If you’re like most people, you’ve probably heard the terms “ellipsoid height” and “elevation” thrown around. But what exactly do these terms mean? And more importantly, what is the difference between them?
To put it simply, ellipsoid height refers to the height above the reference ellipsoid, which is an idealized model of the Earth’s surface. Elevation, on the other hand, refers to the height above mean sea level. The difference between the two is due to the fact that the Earth’s surface is not a perfect sphere, and thus different models are needed to accurately measure height.
So why does this matter? Well, for one thing, understanding the difference between ellipsoid height and elevation is crucial for anyone working in fields such as geodesy or GPS. It can also be important for those who live in areas prone to flooding or other natural disasters, as knowing the precise elevation of their home can help them better prepare for potential hazards.
Geodetic Heights
Geodetic height refers to the height of a point on the Earth’s surface with reference to the Earth’s geodetic datum. It is different from the elevation, which refers to the height of the point above the mean sea level (MSL). The geodetic datum defines the reference surface for all geodetic heights. The heights are usually measured with respect to the ellipsoid or the geoid.
- The ellipsoid is an ideal mathematical model of the Earth’s shape, assuming it to be a regular smooth surface. The ellipsoid height is measured perpendicular to the ellipsoid surface at the point of interest.
- The geoid is a surface that closely approximates the Earth’s gravity field. It accounts for the irregularities in the Earth’s shape and gravity, making it a more accurate representation of the Earth’s surface. The geoid height is the difference between the ellipsoid and the geoid at any given point on the Earth’s surface.
The geodetic heights are used for surveying, mapping, and geodesy. Surveyors use the geodetic heights to establish elevation and vertical control, while geodesists use them for geodetic networks and global models. The geodetic heights are also used for GPS and GIS applications, such as mapping, navigation, and geolocation.
The following is a table of common geodetic datums and their associated parameters:
| Datum | Ellipsoid | Semi-major axis | Inverse flattening |
|---|---|---|---|
| WGS84 | WGS84 | 6378137.000 m | 298.257223563 |
| NAD27 | Clarke 1866 | 6378206.4 m | 294.9786972 |
| NAD83 | GRS80 | 6378137.000 m | 298.257222101 |
In conclusion, geodetic heights are an essential aspect of geodesy and geospatial analysis. They enable accurate measurement and representation of the Earth’s surface, providing crucial information for various applications.
Orthometric Heights
Orthometric height, also known as Mean Sea Level (MSL) height or simply elevation, is the height of a point above the geoid, a model of the Earth’s surface reflecting the distribution of gravity. It is the most commonly known and used height reference system worldwide.
The determination of orthometric heights requires complex geodetic computations and physical measurements such as leveling using precise instruments and observations of gravitational acceleration. Using a benchmark network, surveyors establish a series of height reference points with known orthometric heights by carrying out leveling runs from a known point of origin. The height of any other surveyed point can then be determined by measuring the vertical distance between that point and the nearest benchmark point and then adding or subtracting the difference in height between the two.
Key Differences Between Ellipsoid Height and Orthometric Height
- Ellipsoid height is the height of a point above the earth’s ellipsoidal model while orthometric height is the height of a point above the geoid model.
- The ellipsoid model disregards the effects of gravity, while the geoid model takes into account the variations in gravity and the irregular shape of the Earth. This makes elevation (orthometric height) more accurate than ellipsoid height.
- Ellipsoid height values are typically used in GPS and satellite imagery applications, while orthometric height values are commonly used in surveys, engineering, and mapping applications.
Applications of Orthometric Heights
Orthometric heights are widely used in geodetic and surveying applications, geophysics, geology, and civil engineering. Accurate determination of orthometric heights is essential in precise datum definition, survey control establishment, and mapping of accurate contours and landforms. Orthometric heights can also be used to estimate the volume of stockpiled materials, monitor and control subsidence and uplift, and design infrastructure such as roads, bridges, and tunnels.
Example of Orthometric Heights Measurement
Suppose a surveyor needs to determine the elevation of a point on a construction site above the geoid model. Using a leveling rod, the surveyor measures the vertical distance between the point and a nearby benchmark with a known orthometric height of 25 meters. The reading on the rod indicates a vertical distance of 1.5 meters. Therefore, the orthometric height of the point is 26.5 meters (25 + 1.5).
| Point | Height above Geoid (m) | Height above Ellipsoid (m) |
|---|---|---|
| Point A (Benchmark) | 25.0 | 23.5 |
| Point B (Surveyed Point) | 26.5 | 25.0 |
In the table above, Point A is the benchmark while Point B is the surveyed point. The height above the geoid at Point B is 26.5 meters while the height above the ellipsoid is 25.0 meters. The difference between the two heights (1.5 meters) reflects the gravity variations in the region.
Ellipsoid
The reference ellipsoid is a mathematical representation of the shape of the earth. It is the best fit model of the shape of the earth taking into consideration the slight variations in the earth’s shape such as curvature and irregularity. The ellipsoid is defined by an equation of x, y, and z where the semi-minor and semi-major axis of the earth are expressed. The ellipsoid is essential in the geodetic industry because it provides the necessary reference for all geodetic measurements.
- What is the Geoid? The Geoid is essentially a hypothetical surface of the earth’s gravitational field. It is different from the ellipsoid because it is not a smooth and consistent model. The geoid can be described as the shape of the earth’s surface if the oceans were allowed to settle under the influence of gravity.
- Why is it important? The geoid is important because it allows us to determine the height of land in relation to sea level. It is the point of reference that satellite measurements use to determine elevation. It is critical in the fields of geodesy, surveying, and mapping.
- How do we use the Ellipsoid? The ellipsoidal height is the distance measured from the reference ellipsoid to the object or point on the earth. The primary purpose of this measurement is to provide a reference for satellite measurements of elevation. The ellipsoid height differs from elevation, which is the distance measured from the geoid to an object or point on the earth’s surface.
Ellipsoid Height vs. Elevation
The difference between ellipsoid height and elevation is their reference points. Elevation is the height above mean sea level, while ellipsoid is the height above the reference ellipsoid. Understanding the difference is necessary so that the measurement can be accurately calculated.
In geodetic surveying, satellite measurements are used to calculate the elevation of a point on the earth’s surface. These measurements are only accurate if the reference for calculating the elevation is known. The reference used is the geoid or the reference ellipsoid. The geoid is a complex model that considers factors such as gravity, the shape of the earth, and the fluctuating surface of the ocean. The reference ellipsoid, on the other hand, is a simpler mathematical model that approximates the overall shape of the earth.
The use of the ellipsoid height can cause problems in situations where an accurate height reference is required. For example, in construction, where precise measurements are critical, the ellipsoid height is not a reliable measurement because it does not take into consideration the irregularities of the earth’s surface. However, in geodetic measurements, the ellipsoid height is essential for providing a consistent reference for measurements taken by satellite technology.
| Elevation | Ellipsoid Height |
|---|---|
| Height above Mean Sea Level | Height above a mathematical representation of the Earth’s surface (ellipsoid) |
| Height reference used in most engineering applications | Height reference used in most satellite technology applications |
| Can be used to determine drainage patterns, slopes, and other features | Important in geodetic measurements since it provides a consistent reference for satellite measurements of elevation |
In summary, both ellipsoid height and elevation are measurements of height above a reference point, but their reference points are different. Elevation is the distance measured from the geoid to a point on the earth’s surface, while ellipsoid height is the distance measured from the reference ellipsoid to a point on the earth’s surface. Understanding the difference between the two is crucial in the geodetic industry to ensure accurate measurements are taken.
Mean Sea Level
Mean sea level (MSL) is the average height of the sea surface over a specific period of time. It is used as a reference point to measure elevations and depths. MSL is determined by measuring the fluctuations in sea level over a period of time, usually 19 years, known as a tidal epoch.
All elevation measurements are made in reference to a specific vertical datum, which is a standard reference level used for measuring elevations. In the United States, the most commonly used vertical datum is the North American Vertical Datum of 1988 (NAVD88). NAVD88 is based on the MSL height at a specific location at a specific time and is updated periodically to reflect changes in the MSL height due to natural factors such as sea level rise and land subsidence.
Difference between Ellipsoid Height and Elevation
- Ellipsoid height: Ellipsoid height is the height above the reference ellipsoid, which is a mathematical representation of the Earth’s shape. The reference ellipsoid is an idealized shape that is used for mapping and measuring purposes. It is based on the assumption that the Earth is a smooth, homogeneous shape that bulges slightly at the equator and is flattened at the poles. Ellipsoid height is measured using satellite-based positioning systems such as GPS and is expressed in meters or feet.
- Elevation: Elevation is the vertical distance between a specific location on the Earth’s surface and a specified reference level. The reference level could be mean sea level (MSL) or any other defined level such as a benchmark or geoid. Elevation is measured using surveying instruments such as levels and total stations and is expressed in meters or feet.
How Mean Sea Level is used to determine Elevation
To determine the elevation of a specific location, surveyors use instruments to measure the vertical distance between the location and the reference level. The reference level could be MSL, a benchmark, or a geoid. The surveyor would first establish the elevation of the benchmark or the geoid and then measure the vertical distance between the benchmark or the geoid and the location to determine the elevation of the location. The elevation of the location would be expressed in meters or feet relative to the reference level.
It is important to note that the MSL height is not constant and can vary over time due to natural factors such as sea level rise and land subsidence. This means that elevations based on MSL as a reference level will also change over time. Surveyors and geodesists must take this into account when conducting surveys and making elevation measurements.
Example of the Relationship between Ellipsoid Height and Elevation
| Ellipsoid Height (m) | Elevation (m) |
|---|---|
| 100 | 96 |
| 200 | 196 |
| 300 | 296 |
In the above example, the elevation values are lower than the ellipsoid height values. This is because the reference level used for elevations is mean sea level, which is lower than the reference ellipsoid. As a result, elevations will always be lower than ellipsoid heights.
Gravitational Field
The gravitational field is a fundamental concept in physics that describes the influence of a massive object on its surroundings. It is a vector field that describes the direction and strength of the gravitational force at each point in space. The gravitational field is responsible for the acceleration of objects in free fall, and it is a key factor in determining the shape of the Earth.
- The gravitational field is related to the distribution of mass in the Earth.
- Gravity is a fundamental force of nature that affects all objects with mass.
- The strength of the gravitational field varies depending on the distance from the center of mass of the Earth.
Ellipsoid Height versus Elevation
Ellipsoid height and elevation are two different ways of measuring the position of a point on the surface of the Earth. Ellipsoid height is the distance from a point on the Earth to the reference ellipsoid, while elevation is the distance from a point on the Earth to mean sea level. The reference ellipsoid is an approximation of the shape of the Earth that is used as a reference surface for geodetic measurements.
One of the main differences between ellipsoid height and elevation is that they are based on different reference surfaces. Ellipsoid height is based on the reference ellipsoid, while elevation is based on the geoid, which is a surface that represents mean sea level. The geoid is a more complex surface than the reference ellipsoid, and it takes into account the effects of the Earth’s gravitational field on the shape of the Earth’s surface.
Another difference between ellipsoid height and elevation is that they are affected differently by changes in the Earth’s gravitational field. Ellipsoid height is not affected by changes in the gravitational field because it is based on a fixed reference surface. Elevation, on the other hand, is affected by changes in the gravitational field because it is based on the geoid, which is shaped by the Earth’s gravitational field.
| Ellipsoid Height | Elevation |
|---|---|
| Based on the reference ellipsoid | Based on the geoid (mean sea level) |
| Not affected by changes in the Earth’s gravitational field | Affected by changes in the Earth’s gravitational field |
In summary, the main difference between ellipsoid height and elevation is their reference surface. Ellipsoid height is based on the reference ellipsoid, while elevation is based on the geoid. They are also affected differently by changes in the Earth’s gravitational field, with ellipsoid height being unaffected and elevation being affected.
Geoid
The geoid is defined as an equipotential surface of the Earth’s gravity field that best approximates the mean sea level (MSL) over the oceans. It is a theoretical representation of the Earth’s shape, which is not a perfect sphere or ellipsoid.
- The geoid is used as a reference surface for measuring elevations and heights on the Earth’s surface.
- The geoid is an irregular surface with complex features that vary geographically, caused by the uneven distribution of Earth’s mass.
- The geoid surface has a height range of more than 100 meters from the highest point to the lowest point.
Geoid models are used for precise determination of elevation, measuring the sea level, and determining the shape of the Earth. Different geoid models are produced by several global organizations, such as the International Earth Rotation and Reference Systems Service (IERS), the International Association of Geodesy (IAG), and the National Geospatial-Intelligence Agency (NGA).
Geoid models provide a precise and consistent reference for height measurements and Earth surface measurements, which helps in various applications, such as surveying, mapping, and satellite positioning. The geoid helps to relate the ellipsoid height measured by GPS receivers to the mean sea level elevation of a specific location.
| Geoid Height | Ellipsoid Height |
|---|---|
| The geoid height is the difference between the geoid and the ellipsoid height at a specific point on the Earth’s surface. | The ellipsoid height is the height measured from the reference ellipsoid to the point on the Earth’s surface. |
| The geoid height varies from place to place due to the Earth’s mass distribution and the geoid’s irregular shape. | The ellipsoid height is a measure of the physical distance from the center of the Earth to a point on the surface, assuming an idealized ellipsoid shape. |
| The geoid is an equipotential surface that is used as a reference for calculating gravity. | The reference ellipsoid is an idealized mathematical model that approximates the shape of the Earth. |
The geoid height is essential for determining precise elevations, such as orthometric heights, which are height measurements referenced to MSL. The difference between the geoid height and the ellipsoid height varies significantly from place to place and can significantly affect height measurements used in various applications.
Measuring Devices
In order to understand the difference between ellipsoid height and elevation, one must first understand the measuring devices used to obtain these measurements.
- Global Navigation Satellite System (GNSS): GNSS is a system of satellites that provide positioning and timing services. GNSS receivers can measure the distance between the receiver and the satellite, which can then be used to determine the receiver’s position.
- Total Station: A total station is an electronic theodolite (a device used to measure angles in horizontal and vertical planes) integrated with an electronic distance meter (EDM). Total stations are used in the surveying industry to measure angles and distances.
- Laser Scanners: Laser scanners use laser beams to scan objects and generate a 3D model of the object. The resulting model can be used to measure distances and angles.
- Digital Elevation Models (DEMs): DEMs are digital representations of the Earth’s surface. They are created by using elevation data obtained from various sources, such as airplanes, satellites, and ground surveys.
- Airborne LIDAR: Airborne LIDAR (Light Detection and Ranging) uses laser beams to measure the distance between the sensor and the ground. The resulting data can be used to create high-resolution elevation models of the terrain.
- Satellite Altimetry: Satellite altimetry measures the height of the Earth’s surface by bouncing radar signals off the surface. These measurements can be used to create detailed models of the Earth’s topography.
- Barometric Altimeters: Barometric altimeters measure altitude by measuring changes in atmospheric pressure. They are often used in aviation and hiking applications.
Ellipsoid Height vs. Elevation
Now that we have an understanding of the measuring devices used to obtain these measurements, we can delve into the difference between ellipsoid height and elevation. Simply put, ellipsoid height is the distance between a point on the Earth’s surface and the reference ellipsoid (a mathematical model of the Earth’s shape), while elevation is the height of a point on the Earth’s surface above mean sea level.
The difference between ellipsoid height and elevation can be caused by a number of factors, including changes in the Earth’s gravity field and differences in the reference ellipsoid used.
| Ellipsoid Height | Elevation |
|---|---|
| Based on a mathematical model of the Earth’s shape | Based on distance above mean sea level |
| Used in geodetic and surveying applications | Used in aviation and navigation applications |
| Can be affected by changes in the Earth’s gravity field | Not affected by changes in the Earth’s gravity field |
Understanding the difference between ellipsoid height and elevation is important in a wide range of applications, including surveying, mapping, and navigation.
What is the difference between ellipsoid height and elevation?
FAQs
1. What does ellipsoid height mean?
Ellipsoid height is the height of the point above the ellipsoid, which is an approximate model of the earth’s shape. It is usually measured by using Global Positioning System (GPS) devices.
2. What does elevation mean?
Elevation is the height of the point above the mean sea level, which is the average level of the ocean’s surface. It is usually measured by using traditional surveying methods or by using a radar altimeter.
3. Is ellipsoid height the same as elevation?
No, ellipsoid height and elevation are not the same. They are two different concepts that measure the height of the same point using different reference surfaces.
4. When should I use ellipsoid height?
Ellipsoid height is useful in many fields such as geodesy, cartography, and satellite navigation. It is commonly used in remote sensing applications to measure the terrain height.
5. When should I use elevation?
Elevation is useful in many fields such as civil engineering, land surveying, and aviation. It is commonly used to measure the height of buildings, bridges, and mountains.
Closing Thoughts
Thanks for reading this article to learn the difference between ellipsoid height and elevation. Understanding the difference between these two concepts is important for accurate measurements and calculations in various fields. We hope this article was helpful to you. Please visit again later for more informative articles!