An image is a mesh or grid of cells referenced to actual geography. We call this a raster, and each cell within a raster contains a value. Each image is made up of a series of rows of cells (pixels) that contain a value. The values represented in each cell of an image can be colored pixels like those seen in any digital photo. Often times the value in a raster corresponds to a color, but it can represent elevation information, a land cover type, a land use category, or even an amount of precipitation.
In short, an image is a series of geo-referenced cells that each contains the value of the observed phenomenon. In space science, ground astronomy and Earth observation, ultra-definition images have primary importance. It is the imaging sensor that makes all the difference. If you want to learn more about these sensors then click here to learn more.
The observed phenomenon, what does this mean?
It is important to understand that when we talk about images in GIS we are not only referring to the color images that perhaps we are more used to seeing every day. We also refer to raster data sets that contain radiation reflected or emitted by sensors across the electromagnetic spectrum. A digital photograph only captures reflected light within the visible part of the electromagnetic spectrum. Many devices are capable of capturing portions of the electromagnetic spectrum inside and outside the visible portion, including infrared light and thermal radiation.
An image containing measurements of radiation across the electromagnetic spectrum can provide us with information that would otherwise be invisible to the human eye. Remote sensing enables continuous and widespread monitoring of the Earth, even in inhospitable areas. There are many different types of sensors to capture data. These sensors can be divided into two categories – active and passive.
What are active and passive sensors?
Active sensors provide their own source of energy to illuminate the objects they observe. An active sensor emits radiation in the direction of the target to be investigated. The sensor then detects and measures the radiation that is reflected from the target. Active sensor example includes Radar and Sonar. Passive sensors detect natural energy emitted or reflected by the object or scene being observed. Reflected sunlight is the most common source of radiation measured by passive sensors.
What are the benefits of detecting radiation?
All entities on Earth have unique spectral signatures. A spectral signature is the profile of energy reflected, absorbed, or emitted through the electromagnetic spectrum. Some features have very similar profiles within visible wavebands, but show variations in other areas of the spectrum. This variation allows us to differentiate between entities, ultimately allowing us to visualize objects that would otherwise be indistinguishable through rendering or indexes, or to create new data sets by classifying each entity into a group or set.
So, as we see, with the help of imaging sensors we can monitor Earth’s radiation, to know how it evolves and effects the vegetation, human lives, and Earth itself.