Resolution in remote sensing terms

 
 
It is important to understand how a remote sensing tool operates in order to appreciate its ability to detect features - such as habitats - on the seabed (in intertidal or subtidal areas). There are many excellent texts and websites explaining all the technical aspects of remote sensing; for example Green et al.,  describe the use of satellite and airborne remote sensors for marine mapping in tropical waters. The following text is based on information available on the US NASA website with some definitions taken from Wikipedia.
 
Remote sensors measure and record the magnitude and frequency of reflected energy from an object where the ‘energy’ is generally either electromagnetic radiation (light) or acoustic (sound). Remote sensing devices mounted on aircraft and satellites normally use imaging sensors that measure reflected energy from objects under surveillance; the mostly commonly used sensors for underwater detection use acoustic systems although the results are often presented as images. Imaging sensors fall into two general categories: active sensors and passive sensors. Passive sensors monitor only the natural solar reflected light or electromagnetic energy from an object and form the majority of the airborne and satellite based sensors in use today. Active image sensors provide their own energy which is transmitted to the object and then reflected back to the sensor. Acoustic systems, RADAR and LiDAR (based on a laser) are all active sensors.
 
Early remote sensing devices recorded photographic images on film (taken by cameras) or traces printed onto paper rolls (sonar devices). Both routes created an image in analogue format. These images were fixed and could not be subject to very much manipulation (correction, change of contrast or colour etc); more recently, they can be converted into an electronic digital format for limited manipulation. Most modern sensors now record their information in digital format, often as digital images. A digital image is made up of numbers, which represent image attributes such as brightness, colour or radiated energy frequency wavelength, and position location for each point or picture element in the image. The smallest sized picture element on an image is called a pixel; a digital image is made up of pixels arranged in rows and columns commonly known as a raster image. The dimensions and the information content of these pixels are both aspects of the resolution of the image.
 
Resolution has a popular meaning but is best defined in a technical sense. We normally think of resolution as the ability to separate and distinguish adjacent objects or items in a scene, be it in a photo, an image or real life. We often specify the resolution in terms of the linear size of the smallest features we can discriminate (often expressed in meters). But, contrast influences our ability to resolve between objects: if two items are the same colour, they may be hard to separate, but if they are sharply different in colour, tone, or brightness we can identify them more easily. Remote sensors measure differences and variations of objects that are often described in terms of three main resolutions, each of which affect the accuracy and usefulness of remote sensors to habitat mapping.
 
  • Spatial resolution describes the ability of a sensor to identify the smallest size detail of a pattern on an image. In other words, the distance between distinguishable patterns or objects in an image that can be separated from each other and is often expressed in meters.
  • Spectral resolution is the sensitivity of a sensor to respond to a specific frequency range (mostly for satellite and airborne sensors). The frequency ranges covered often include not only visible light but also non-visible light and electromagnetic radiation. Objects on the ground can be identified by the different wavelengths reflected (interpreted as different colours) but the sensor used must be able to detect these wavelengths in order to see these features.
  • Radiometric resolution is often called contrast. It describes the ability of the sensor to measure the signal strength (acoustic reflectance) or brightness of objects. The more sensitive a sensor is to the reflectance of an object as compared to its surroundings, the smaller an object that can be detected and identified.

 

When selecting a remote sensing tool for habitat mapping, it is clearly essential that you consider the capability of the tool in terms of its ability to both resolve spatial and textural differences in the habitats you wish to display on your final map.
 
Links to websites
 

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