4 The nature of spatial data and its portrayal in a GIS and on a map
4.1 Consider the following word picture:
Imagine standing on a hilltop. To your left is a broad sandy beach bordering on the ocean. A meandering tidal river slices the beach in two. Skirting the base of your hill, a freeway and a railway wind their way arm in arm towards the town that sprawls across the distant landscape to your front. To your right, fenced paddocks[1] of crops undulate into the distance until they blend into a forest that erupts out of the foothills.
4.2 The word picture above sketches the barest outline of the geographic information contained in a real world scene, a scene that is infinitely complex in detail that has yet to be described. The interpretation of the features observed and of the features ignored will be influenced by the experience and cultural background of both the observer and the audience with whom it is intended to communicate the scene (Burrough and McDonnell, 1998, p19 and Egenhofer and Herring, 1991, p228). For example, to an elder Aboriginal Australian, a tree that is vitally important in communicating a dreaming story to the youth of his clan may be the only feature worth noting on a particular hillside. Whereas this geographic phenomenon may be totally ignored by a geologist who is more interested in the adjacent rock outcrop that contains a gossan, indicating the potential of an underlying orebody.
4.3 A method often used to communicate geographic information is a map. If properly designed, a map can be an elegant medium for communicating a wealth of information about the spatial relationships of geographic phenomena, in a compact, convenient and familiar format. However it is easy to forget that a map is merely an abstraction of reality, a model of the real world as it once was at a particular time. It is not a miniature version of the real world and does not show every nuance of detail that exists in reality, nor can it be expected to (DeMers, 1997, p52). For example, at a scale of 1:250,000, 1 millimeter on a map represents 250,000 millimeters on the ground. Put in a more easily understood way, 1 millimeter on the map represents 250 meters on the ground (there are one thousand millimeters per meter). If a road had five curves within a stretch two hundred and fifty meters long, it is not realistic to expect a 1:250,000 map to display all five curves in the space of one millimeter. In reality only the trend of the curves and adjacent portions of the road could be intelligibly displayed. This same road may also have a line width of one millimeter on the map. Using the above calculation this represents a road two hundred and fifty meters wide on the ground, which is probably not the case. Conversely on a 1:25,000 map, one millimeter represents twenty-five meters on the ground. Therefore, it is realistic to expect all five curves to be portrayed within the space of ten millimeters on a 1:25,000 map. It is also important to understand that merely enlarging the data captured at 1:250,000 and displaying it at 1:25,000 does not make the original data more accurate. The above concepts apply equally to a GIS as they do to a map.
4.4 The geographical phenomena or features that appear on a map, or in a GIS, require at least two attributes that represent what is present in the real world. These are a descriptor (what is present?) and a location (where is it?) (Burrough and McDonnell, 1998, p19). Transforming real world geographical phenomena into an abstract, generalised and approximated feature for inclusion on a map or GIS is not a trivial process. Consider some limited examples of the types of questions that need to be asked prior to capturing the relatively simple features described in the word picture at paragraph 5.2:
4.4.1 Coastline How is the coastline to be portrayed? Do we use the high water mark, the low water mark or mean sea level? Do we take into account neap, spring, or king tides? How do we determine where that line is on the ground? Do we just represent the current waterline? Do we draw a line across the mouth of the tidal river to represent the coastline, or follow the riverbanks upstream? How far upstream do we go before we classify the riverbank as a riverbank and not as a coastline?
4.4.2 River How is the river to be portrayed? Do we show it as a double line stream with a line for each riverbank or do we only show the centerline of the river? Do we show every curve faithfully or only generalise to show the general direction of the river? Do we show all tributaries to the river or only the major ones? How do we determine if a watercourse is a major or a minor tributary?
4.4.3 Freeway How is the freeway to be portrayed? Do we only show the road reserve? Do we faithfully reproduce each edge of the road for both carriages? Do we show the centerline of each carriage or only the centerline of the freeway? Do we offset one carriage to show the dual carriages?
4.4.4 Town How is the town to be portrayed? Do we display it as a polygon using the Municipal Boundary, or the edge of development? Do we only show the town as a point symbol?
4.4.5 Paddocks How do we portray the paddocks? Do we show them as a paddock or do we portray the use to which each paddock is put? Do we need to show them at all?
4.4.6 Forest How do we portray the forest? How do we determine where the edge of the forest is? Do we take into account the fact that the forest encroaches into the paddocks or do we arbitrarily draw a line along the paddock boundary? Do we take into account the scattered trees at the edge of the forest?
4.4.7 Terrain How do we portray the terrain? Do we show it as a continuous surface? If so do we show it is a Digital Elevation Model in regular or triangulated form, or do we show the terrain as a hill shaded model? Do we show the terrain using contours, if so at what contour interval? Do we show the terrain as a thematic surface based on elevation? What elevation ranges do we use? Do we only use spot heights? Do we need to show the terrain at all?
4.5 Determining what information to include and portray in a GIS or a map depends on a number of factors, for example, the cost to capture the data; the intended use of the data; the scale at which the data will be used. Consider the difference in effort and costs between a detailed cadastral plan at 1:1,000 scale showing survey accurate cadastral boundaries, roads, drainage etc and a regional tourist map at 1:250,000. The tourist map may show the area covered by the cadastral plan as part of yellow blob representing a city. The cadastral plan is used to accurately determine property boundaries while the tourist map is used to find places of interest within a region. When compared to the cadastral plan, the tourist map covers a larger area, is considerably cheaper to produce and is a more functional document for visitors to find their way around. The tourist map should not however, be used to determine whether the house being purchased is in the correct parcel of land.
4.6 Similarly, determining how to portray a geographic feature is dependent on the same factors. For example a city may be portrayed as a point, as a symbol or as a polygon, depending in the function that the data is to be used for. It may be portrayed as a point feature, in a small-scale representation of the location of all the cities in a country. It may be portrayed using a symbol, if the same data set was classified by population count, where the greater the population of a city, the larger the symbol used to portray it. The same city may also be portrayed as a polygon, where it is important to portray the geographic extents covered by that city.
4.7 In some situations it may not be possible to display two geographic phenomena adjacent to one other, due to the size of the features when portrayed at scale. For example a railway line and a road are one hundred meters apart and parallel to each other in reality. The road and railway when displayed at scale may each appear with a line width of one millimeter. There is also usually spacing between the two features on the map of for example one half a millimeter. For this example two and a half millimeters width is required to portray the features. As discussed previously, one millimeter at a scale of 1:250,000, represents two hundred and fifty meters on the ground. Therefore the two features side by side on the 1:250,000 map will displace the equivalent of six hundred and twenty five meters. As the actual width on the ground of the two features is approximately one hundred meters, there is a problem. This type of situation often occurs, for example, buildings located adjacent to a road, or a road located beside a river. The situation described above is usually resolved by the juxtaposition (offsetting) of one feature from the other. This allows both features to be portrayed, even if one is in the incorrect location.
4.8 Historically, areal geographic phenomena have been portrayed on a map and in a GIS using crisp boundaries. While this works well for such features as land parcels and administrative boundaries, not all areal features can be accurately portrayed in such a way. Consider the questions asked at paragraph 4.4.1 in trying to determine where to locate the coastline. Similar problems are experienced in many other phenomena where there is a gradual change from one type to another, for example vegetation, geological or soil types. (Burrough and McDonnell, 1998, p19). In addition, there is also variation within an areal boundary. Research is being conducted as to how to model the variation between such features using ‘Fuzzy Sets and Fuzzy Geographical Objects’ (Burrough and McDonnell, 1998, pp265-291). For simplicity in portrayal these features are usually defined using arbitrarily determined boundaries.
4.9 Many geographic phenomena change over time, e.g. roads, cities, forests etc. Even the location of a river will change as its path migrates due to earth movements, changes in climate and flooding etc. (Clark and Cook, 1983, pp178-180, pp184-189). Consider also the problems of defining a billabong[2] in northern Australia, where its location and extent change with the seasons.
4.10 In Summary The concepts outlined in this section are best summarised using the words of Maguire, Goodchild and Rhind, 1991, p112:
“…geography is infinitely complex and must be generalised, approximated or abstracted in order to be represented within the finite dimensions of a discrete computing device.”
It is important to remember that a map and a GIS dataset are merely an abstraction of reality, a model of the real world as it once was at a particular time.
[1] Fields.
[2] Billabong: “river branch that forms backwater or stagnant pool” (Oxford, 1987).
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