Descripción y análisis del proceso

PRACTICE

TABLE 2.2 Examples of spatially referenced survey data

Home improvement grants Derelict land in 1982 English house condition Land use change statistics survey 1981

Land register database Lead concentrations in drinking water Urban development grants Noise measurements

1975–79 Register of buildings of Gypsy sites historical interest Lifestyle surveys

Source: Adapted from Department of the Environment, 1987

land use, vegetation type, moisture or heat levels or other aspects of the landscape from the photograph.

Aerial photographs are particularly useful for moni-toring change, since repeated photographs of the same area are relatively inexpensive. For example,

Gunn et al. (1994) have monitored changes in land use, particularly peat extraction, in County Fermanagh, Northern Ireland, from a time series of photographs. Interpretation of a sequence of photo-graphs may allow the dating of events such as major

(a) Infrared vertical aerial photograph

(c) Oblique colour aerial photograph

(b) Vertical colour aerial photographs showing archaeological remains

(d) Vertical black and white aerial photograph

Figure 2.19 Aerial photographs

(Sources: (a) U.S. Department of Agriculture; (b) Copyright © Bluesky, used by permission; (c) National Oceanic and Atmospheric Administration (http://boulder.noaa.gov/gifs/aerial.jpg); (d) United States Geological Survey)

floods which cause changes to the landscape.

Curran (1989) identifies six characteristics of aerial photographs that make them of immense value as a data source for GIS:

 wide availability;

 low cost (compared with other remotely sensed images);

 wide area views;

 time-freezing ability;

 high spectral and spatial resolution; and

 three-dimensional perspective.

Additionally, aerial photographs can be used to obtain data not available from other secondary sources, such as the location and extent of new housing estates, or the extent of forest fires. One characteristic of aerial photographs that constitutes a possible disadvantage is the fact that they do not provide spatially referenced data. Spatial referencing has to be added to features on the image by refer-ence to other sources such as paper maps. Several different types of aerial photographs are available, from simple black and white, which may be used for a wide variety of purposes, to colour and thermal infrared for heat identification.

The angle at which the photograph was taken is important. A photograph is referred to as vertical if taken directly below the aeroplane, and oblique if taken at an angle. Oblique photographs generally cover larger areas and are cheaper than vertical pho-tographs. Vertical photographs are, however, the most widely used for GIS applications. Figure 2.19 contains examples of black and white, colour, infrared, vertical and oblique aerial photographs.

Before any aerial photograph information can be used in a GIS a number of factors must be consid-ered. The first of these is scale. Scale varies across an aerial photograph, owing to the distance of the camera from the ground (Figure 2.20). The scale will be constant only at the centre of the image, and the greater the flying height, the greater the scale differ-ence between the centre and edges of the image.

This will also affect the angle of view towards the edge of the image creating the effect whereby tall vertical features such as mountains, buildings and trees appear to lean away from the centre of the image or the ‘nadir’ (that point that is vertically beneath the camera). This is particularly noticeable

in the latest generation of high-resolution digital aerial photography and especially in urban areas where the ‘lean’ of tall buildings can obscure the streets below. Second, factors that may influence interpretation need to be considered. These include time of day and time of year. On photographs taken in winter, long shadows may assist the identification of tall buildings and trees, but may obscure other features on the image. Conversely, in summer, when trees are in full leaf, features that may be visi-ble from the air in winter will be obscured.

Aerial photography has been successfully used in archaeological surveys. Subtle undulations, indicat-ing the presence of archaeological features such as the foundations of old buildings and the outlines of

1:5,000 1:10,000 1:20,000

500 m 1000 m 1500 m Height above ground

Flight path

Increased scale distortion towards edges of photographs

Figure 2.20 Varying scale on aerial

ancient roads and field systems just below the sur-face, can standout in sharp contrast on winter or late evening images. Photographs taken during drought periods may reveal the detail of subsurface features as alternating patterns of green and dry vegetation caused by variations in soil depth.

Aerial photographs represent a versatile, rela-tively inexpensive and detailed data source for many GIS applications. For example, local government bodies may organize aerial coverage of their districts to monitor changes in the extent of quarrying or building development. At a larger scale, photo-graphs can be used to provide data on drainage or vegetation conditions within individual fields or parcels that could not be obtained from conven-tional topographic maps (Curran, 1989).

Satellite images

Satellite images are collected by sensors on board a satellite and then relayed to Earth as a series of elec-tronic signals, which are processed by computer to produce an image. These data can be processed in a variety of ways, each giving a different digital version of the image.

There are large numbers of satellites orbiting the Earth continuously, collecting data and returning them to ground stations all over the world. Some satellites are stationary with respect to the Earth (geo-stationary), for example Meteosat, which produces images centred over Africa along the Greenwich meridian (Curran, 1989). Others orbit the Earth to provide full coverage over a period of a few days. Some of the well-known satellites, Landsat and SPOT, for example, operate in this way. Landsat offers repeat coverage of any area on a 16-day cycle (Mather, 1991).

Figure 2.21 shows examples of images from earth observation satellites.

Most Earth observation satellites use ‘passive’

sensors that detect radiation from the Sun that is reflected from the Earth’s surface. Sensors may operate across different parts of the electromag-netic spectrum, not only those portions visible to the human eye. The multispectral scanner (MSS) on board Landsat simultaneously detects radiation in four different wavebands: near infrared, red, green and blue (Curran, 1989). After processing, the images can be used to detect features not read-ily apparent to the naked eye, such as subtle

changes in moisture content across a field, sedi-ment dispersal in a lake or heat escaping from roofs in urban areas.

A smaller number of satellites use ‘active’ sensors that have their own on-board energy source and so do not rely on detecting radiation reflected from the surface of the Earth. Examples are radar-based sen-sors such SAR (Synthetic Aperture Radar). These have the advantage of being able to explore wave-lengths not adequately provided for by the Sun, such as microwave, and are both able to work at night and penetrate cloud layers. A new form of active remote sensing is LiDAR (Light Detection and Ranging). LiDAR is a remote sensing system that uses aircraft-mounted lasers to collect topographic data from low altitude. The lasers are capable of recording elevation measurements with a vertical precision of 15 cm at spatial resolutions of around 2 m. Measurements are spatially referenced using high-precision GPS. The technology creates a highly detailed digital elevation model (DEM) that closely matches every undulation in the landscape. Even change in ground surface elevation detail caused by buildings and trees can be detected. This makes LiDAR extremely useful for large-scale mapping and engineering applications. The images shown in Figure 2.22 are derived from LiDAR data. Box 2.8 outlines the use of LiDAR data in a hydrological application.

Scanned images are stored as a collection of pixels, which have a value representing the amount of radia-tion received by the sensor from that porradia-tion of the Earth’s surface (Burrough, 1986). The size of the pixels gives a measure of the resolution of the image. The smaller the pixels the higher the resolution. The Landsat Thematic Mapper collects data for pixels of size 30 m by 30 m. Much greater resolution is possible, say 1 m by 1 m, but this has in the past been restricted to military use. Recent changes in US legislation and the availability of Russian military satellite data have made access to very high-resolution data easier such as that provided by the QuickBird and IKONOS satel-lites. Resolution is an important spatial characteristic of remotely sensed data and determines its practical value. A Landsat Thematic Mapper image with a pixel size of 30 m by 30 m would be unsuitable for identify-ing individual houses but could be used to establish general patterns of urban and rural land use. Box 2.9 provides further discussion of resolution.

For GIS, remotely sensed data offers many advan-tages. First, images are always available in digital form, so transfer to a computer is not a problem. However, some processing is usually necessary to ensure

inte-gration with other data. Processing may be necessary to reduce data volumes, adjust resolution, change pixel shape or alter the projection of the data (Burrough, 1986). Second, there is the opportunity to

Figure 2.21 Examples of satellite imagery (Sources: (a) CNES 1994-Distribution SPOT Image;

(b,e) i-cubed: LLC; (c) Science Photo Library/Earth Satellite Corporation; (d) Wetterzentrale, Germany)

(a) SPOT (b) Landsat TM

(c) MSS

(e) IKONOS satellite imagery

(d) Meteosat

Stuart Lane and Joe Holden

Rainfall leads to changes in stream flow. To understand these changes we need to understand hydrological connectivity between hillslopes and river channels.

Soils on saturated hillslopes that are hydrologi-cally connected to a river channel will demonstrate a rapid response to rainfall inputs. This is because the water can enter the channel more quickly than from

saturated soils on slopes that are not connected (Figure 2.23). Catchments where the response to rainfall inputs is rapid (where saturated soils exist and there is a high degree of connectivity) a ‘flashy’ or rapid discharge response is seen. These catchments are prone to flooding, erosion and diffuse pollution problems. For effective land management in flood prone catchments information about the following factors is important:

BOX 2.8 High resolution