During the past two decades, GIS has grown from a specialist technique with only a small number of practitioners to become one of the most important tools in envir- onmental science and management. Use of GIS in forest ecology and conserva- tion is now very widespread. Forest managers use GIS to manage and display inventory data, and as a basis for management planning and monitoring.
Ecological researchers use GIS to visualize and analyse spatial patterns in ecological communities, and increasingly to investigate the spatial processes responsible for generating such patterns (Figure 2.4). The increased ability to collate and process spatial data afforded by GIS has led to the development of landscape ecology as a subdiscipline in its own right, with its own specialist journals (Landscape Ecology), organizations (the International Association for Landscape Ecology (IALE), 具www.landscape-ecology.org/典and textbooks (Gutzwiller 2002, Turner et al. 2001).
GIS is also widely used to support conservation planning and priority setting, and for mapping biodiversity.
GIS can be defined most simply as a tool for the collection, integration, process- ing, and analysis of spatial data (DeMers 2005). A common feature of GIS is the ability to present data in different layers, which can be overlaid on top of each other (I once heard GIS described, by no less than a professor of geography, as ‘glorified tracing paper’). Most importantly, GIS can be used to produce maps, which differentiates it from most computer-aided drafting (CAD) systems that otherwise share some features with GIS. The technologies associated with GIS, both hardware and software, continue to develop rapidly. For a thorough introduction to GIS, specialist textbooks should be consulted. Many such books are now available (Borrough and McDonnell 1998, DeMers 2005, Longley et al. 2005). Johnston (1998) provides a useful introduction to the use of GIS in ecology. Informative resources are also available via the Internet 具http://gislounge.com典.
Although GIS software is becoming more accessible and user-friendly, it is also becoming more powerful, which can increase the length of the learning curve. As noted by Johnston (1998), ecologists should not assume that mastering GIS soft- ware will be as rapid as learning word processing or spreadsheet software. Problems that arise can be difficult to solve without access to an experienced practitioner.
Such people can be difficult to find, even in academic environments, because they
can usually earn a lot more in the private sector (Johnston 1998). Extensive resources are available via the Internet that can help solve specific problems, but there is no substitute to discussing a technical issue with a colleague, so before embarking on a GIS project it is worth spending time searching for people who might be able to help. Johnston (1998) provides some further advice for those about to start using GIS (Table 2.7).
Implementation of GIS requires appropriate software and computer hardware on which to run it. As spatial data sets can be very large and require a lot of processing power, the computer should be equipped with large amounts of RAM and hard disk memory, as well as a reasonably fast processor. Most commercially Geographical information systems (GIS) | 69
Fig. 2.4 Example of application of GIS to support a forest conservation project in the Isle of Wight, southern England. In this example, a map of woodland cover has been produced by creating a separate polygon (vector data) for each individual woodland (coloured black on the figure) in the landscape. This was achieved by digitizing woodland boundaries identified on an aerial photograph. To achieve this, the photograph was georeferenced and included as a separate data layer in the GIS database. The white points were created as an additional data layer, and represent the locations of a threatened insect species determined in a field survey using a GPS device. (Courtesy of Niels Brouwers.)
|Forest extent and condition Table 2.7 Advice that should be heeded before embarking on a GIS project (adapted from Johnston 1998).
Keep it simple Begin with relatively simple data and software. Although high-specification GIS software may appear attractive, a simpler package may be adequate for the task at hand, and is likely to be easier to learn.
Read the Manuals provided with the software should be carefully consulted. Extensive online help is now available, for documentation example on the websites of the companies producing the software, via user groups for particular software packages,
and from online teaching resources developed by educational establishments.
Use existing data Much spatial data can now be accessed online, and downloaded free of charge in many cases. Develop collaborative partnerships with others investigating the same study area so that data can be shared, duplication of effort avoided, and considerable time saved.
Plan ahead GIS analysis usually requires a series of steps to be undertaken, which should be planned beforehand Keep good records This is a crucial and often neglected aspect of GIS work. Each data set used should be carefully described and
documented, and the source noted. (Such information about data is often referred to as metadata.) Keep detailed notes of each step undertaken in the compilation and analysis of the data.
Check results It is important to apply quality control throughout the process; inspect the data and compare different data sets against each other to check on their accuracy. Check that simple operations, such as distance and area operations, are giving correct results.
Consult with experts Before starting any GIS project, consult experienced practitioners for advice on database management and GIS procedures. If no experts are readily available, form a user group with interested colleagues to share tips and techniques.
Geographical information systems (GIS) | 71 available GIS software will perform well on most modern desktop personal computers. Facilities for data storage and backup are an important part of any GIS computer system; writable CD and DVD drives and additional hard disks are all potential options that should be explored. Once appropriate hardware is available, the main choices that need to be made concern selection of appropriate software, types of data, map projection, and the analytical procedures to be used. These are each considered in the following sections.
2.6.1 Selecting GIS software
Several different GIS software programs are available (Table 2.8), which vary in their specifications and ease of use. How to choose which is most appropriate?
This will depend on the precise objectives of the task set, and also the previous experience of the user. The best approach is to try using demonstration copies of different software packages, which are often available as a free download via the Internet. Information about the products provided by the manufacturers should also be consulted. Most GIS software programs provide the same basic facilities, for example the ability to enter geographic objects such as lines, polygons, and points, together with their attributes, and to overlay different data layers. Where software programs differ most markedly is in their ability to process and analyse spatial data, rather than basic mapping. Also, some are much easier to use than others: pay careful attention to the documentation, tutorials and help facilities provided.
Some points to consider when selecting GIS software include the following:
● Will the software run on the hardware that is available? Is it compatible with the computer’s operating system? Does the hardware have sufficient capacity (in terms of RAM, hard disk memory, and processor speed) to run the software adequately?
● Which file types are supported? Digital data are available in a very wide variety of formats, which are changing all the time. If there are specific data that you wish to use, check which format they are available in, and ensure that you choose software that can import data in this format. Although some software products provide facilities to convert from one file type to another, these do not always work entirely satisfactorily.
● Which type of software is being used by others with whom you wish to col- laborate or share data? Given that tools for converting between file types are widely available, it may not necessarily present a problem if you wish to share data with others using a different type of software. However, a lot of potential problems can be avoided if a common type of software is used.
● Do you wish to carry out image-processing operations, such as orthorectifica- tion of aerial photographs, or classification of satellite remote sensing imagery? Although some GIS packages provide these facilities, many do not.
Although most provide tools for georeferencing images, the ability to warp images is often very restricted.
|Forest extent and condition 具http://gislounge.com典
Product Comments URL
AGISMap A simple, easy-to-use GIS package that is distributed as shareware via the internet ESRI www.agismap.com/
ArcGIS, ArcView, ESRI (Environmental Systems Research Incorporated) is the market leader in GIS software, www.esri.com/
ArcInfo and is widely used to support practical forest management and planning. The main products provided by ESRI are ArcView and ArcInfo, which are now combined in a single software package called ArcGIS. Additional functionality is provided by a suite of extensions and additional tools; see website for further details. A free version of the software (ArcExplorer) is also available, which allows basic mapping and spatial querying.
GRASS A well-known and widely used program that can be downloaded free of charge. Can be used www.geog.uni-hannover.de/
for analysis and presentation of both vector and raster data, and for image processing. Not grass/index.php easy to use at first.
Idrisi32 Another popular GIS package, with a large community of users interested in forests. www.clarklabs.org/
Although it can be used to analyse both vector and raster imagery, it is particularly designed for the latter, and is widely used by the remote sensing and modelling community for this reason. Possesses many powerful analytical features. For example, Idrisi Kilimanjaro includes tools for spatial modelling of forest cover change, such as GEOMOD.
MapInfo A leading GIS software package, with a wide range of basic functions. Although marketed www.mapinfo.com/
towards the business sector, and lacking some of the more sophisticated analytical functions of IDRISI or ArcGIS, this package is widely used by land managers and planners.
Map Maker A low-cost and easy to use programme, yet powerful enough to produce sophisticated maps. www.mapmaker.com An excellent option for those new to GIS, wanting to learn the basic techniques quickly.
A basic version is available for free download, and a more powerful version (Map Maker Pro) is available free of charge to non-profit organizations, educational establishments, and African students.
● How do you wish to analyse your data? Does the GIS software offer appropriate tools for the analysis method that you have in mind? Most GIS programs allow different data layers to be integrated and simple analytical procedures to be performed, such as buffering around objects, calculation of areas and distances, or querying of the data. However, some offer much more sophisticated analyt- ical tools, including tools for geostatistics, spatial analysis, and modelling. In some cases, these tools are available as additional software modules or extensions that can be purchased separately. An alternative approach is to export the data from the GIS software to a statistical program (such as SPSS, Statistica, SAS, or S Plus), which can be used to further analyse and process the data.
2.6.2 Selecting data types
A key principle relating to the use of GIS is that digital data are generally available in one of two forms: raster and vector.
● Raster datarepresent an area as a grid of (usually square) cells. Various proper- ties or attributes may be assigned to these cells to produce maps. These cells are equivalent to the pixels that form a digital image, such as those generated from satellite remote sensing data or digitized air photos. It is important to note that when features are represented in this way, information about vari- ation within the pixel is lost. Selection of a particular grid or pixel size therefore has an important bearing on how information on some object of interest is represented within the GIS.
● Vector datafocuses on mapping objects as points (or vertices), lines, or poly- gons, which are areas bounded by points that are joined by lines. The term polyline, which is widely used, refers to a curved line represented by a series of straight lines joined together.
With respect to mapping forests, both types of data have their uses. Forest stands can usefully be mapped as polygons, perhaps derived from an aerial photograph, enabling precise estimates of distances and area to be obtained. Such measure- ments are less reliable if derived from raster data, because of reduced spatial accuracy. A vector-based map of forest stands might also look more presentable, or more accurate, than a map of forest stands based on raster data. Yet in practice the boundaries of forest stands are often not sharp, and may in fact be more accurately represented as raster data (for example, if there is a gradient in species composition of a forest across an area). The main advantage of raster data is that many analytical functions, including overlay operations, spatial analysis, and modelling are far more easily done with raster than with vector data. The fact that satellite remote sensing data are provided in raster format is another main reason why raster data are widely used in forest mapping. In general, file sizes tend to be significantly larger for raster data than for vector data, and this has implications for data storage and the time required for analytical procedures.
Many GIS software packages now provide tools for the display and analysis of both types of data, and even enable data to be converted from one type to another.
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However, most software programs are biased towards one or other data type, in terms of the functionality provided. Consideration should therefore be given to whether data are to be represented as raster or as vector data layers within the GIS, and the ability of the software to process the selected data type should be checked.
If satellite remote sensing data are to be used as a basis for forest mapping, for example, it would make sense to choose GIS software that is explicitly designed to analyse and present raster data.
2.6.3 Selecting a map projection
Given that the Earth’s surface is curved, any representation of features on the Earth’s surface as a two-dimensional map inevitably results in some form of distor- tion. A wide variety of different map projections are available that differ in how the Earth’s surface is represented on a flat surface. These projections distort features in different ways, and therefore the choice of map projection has a major influence on the results of any calculations or analysis performed on the mapped data. A funda- mental issue is that areas and angles cannot be preserved at the same time: if a pro- jection is selected that accurately represents angles, then measurements of area will be inaccurate. Conversely, if a projection is used that preserves area, then measure- ments of angles will be inaccurate.
Many GIS software packages enable data to be mapped according to a range of different projections. Choice of an appropriate map projection therefore depends on the type of calculation that is to be made, and can be summarized as follows (DeMers 2005):
● If the objective is to make measurements of area, for example when analysing changes in forest area over time, then some form of equal area projection should be selected. Examples include Alber’s equal area and Lambert’s equal area projections. The size of the area to be mapped will influence how much angular distortion occurs; small areas display less angular distortion than large ones when equal area projections are used. This is important if both the area and shape of forest areas are of interest. The Universal Transverse Mercator (UTM) projection is widely used in the production of large-scale maps; it preserves the shape of mapped features (Johnston 1998) and provides accurate estimation of distances.
● If the objective is to analyse motion or the changing direction of objects, for example when the movements of animals are detected by using radio telemetry, then a conformal projection is most appropriate. This type of projection is preferred whenever angular information is important, such as in navigational maps and with topographic data. Examples of this type of projection include the Mercator, Lambert’s conformal conic, and conformal stereographic projections.
● Azimuthal projectionsare used are used when the determination of shortest routes is required, particularly over long distances. Examples include Lambert’s equal area, azimuthal equidistant, and gnomonic projections.
2.6.4 Analytical methods in GIS
Typically, spatial data incorporated in a GIS will be georeferenced, enabling maps to be produced according to an appropriate coordinate system. GIS software pack- ages usually provide georeferencing tools that enable coordinates to be entered for an image or any data that have been collected, as described in section 2.2.2.
However, an appropriate coordinate system must be selected. Cartesian coordinate systems are widely used, which represent coordinates as pairs of x, yvalues that indicate the position on a grid. For example, the UTM projection uses a Cartesian coordinate system, in which easting (x) and northing (y) distances are measured in metres relative to the origin of the coordinate system, which lies at the intersection of the equator and the central meridian of each 6 zone of the Earth’s surface (Johnston 1998). Although Cartesian coordinates are appropriate for relatively small areas of the Earth’s surface, they cannot be used for the entire planet, because of its (approximately) spherical shape. The latitude–longitude system provides Earth coordinates according to a spherical system, with lines of longitude defined as circles that pass through both poles, and lines of latitude defined as concentric circles around the poles (DeMers 2005). Longitude and latitude coordinates according to this system are presented as degrees from the prime meridian and equator, respectively.
Generally, a coordinate system will be selected that is typically used on pub- lished maps available for a particular area. Individual countries use different coord- inate systems based on different datums, which are ways of describing the shape of the Earth. Awareness of different datums and their associated coordinate sys- tems is particularly important with respect to the use of GPS (see section 3.4.2), which typically provide options for location data according to a large number of different datums. It is therefore essential to know which datum is used for georef- erencing a map. The same datum and associated coordinate system must be used for each map included in a GIS if they are all to be located at precisely the same places on the Earth’s surface (DeMers 2005). A widely used datum is the world geodetic system, WGS84.
Once data have been incorporated within a GIS, a wide range of analytical procedures can be carried out. The methods available depend on the software being used, but a number of basic procedures that are common to most GIS packages are listed below. Many of these operations can be carried out on either raster or vector data.
● Querying. The data incorporated in a GIS are generally organized in a database, which permits a variety of operations to be performed on the data, including filtering and sorting. Querying refers to the process of selecting specific data- base entries according to their characteristics, and this may be done by using a variety of approaches: interactively, by using a look-up table, by specifying numerical thresholds, or through Boolean logic (Johnston 1998). For example, different forest areas could be classified according to the tree species present by using these different approaches. An interactive query might select those areas Geographical information systems (GIS) | 75