins are available for Google Earth Pro that streamline the process of extracting elevation data. These plugins often provide more advanced features, such as direct export to CSV files and automated elevation sampling.
Example Plugin: GPS Visualizer
GPS Visualizer is a popular online tool that can be used in conjunction with Google Earth Pro to extract elevation data. It provides a simple interface for converting GPS data (which can be created in Google Earth Pro) into various formats, including GPX and CSV.
Steps using GPS Visualizer (as an example):
- Create a Path in Google Earth Pro: Follow the steps outlined in Method 1 to create a path along the area of interest. Ensure the path has sufficient vertices for the desired accuracy.
- Save the Path as KML: Save the path as a KML file (not KMZ).
- Upload the KML to GPS Visualizer:
- Go to the GPS Visualizer website (https://www.gpsvisualizer.com/).
- Look for the “Convert to Plain Text” tool.
- Upload the KML file you saved from Google Earth Pro.
- Under “Output options”, select “plain text”. You can customize the output format, including the delimiter (e.g., comma or tab).
- Click the “Convert” button.
- Download the Text File: GPS Visualizer will generate a plain text file containing the latitude, longitude, and elevation for each point along the path. Download this file.
- Open in Spreadsheet Software: Open the downloaded text file in spreadsheet software like Microsoft Excel, Google Sheets, or LibreOffice Calc. The data will be organized into columns, allowing you to easily analyze and manipulate the elevation data.
Other Plugins to Explore:
- Earth Point: Offers various tools for working with geographic data in Google Earth, including elevation extraction capabilities.
- GE Path Profile: A specialized tool for generating elevation profiles along paths in Google Earth.
Disclaimer: The availability and functionality of third-party plugins may vary. Always check the plugin’s documentation and user reviews before use.
Accuracy Considerations
It’s crucial to understand the limitations regarding accuracy when extracting topography from Google Earth:
- Resolution of Underlying Data: Google Earth uses various sources of elevation data, with varying resolutions. The accuracy of the extracted data is limited by the resolution of the source data. In general, expect errors on the order of several meters, and possibly more in areas with steep terrain or poor data coverage.
- Data Processing and Interpolation: Google Earth employs algorithms to process and interpolate the elevation data, which can introduce errors.
- Vegetation Cover: In areas with dense vegetation, the elevation data may represent the top of the canopy rather than the ground surface. This is especially true for areas using older data and techniques.
- Water Bodies: Elevation data over water bodies is often unreliable.
When to Use Google Earth Topography:
Google Earth topography is suitable for:
- Preliminary site assessments
- Regional-scale studies
- Visualization and mapping
- Educational purposes
When to Use More Accurate Data:
For projects requiring high accuracy, such as:
- Detailed engineering design
- Precise volume calculations
- Legal surveys
Consider using alternative data sources, such as:
- LiDAR (Light Detection and Ranging): Provides highly accurate elevation data, especially in vegetated areas.
- Traditional Surveying: Ground-based surveying methods offer the highest accuracy but can be time-consuming and expensive.
- High-Resolution Digital Elevation Models (DEMs): Available from various government agencies and commercial providers.
Tips for Improving Accuracy
While Google Earth’s accuracy has limitations, you can take steps to improve the quality of the extracted data:
- Use Google Earth Pro: The Pro version often has access to higher-resolution imagery and elevation data.
- Increase Vertex Density: When creating paths or polygons, use a higher density of vertices, especially in areas with complex terrain.
- Compare with Other Data Sources: Cross-validate the extracted data with other available data sources, such as topographic maps or online DEMs.
- Apply Smoothing Techniques: GIS software offers various smoothing algorithms that can reduce noise and improve the overall accuracy of the elevation data.
- Check for Recent Imagery: Use the historical imagery feature in Google Earth to see if newer imagery is available for your area of interest. Newer imagery may contain updated elevation data.
Conclusion
Extracting topography from Google Earth is a valuable skill for anyone working with geographic data. By understanding the various methods, accuracy considerations, and limitations, you can effectively utilize this readily available resource for a wide range of applications. While Google Earth topography may not be suitable for all projects, it provides a convenient and accessible way to obtain elevation information for preliminary assessments, visualization, and educational purposes. Remember to always critically evaluate the accuracy of the data and consider using alternative data sources when higher precision is required. By mastering these techniques, you can unlock the hidden topographic potential within Google Earth and enhance your understanding of our planet’s diverse landscapes.