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
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
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
; a digital image is made up of
pixels arranged in rows and columns commonly known as a raster
. 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
Links to websites