Ground truth
Encyclopedia
Ground truth is a term used in cartography
, meteorology
, analysis of aerial photographs
, satellite imagery
and a range of other remote sensing
techniques in which data are gathered at a distance. Ground truth refers to information that is collected "on location." In remote sensing, this is especially important in order to relate image data to real features and materials on the ground. The collection of ground-truth data enables calibration of remote-sensing data, and aids in the interpretation and analysis of what is being sensed.
More specifically, ground truth may refer to a process in which a pixel
on a satellite
image is compared to what is there in reality (at the present time) in order to verify the contents of the pixel on the image. In the case of a classified image, it allows supervised classification to help determine the accuracy of the classification performed by the remote sensing software and therefore minimize errors in the classification such as errors of commission and errors of omission.
Ground truth is usually done on site, performing surface observations and measurements of various properties of the features of the ground resolution cells that are being studied on the remotely sensed digital image. It also involves taking geographic coordinates of the ground resolution cell with GPS technology and comparing those with the coordinates of the pixel being studied provided by the remote sensing software to understand and analyze the location errors and how it may affect a particular study.
Ground truth is important in the initial supervised classification of an image. When the identity and location of land cover types are known through a combination of field work, maps, and personal experience these areas are known as training sites. The spectral characteristics of these areas are used to train the remote sensing software using decision rules for classifying the rest of the image. These decision rules such as Maximum Likelihood Classification, Parallelepiped Classification, and Minimum Distance Classification offer different techniques to classify an image. Additional ground truth sites allow the remote sensor to establish an error matrix which validates the accuracy of the classification method used. Different classification methods may have different percentages of error for a given classification project. It is important that the remote sensor chooses a classification method that works best with the number of classifications used while providing the least amount of error.
Ground truth also helps with atmospheric correction. Since images from satellites obviously have to pass through the atmosphere, they can get distorted because of absorption in the atmosphere. So ground truth can help fully identify objects in satellite photos.
opposed to what intelligence reports and mission plans assert the reality to be. The term is
reflected in the title of the 2006 Iraq War documentary The Ground Truth
and is used in military publications, for example Stars and Stripes
saying "Stripes decided to figure out what the ground truth was in Iraq."
The military usage of the term is long-standing but its origins are obscure. It is plausible but difficult to prove that "ground truth" began life as military terminology and then was applied to other domains such as remote sensing control.
Ground truthing (verb) in science is gathering objective data to assess whether a theoretical model is accurate. This is the most basic test of the scientific method.
Cartography
Cartography is the study and practice of making maps. Combining science, aesthetics, and technique, cartography builds on the premise that reality can be modeled in ways that communicate spatial information effectively.The fundamental problems of traditional cartography are to:*Set the map's...
, meteorology
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
, analysis of aerial photographs
Aerial photography
Aerial photography is the taking of photographs of the ground from an elevated position. The term usually refers to images in which the camera is not supported by a ground-based structure. Cameras may be hand held or mounted, and photographs may be taken by a photographer, triggered remotely or...
, satellite imagery
Satellite imagery
Satellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...
and a range of other remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
techniques in which data are gathered at a distance. Ground truth refers to information that is collected "on location." In remote sensing, this is especially important in order to relate image data to real features and materials on the ground. The collection of ground-truth data enables calibration of remote-sensing data, and aids in the interpretation and analysis of what is being sensed.
More specifically, ground truth may refer to a process in which a pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
on a satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
image is compared to what is there in reality (at the present time) in order to verify the contents of the pixel on the image. In the case of a classified image, it allows supervised classification to help determine the accuracy of the classification performed by the remote sensing software and therefore minimize errors in the classification such as errors of commission and errors of omission.
Ground truth is usually done on site, performing surface observations and measurements of various properties of the features of the ground resolution cells that are being studied on the remotely sensed digital image. It also involves taking geographic coordinates of the ground resolution cell with GPS technology and comparing those with the coordinates of the pixel being studied provided by the remote sensing software to understand and analyze the location errors and how it may affect a particular study.
Ground truth is important in the initial supervised classification of an image. When the identity and location of land cover types are known through a combination of field work, maps, and personal experience these areas are known as training sites. The spectral characteristics of these areas are used to train the remote sensing software using decision rules for classifying the rest of the image. These decision rules such as Maximum Likelihood Classification, Parallelepiped Classification, and Minimum Distance Classification offer different techniques to classify an image. Additional ground truth sites allow the remote sensor to establish an error matrix which validates the accuracy of the classification method used. Different classification methods may have different percentages of error for a given classification project. It is important that the remote sensor chooses a classification method that works best with the number of classifications used while providing the least amount of error.
Ground truth also helps with atmospheric correction. Since images from satellites obviously have to pass through the atmosphere, they can get distorted because of absorption in the atmosphere. So ground truth can help fully identify objects in satellite photos.
Errors of commission
An example of an error of commission is when a pixel reports the presence of a feature (such as trees) that, in reality, is absent (no trees are actually present). Ground truthing ensures that the error matrices have a higher accuracy percentage than would be the case if no pixels were ground truthed.Errors of omission
An example of an error of omission is when pixels of a certain thing, for example maple trees, are not classified as maple trees. The process of ground truthing helps to ensure that the pixel is classified correctly and the error matrices are more accurate.Military Usage
In US military slang, "ground truth" is used to describe the reality of a tactical situation asopposed to what intelligence reports and mission plans assert the reality to be. The term is
reflected in the title of the 2006 Iraq War documentary The Ground Truth
The Ground Truth
The Ground Truth is a 2006 documentary film about veterans of the Iraq War...
and is used in military publications, for example Stars and Stripes
Stars and Stripes (newspaper)
Stars and Stripes is a news source that operates from inside the United States Department of Defense but is editorially separate from it. The First Amendment protection which Stars and Stripes enjoys is safeguarded by Congress to whom an independent ombudsman, who serves the readers' interests,...
saying "Stripes decided to figure out what the ground truth was in Iraq."
The military usage of the term is long-standing but its origins are obscure. It is plausible but difficult to prove that "ground truth" began life as military terminology and then was applied to other domains such as remote sensing control.
Vernacular Usage
Somewhat analogous to military slang, "ground truth" may be used in the scientific vernacular to refer to the presumed objective reality of a statement or situation, in contrast with some predicted or computed expectation of reality. "Is (x) ground truth?" is a way of asking "Is x really what is going on, objectively, versus some evidential claims we are making?"Ground truthing (verb) in science is gathering objective data to assess whether a theoretical model is accurate. This is the most basic test of the scientific method.
External links
- Forestry Organization Remote Sensing Technology Project (includes an example of an error matrix)