This data is collected from a vantage point using measurement devices designed with remote sensing in mind. One of the original examples of remote sensing still in use today is the use of cameras mounted on aircraft to produce aerial photography.

The objects on these photographs are then identified (identification is discussed on page Error: Reference source not found) after close examination and are then digitised according to their features and significance. The data itself can contain any feature or characteristic about the observed phenomenon, for example, the features of a particular object in a group, or the characteristics of the group as a whole.

The devices used for remote sensing (referred to as sensors) can be mounted on orbital and suborbital platforms. Orbital platforms are those situated in Earth’s orbit, such as satellites. Suborbital platforms are located closer to Earth, such as aircraft. There is no specific distance defined for remote sensing, meaning that the sensor could be, for example, one-hundred kilometres or one centimetre from the phenomenon observed and still qualify as remote.

Most sensors record EMR (electromagnetic radiation), which changes depending on the properties and quantities of the phenomenon observed, allowing objects and their attributes to be differentiated. Sonar is another form of remote sensing, but it is not used nearly as often since EMR is usually better suited.

Remote sensing has several advantages to other methods of data gathering; for example, the data is often recorded passively, meaning that it does not influence or disturb the phenomena of interest, which could possibly lead to incorrect or misleading results.

Furthermore, remote sensing data is recorded in a systematic manner, which would remove any sampling bias that is often introduced with data that is gathered in situ. The data itself could be entirely new and informative, and could provide combinations and correlations not previously available. It could also be on a larger scale that was previously impossible with data gathered in situ.

Remote sensing also has a series of limitations, with the most prominent being the fact that it is often seen as an overarching solution to all data gathering problems, providing all the necessary data and information. This is not the case, remote sensing is not suitable in all situations and trying to apply it to an inappropriate problem will result in failure.

Not all remote sensing systems are passive, and active systems emit their own radiation which could influence results and lead to incorrect data and interpretations. This issue has not yet been researched thoroughly enough to exactly determine the extent to which the gathered data is influenced.

Human error could also be introduced into the data captured by remote sensing devices since they are managed entirely be people who are prone to mistakes and fatigue. The devices could also lose their calibration over time, resulting in data that is slightly more incorrect each time it is gathered. Remote sensing is also expensive in both the collection and analysis of the data, which means that it is not always a viable option.

The elements that can be used to identify objects on aerial photographs

Location

The exact location of a phenomenon can be obtained using GPS or surveying techniques, or by retrieving sensor data and applying it to a basemap from which coordinate information can be obtained in relation to other features in the basemap, as well as its origin.

GPS is often used since it is inexpensive and allows the user to obtain an exact location in longitude and latitude. Remote sensing devices often have GPS equipment that allows the collected data to be mapped automatically according to location data.

Tone and colour

Different surfaces reflect different amounts of radiation in various wavelengths, which means that it is possible to isolate different colours and shades in remotely sensed EMR data.

A spectral signature can be obtained by mapping the amount of energy each type of material reflects, creating a simple way to reference different materials to the energy they radiate. EMR data can be interpreted according to the shade of black or white illustrated on an image representing a particular band of wavelength (a certain colour for example), this is referred to as the tone of the image.

Since certain wavelengths penetrate surfaces more successfully than others, this tone can be used to identify objects and their attributes more accurately. A certain range of green can penetrate water better than other wavelengths, allowing underwater observations to be made based on this data. Foliage and other objects can similarly be isolated according to the wavelengths they radiate. In fact, plant health and species can be differentiated using this method. One should however avoid analysing only a single band as it could lead to an incorrect interpretation; all bands should be compared during inspection to ensure that the tone illustrates the expected phenomenon as indicated on the spectral signature.

Various wavelengths can be combined to produce a colour image, which often aids interpretation substantially, allowing features to be differentiated easily. Colour blindness tests need to be done to ensure that all analysts perceive colour in the same manner to avoid confusion.

Size

The size of an object determines its length, width, perimeter and area. These attributes are often measured by analysts to classify unknown objects found in spatial data. In order to calculate the size of an object, the scale of the image must be known. An unknown scale could be approximated using the known measurements of an object in an aerial photograph (for example a baseball pitch).

The scale of digital data is represented by the spatial resolution which determines the size of a pixel (or cell of data) in ground units. All objects smaller than this resolution will appear to be the same size (the minimum ground size that the resolution can contain).

Aerial photographs often need to be adjusted to convert them to orthophotographs since features at different elevations will appear to be a different size. Measurements made using unmodified aerial photographs could be very inaccurate due to this.

Shape

The shape of an object is often useful in its identification since many types of objects share a similar form; cars and aircraft are good examples of this. Shapes are usually described with a variety of adjectives, e.g. elongated, triangular and radial.

Man-made objects often appear more rigid and Euclidean (buildings, roads) than natural objects (streams, mountains), which also aids identification.

Texture

Texture refers to the homogeneity of a surface; from a distance (e.g. an aerial photograph), roads seem “smooth” in relation to forests. It usually involves the repetition of a feature, creating a pattern from which the smoothness of the phenomenon can be deducted based on the height (interpretation of shadows) and frequency of the repetition. The interpretation of texture depends upon the scale of the image from which it is inspected; a road is very coarse upon close inspection, and yet seems smooth when viewed from afar.

Pattern

The arrangement of features produce a pattern; this pattern can be ordered or seemingly random, man-made structures usually follow some kind of ordering while nature appears random in relation. Well known patterns can aid the interpreter in identifying objects.

Shadow

Shadows can be very helpful in the identification and categorisation of objects in aerial photographs; they can indicate the height of objects in relation to others and sometimes provide more information than the objects themselves. They allow the interpreter to view some features only visible when the object is seen from the side, as is the case with some architecture such as bridges and towers.

Height and depth

The height and depth of an object is referred to as its elevation and bathymetry, respectively. Analysts often use stereoscopic equipment that allows them to view two aerial photographs taken from different angles simultaneously to interpret height and depth.

Objects with exceptional height in relation to their surroundings will display relief displacement, which means that it will mask some of the features around it due to the angle at which the photo was taken.

Read more: http://www.ukessays.com/essays/philosophy/the-site-situation-and-association-philosophy-essay.php#ixzz33f0gK6IX