Let's begin the examination using satellite images of the changes in the cities with the national capital, Canberra. Canberra is a useful beginning because many important points of this chapter are illustrated with this small city.
The baseline image used for Canberra was acquired mid-winter, on August 8, 1972. A small part of this image shows the city of Canberra (and its New South Wales neighbour, Queanbeyan) in its geographic context - an inland city nestled in a gently undulating landscape astride a human-made lake.
Use your browser to open each image in a new window to compare them.
This context reflects Canberra's history; it was located by political direction to placate the rival claims of Sydney and Melbourne, and to be sufficiently far from the coast to be safe from naval bombardment.
The imposition of this city on the landscape was just one of a sequence of landuse-driven landcover changes in this part of Australia. The sequence of all three of these changes can be guessed at by examining the 1972 Landsat MSS image.
The natural landcover of this area was a eucalypt woodland with a grassy understorey (eM2G, AUSLIG 1990). This landcover was progressively modified by the changing landuse patterns of the European Australian settlers. First the trees were thinned by ringbarking to encourage the growth of the native grasses. Then the remaining trees were thinned still further, or entirely removed to allow mechanised cultivation or the establishment of fertilised, exotic pastures.
It is this landcover type that is the background to the city of Canberra. The only extensive eucalypt woodlands remaining are on the steeper parts of the landscape where cultivation was not possible. Their cover, dark in the 1972 image, forms two largely continuous strips on either side of the city. Smaller remnant outliers are scattered across this scene.
Two remnants of the native woodlands remain close into the heart of the city: Mt. Ainslie and Mt. Majura on the east and Black Mountain on the west. You can detect a large dual carriageway road cutting through the woodland on the Northern end. These woodland remnants are much loved and valued by Canberrans because they provide a constant source of enjoyment and visual contrast to the built environment as well as a reminder of how things once were.
Between these (dark) eucalypt covered uplands is a landcover that is typical of winter-rainfall agricultural landuse - extensive, largely treeless grasslands. Because of contrasts in the type or amount of pasture or crop across paddock or property boundaries, a patchwork effect is noticeable in the image. A close examination of this landcover type all over the image indicates that in some places, the clearing of the eucalypt woodland has extended well up steep slopes and entire hills are completely cleared. You can also interpret that here the cover of pasture is much sparser than on the bottomlands. You might also (correctly) guess that soil erosion may be a problem on these uplands.
There is one other human-made landcover created after the clearing of the eucalypt woodlands. This landcover type is plantation forestry. In the 1972 image, these exotic conifers are identified by their geometric shapes and reddish colour compared with the dark remnant eucalypt woodlands or the pinkish crops and pastures. We will examine the sequences of landcover change characteristic of agriculture and forestry in much more detail in the chapters that follow.
The last landcover type to be created in this Canberra scene was the urban one, the city and its suburban surrounds. The MSS spectral characteristics of urban areas are such that in a FCC image, the high-density central business district (CBD) usually appears as a pale blue colour that grades towards the reddish colours of vegetation.
Canberra is somewhat different from older and larger Australian cities. The area of the CBD is small compared with that of the residential areas, the suburbs. Both city and suburbs contain many vegetated recreation tracts; parks, playing fields and so forth. These areas appear as crimson dots.
The general spatial pattern of Canberra is readily interpretable from space. First, it is in relatively flat terrain. The satellite city of Queanbeyan lies just over the ACT-New South Wales border; a border that is visible only because of a railway line. The general spatial pattern of the Canberra urban area is one of uniformity. Canberra was a carefully planned city from its inception. Of all the Australian cities, Canberra is the only one planned for the automobile. The clearly visible contrast between Canberra and Queanbeyan is a reflection of the difference in planning history and investment.
Most of Canberra's suburbs lie along a north-south axis that straddles Lake Burley Griffin, created by damming the east-west flowing Molonglo River. The spectral (colour) contrast between the urban areas and the surrounding agricultural landcover is greatest in the 1988 image.
Even a visual comparison between the 1972 and the 1988 images shows the expansion of the suburban areas of Canberra in the west and in the south. At the southwest tip of Canberra, the Tuggeranong Valley, the extensive clearing of agricultural landcover in preparation for conversion to urban landcover, is clearly visible from space in the 1988 FCC image.
Using these two satellite images and our capacity to interpret them by eye, we can look back from 1988 to 1972. Let's ask - is the landcover change that has occurred during this 16 year period detectable from space? Let's also ask - how much change was expected?
Let's try to answer the first question by interpreting the Difference FCC image. As explained in a previous Chapter, the difference image is interpreted using a set of rules that, though few, are not necessarily simple.
Landcover that has changed little will appear in neutral grey colours. The change of a dark landcover (eg vegetation) present in 1972 to a brighter cover (eg bare soil) in 1988 will be indicated by dark tones. In contrast, the conversion of a bright landcover in 1972 to a darker one in 1988 is indicated by white tones. Finally, a decrease in the greenness of the landcover from 1972 to 1988 appears as red tones.
The answer to the first question - can urban landcover change be detected using satellite data - is clearly yes. Dark colours ring the urban fringes of the western and southern suburbs of Canberra and the southern suburbs of Queanbeyan. These colours show where the suburbs have expanded out into previously agricultural land. In some areas, such as the Belconnen Valley in the north-western edge of Canberra, much of the clearing for new suburbs was apparent in the 1972 image, so the dark colour indicating their conversion is less intense than it might otherwise be.
The spatial patterns of urban landcover conversion vary. Within the boundaries of the established suburbs, urban infill programs can be detected. On the fringes, whole suburbs appear as blocks of change and new roads appear as dark slivers.
In contrast to the dark tones, which indicate a loss of vegetation cover, are the many light areas that indicate a gain in vegetation cover or equivalent. The most obvious of these is the south-eastern corner, which from its shape, can be inferred as a water storage, the Googong reservoir, that was not present in the 1972 image. Similar bright areas to the east and west of Canberra can, after viewing the 1972 and 1988 images, be interpreted as the replacement of relatively bright dryland pastures with dense, dark plantation forests of conifers.
Since we are looking at the changes in the national capital that just span the celebration of the Bicentenary, a very localised and central area of marked change is the new Parliament House on Commonwealth Hill, just across the lake from the CBD.
How much change can we detect using our Difference space image?
My assessment of the predominant colours in the Difference image is that it is the reds that occupy the largest area in total. These colours indicate changes in greenness and therefore represent only reversible seasonal change.
The next largest area of change is flagged by the dark tones that are almost entirely generated in the near-urban areas of Canberra and Queanbeyan. These tones are the signature of the irreversible conversion of agricultural to urban landcover; of grasslands to houses. Very few dark patches are found in the surrounding agricultural areas. In this landscape context, this signature most probably indicates the clearing of eucalypt woodland remnants. One interesting exception is the snake-shaped enlargement of the Federal Highway in the north-east corner of the image that indicates how small landcover changes can be detected from space.
The third area of landcover change is displayed as light tones indicating, except Googong, an increase in vegetation cover. These quite large areas appear to entirely result from the creation of plantation forests of exotic softwoods both in the ACT and New South Wales.
How much change could we have expected?
We can attempt this question using the population census data collected in 1971, just before the launch of Landsat 1, and those collected in 1986, two years before the latest MSS image. During that period, the population grew from 141,000 to 259,000; an increase of 118,000 and a change of +84% in the 15 years. This is a high rate of relative change (growth), but the absolute growth in numbers of people is small compared with the other cities we are yet to examine. Even so, this increase in population resulted in the creation of 31 new suburbs in the interval between 1972 and 1986.
It would be wise to leave our assessment of the magnitude and significance of the landcover change we have detected around Canberra until we have considered the larger cities.
Canberra is the youngest capital city in Australia and the only one not located on the coast. The remaining cities all have a longer and very different history. That history of development, which began with European settlement and was shaped by the colonial era of this nation, can still be interpreted today in the view of each city from space. The road map of historical change over two centuries can be interpreted today with just one image, and the constraints of the past interpreted using two images.