Google Earth’s Ground Elevation: Understanding the Height Above Sea Level

Google Earth presents the ground elevation data as the height above sea level, which represents the vertical distance of the earth's surface from the average sea level. This reference point serves as a standard measure in topographic mapping and geographic information systems (GIS). If you need to find the elevation for a specific location, Google Earth's elevation tool offers a straightforward method to determine the terrain height at that point.

Types of Elevation Models Used by Google Earth

The elevation model used by Google Earth is primarily based on a digital elevation model (DEM).

Digital Elevation Model (DEM): A DEM is a representation of the Earth's surface created from a grid of elevation points. Google uses these models to provide accurate topographical information for its users. According to some sources, Google's DEM closely approximates models such as the Shuttle Radar Topography Mission (SRTM) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) datasets provided by NASA and the Japanese Space Agency (JAXA). These models are known for their precision and reliability.

Orbital Variations and Geoid Model: The elevation data in Google Earth is derived from satellite imagery, but satellites do not always orbit at a fixed altitude. As the satellites move around the Earth, they experience slight variations in gravitational pull, leading to small changes in their orbital height. Additionally, the Earth's surface is not perfectly spherical, and the geoid (a mathematical model of the Earth's gravitational field) does not perfectly match the physical surface. These factors can introduce small discrepancies in the elevation data.

Exactness and Verification

The accuracy of the elevation data in Google Earth can vary depending on the location. For instance, near coastlines, the model may not align perfectly with the exact shoreline. This is due to the fact that the geoid does not precisely match the physical surface, especially near water bodies. To illustrate, if you move your cursor over the shoreline of the Big Island in Hawaii, you might notice a slight discrepancy between the onshore and offshore elevations.

Mean High Water Spring (MHWS) and Elevation Measurements

The heights provided by Google Earth are usually referenced to the Mean High Water Spring (MHWS). This measurement reflects the average level of high spring tides. However, it is important to note that this average does not guarantee that sea level will not exceed the MHWS. Various factors can cause a tide to rise above this level, including:

Wave setup and strong onshore winds Low atmospheric pressure caused by cyclones The moon being closer to Earth during spring tides, particularly around new and full moons The Earth being closer to the sun during spring tides Tsunamis

While the MHWS provides a reliable benchmark for general purposes, it is not without its limitations. Users should be aware that extreme conditions can sometimes cause sea levels to rise above this measurement.

Conclusion

Google Earth's elevation data is a powerful tool for GIS and topographical mapping. However, users should be aware of the potential discrepancies and limitations, especially near shorelines. By understanding the methodology and the factors that can affect the elevation data, users can make more informed decisions when using this resource.