Aerial photography has been widely used for assessment of forests for more than 50 years; it therefore has the benefit of being a tried and tested technique (Lachowski et al. 2000). Black-and-white, colour, and infrared aerial photographs are routinely collected over many forest areas, and are used for forest mapping, assessment of forest condition, forest management planning, and conservation assessments (Figure 2.1).
Despite the development of satellite technologies, aerial photographs are still the most common form of remote sensing used to assess and map forests, primar- ily because they can provide high-resolution images at relatively low cost, and are relatively easy to use. They are also flexible: photographs are available at a range of scales, and can be produced by using a variety of different films, lenses, and cameras (Franklin 2001). The most significant advantage of aerial photographs Aerial photography | 33
Fig. 2.1 Aerial photograph of part of the New Forest National Park in southern England. Such photographs are an extremely useful tool for assessing forest extent and distribution. On this image, the boundaries of forest fragments and even the location of individual trees can readily be determined. Comparison of such images taken at different times enables vegetation change to be assessed. (Courtesy of Getmapping plc, with permission.)
over satellite images is that they can be interpreted with little or no processing, which greatly increases their practical value and reduces their cost. With relatively little practice or training, most people are able to readily interpret many of the fea- tures illustrated in a typical aerial photograph. Useful introductions to the use of aerial photography in forest assessment are provided by Franklin (2001) and Hall (2003). Details of the methods used for estimating stand variables from aerial photography, including appropriate algorithms, are provided by Stellingwerf and Hussin (1997). Wolf and Dewitt (2000) provide a detailed account of the prin- ciples of photogrammetry (or the methods by which information can be derived from remote sensing imagery, including photographs) and its links with GIS.
2.2.1 Image acquisition
If financial resources are available, it may be possible to commission an organiza- tion or company to provide the photographs required. Specialist companies are now available in many areas that will acquire specific imagery on request. However, it is often the case that aerial photographs are already available for the area of interest. National forest services or conservation agencies may possess extensive archives of air photo imagery, although it is important to remember that interest in such photographs goes well beyond the forestry and conservation sectors. Land planning and rural development agencies, local or regional government administrations, hydrological surveys, and agricultural departments may all have commissioned aerial surveys at various times. Failing that, national military or defence organizations can usually be relied upon to possess comprehensive air photo coverage, which in some cases is made available to the public sector.
Some companies, such as those involved in mining or the construction of oil, gas, or water pipelines, also invest in developing extensive archives of air photos.
Increasingly, comprehensive coverage is offered by specialist companies who can provide specific images to order, from archives that they have already developed.
The Internet resource Google Earth 具www.earth.google.com典is a particularly useful source of such imagery.
Whatever the source of the imagery, a key consideration is scale, which deter- mines the area on the ground that the photograph can detect. The spatial resolution or resolving power of a photograph describes the degree of detail on the ground that can be observed, and is influenced by the properties of the camera lens and film used in taking the photograph, as well as the characteristics of the object itself, such as its degree of contrast with its surroundings. The resolution of the image is often expressed in the form of line pairs per millimetre, values of which can be calculated based on the finest set of parallel lines that can be clearly resolved when the image is examined (Wolf and Dewitt 2000). These values can be converted into an effective spatial resolution at a given scale, giving an indication of the size of objects on the ground that can be differentiated. Resolution values that are obtained for photography systems used to assess forests generally fall within the range 0.25–0.8 m at a scale of 1 : 20 000 (Hall 2003), although higher-resolution imagery can sometimes be obtained.
Thespectral sensitivityof an aerial photograph refers to the sensitivity of the film to different parts of the electromagnetic spectrum, which depends on the type of film used. The main types of film used are black-and-white, black-and-white infrared, colour, and colour infrared (Lillesand and Kiefer 1994). Radiometric resolution refers to the image contrast or density on the film, and is influenced by the dyes and metallic silver used in the manufacture of the film, and its degree of exposure. Films also differ in terms of their types of emulsion and how they are processed, which can affect the graininess of the image, and therefore its spatial resolution. The characteristics of films used in aerial photography can most readily be obtained by reference to the websites of the relevant manufacturers (Hall 2003).
The quality of the image produced can also be influenced by the type of paper used in printing the image from the negative.
Often, aerial photographs are digitized by using a scanner, to create digital images that can be viewed on a computer and incorporated within a GIS. It is important that the scanner used has sufficient geometric and radiometric reso- lution, as well as high geometric accuracy, as otherwise the scanning process can introduce artefacts into the image. A number of scanners specially designed for photogrammetry are commercially available (Wolf and Dewitt 2000). Although relatively low-cost desktop scanners can also be used, they are generally less accur- ate and may distort the image. The minimum radiometric resolution of the scanner should be 8-bit (256 levels), although most modern scanners are able to capture images at 10-bit (1024 levels) or higher. Minimum pixel sizes should be of the order of 5–15 mm, and the positional accuracy should be around 2–3 mm (Wolf and Dewitt 2000). Scan resolution is often given in the form of dots per inch (dpi). The size of a single pixel on the ground can be calculated by expressing the scale as 1 cmxm, then using the following simple formula (Hall 2003):
As a general rule, images should be scanned so that the pixel size is no larger than 20–25% of the size of the object to be resolved. For example, at a map scale of 1 : 20 000, 1 mm on the map is equivalent to a distance of 20 m on the ground. In order to be able to map this level of precision with a scanned image, the pixel size should be no larger than 4–5 m (Hall 2003).
It is important to check the date when the photograph was taken. The time of year influences the solar angle and therefore affects how objects within the photo- graph are illuminated by the sun, and the size of shadows that are cast. The phen- ology of the vegetation changes through the growing season, even in evergreen forests, and this influences the characteristics of the vegetation on the photograph.
Details of the flightpath may also be useful as an aid to interpretation.
It is also important to note that aerial photographs vary in terms of the angle above the ground at which they are taken. Whereas vertical photographs are taken with the axis of the camera arranged vertically, oblique photographs will result if the camera axis is tilted. In forest ecological work, vertical air photographs are
pixel size (m)2.54scale (m) dpi
Aerial photography | 35
almost always preferred. If obliques are available, they may be of some value in terms of interpreting the characteristics of a forest area, but are likely to be of lim- ited value with respect to development of forest maps unless they can be adequately converted through some form of digital processing (orthorectification).
2.2.2 Image processing
Much useful information can be gained by examining aerial photographs visually, for example by using a hand lens or binocular microscope. Stereoscopes can be used to view stereo pairs of photographs, enabling a three-dimensional image to be viewed. This is an important technique for photogrammetry, and is described in detail by Wolf and Dewitt (2000). However, at least for most ecological applica- tions, photographs are generally viewed on a computer screen following digitiza- tion (it should be noted that instruments for on-screen stereoscopic examination of digitized images are now available; Hall 2003).
Once in the digital domain, the photograph will often need to be rectified if it is to be used as a basis for spatial analysis or mapping. Image rectificationrefers to the process of producing an image that is geometrically corrected, removing any dis- tortions introduced during the photographic process. The process of correcting for distortion caused by variation in topography is called orthorectification, and photo- graphs that have been processed in this way are often referred to as orthophotos.
Orthorectification is almost always necessary, unless photographs are obtained that have already been processed in this way.
There are two main methods by which an aerial photograph can be orthorecti- fied. First, ground control points(GCPs) are taken at selected locations within a land- scape; these may be obtained from field surveys, perhaps by using a global positioning system(GPS), or directly from a published map. These points are then located on the image and their coordinates entered. At least 3–5 GCPs must be established in this way, but this is a minimum, and more accurate results are obtained if a larger number of GCPs is used. An alternative method is to use digital elevation models(DEMs), which may be derived from digital maps, some remote sensing data (such as lidar) or from stereoscopic models by photogrammetric methods. As with GCPs, a relationship is determined between the map coordinates in the real world and locations on the digitized aerial photograph, and the digital image is then resampled to create the rectified image. This resampling involves warping the image so that distance and area measurements made on the image are closely related to those in the real world. Orthorectification is generally done by using specialist software; some widely used software packages are listed in Table 2.1.
Whether or not a photograph is rectified, it will certainly need to be referenced if it is to be used as a basis for producing maps. The process of georeferencing(some- times called ground registration) involves processing an image so that it is aligned according to a ground coordinate system (Wolf and Dewitt 2000). Although the process shares some similarities with orthorectification, it is important not to confuse the two processes: whereas orthorectification corrects for distortion in the image, georeferencing enables the image to be related to existing map coordinate
Aerial photography|37 Table 2.1 Selected computer software packages appropriate for processing aerial photographs.
Product Comments URL
Aerial Image A relatively cheap product that enables aerial images to be rectified, www.tatukgis.com/
Corrector (AIC) referenced and mosaiced.
ArcView Image An extension to the widely used ArcView GIS software that enables www.esri.com/software/arcview/extensions/imageanalysis/
Analysis imagery to be manipulated and viewed. Can be used for data visualization, data extraction/creation, and analysis.
ER Mapper Very powerful, relatively expensive, high-specification software www.ermapper.com/
package, enabling a wide range of digital image analyses including orthorectification.
ERDAS Another powerful, and relatively expensive, but widely used http://gis.leica-geosystems.com/
IMAGINE software package. The same company also markets simpler versions of IMAGINE software, and other sophisticated products such as the Leica Photogrammetry Suite.
Orthoengine A flexible software package specifically designed for www.pcigeomatics.com/product_ind/orthoengine.html orthorectification; can also be used to produce digital elevation
models (DEMs).
SmartImage An application that provides integrated spatial analysis and www.mappingandbeyond.com/
visualization, including rectification, aimed at MapInfo or ArcView users.
3D Mapper A fully featured, desktop soft photogrammetry package solution www.3dmapper.com/3dmapper.htm that enables users to capture 3D vector data and orthophotos from
(scanned) digital photography.
systems. Again, GCPs are selected in the image for which coordinates on the ground are available. These are then entered, usually by clicking on the appropri- ate points on a computer screen using a mouse. Most GIS software packages have the capability to georeference images in this way, although it should be noted that their ability to warp images (as required during orthorectification) is often limited.
As a result, a GIS package cannot be relied upon to provide all of the tools that may be required to process a photograph to the degree necessary for its use in mapping activities; additional image-processing software (Table 2.1) may also be needed.
Once an image has been georeferenced it can be combined with other spatial data in a GIS with the same coordinate system. Measurements of variables such as distance and area can now be made, enabling different forms of spatial analysis to be carried out. The accuracy of these measurements depends on the accuracy of the georeferencing process, as well as the characteristics (such as spatial resolution) of the image, so care should be taken throughout the processing procedure, and attention paid to the levels of accuracy achieved.
2.2.3 Image interpretation
Aerial photographs are usually interpreted visually, a skill that can be developed through training and practice. Guidance on the interpretation of air photographs is provided by Avery (1968, 1978) and Avery and Berlin (1992). Areas or objects can be differentiated by inspection of characteristics such as tone, texture, pattern, size, shadows, shapes or associations (Lillesand and Kiefer 1994). These may be defined as:
● Image tone or colour. Many objects have a characteristic colour or tone, depending on the specific signatures of electromagnetic radiation that are reflected or emitted. Different types of vegetation may therefore vary in how they appear on either black-and-white or colour images, according to their species composition, phenological state and canopy characteristics.
Coniferous tree canopies, for example, often appear darker than those of broadleaved tree species. Usually, similar objects emit or reflect similar wave- lengths of radiation. The types of camera and film used can also influence how objects appear on an image. For example, on colour infrared images forest canopies tend to appear pink or red rather than the usual tones of green.
● Texture. Vegetation canopies differ in their surface texture, or whether they appear rough or smooth. Texture can readily be used to differentiate between different types of land cover, such as forest and cropland; for example, agri- cultural crops often appear to have a smoother, more homogeneous texture than most natural forest canopies. Forest canopies with many strata or canopy gaps can appear rougher than even-aged stands. Texture, as with the size of the objects being observed, is related to the scale of the image.
● Pattern. How objects are arranged within an image can help aid their identifi- cation. Plantation forests or orchards, for example, tend to be characterized by a more regular pattern of tree distribution than natural forests.
● Size. It is important to note the scale of the image during visual inspection, as this has a major bearing on how objects are interpreted. Both the relative and absolute sizes of objects are important in their identification. The absolute size of an object is determined by reference to the scale of the image.
● Shadow. The presence of shadows can greatly complicate image interpreta- tion, as shaded features generally appear to be dark and difficult to discern.
On the other hand, shadows can be used to help interpret features: for example, the length of a shadow cast by an individual tree can give an indication of its height relative to other objects on the image. Shadows can also display the shape of an object on the ground.
● Shape. The shape of objects can be highly diagnostic. Roads, rivers, and urban development can readily be identified because of their characteristic shapes, but it is also possible to differentiate some individual tree species on the basis of crown shape, as well as different kinds of disturbance to which a forest has been subjected—a fire or logging coup may produce a canopy gap with a very different shape to one caused by a natural windthrow.
Visual interpretation of aerial photographs can be used to map a wide variety of forest features, including variation in species composition, degree of crown clos- ure, height class and density, and pattern of disturbance. Typically, forests are mapped as stands, which may be defined as areas with relatively homogeneous characteristics. Determining the boundaries between forest stands can, however, be difficult in practice. As with all other aspects of visual interpretation, decisions made by the analyst are subjective and may therefore be subject to a degree of error that can be difficult to quantify. However, experienced practitioners are able to discern features with a very high degree of accuracy, which can potentially be validated by reference to field surveys, and for this reason aerial photography is still the main technique of choice for producing maps in support of forest manage- ment. For example, in Finland, data for forest management planning are generally gathered by field surveys of forest stands that are first delineated by reference to air photos (Pekkarinen and Tuominen 2003). Many studies have shown that digitized aerial photographs can do better than satellite remote sensing data for a variety of forest mapping applications (Franklin 2001, Hyyppäet al. 2000, Poso et al. 1999).