Ngā Tohuwhenua Mai Te Rangi: A New Zealand Archeology in Aerial Photographs
Aerial photographic techniques
Aerial photographic techniques
Vertical aerial photographs are most commonly used for survey purposes. They are taken with large cameras fixed in the underside of an aeroplane capable of maintaining a stable flight path. As the name implies, the view is straight down, providing an image in the horizontal plane. Most vertical aerial photography is done with 'large -format' cameras, with single negative images as large as 200 by 200 mm—60 times the area of a conventional 35-mm camera negative. Very fine detail may be recorded on the film, hence its value for military reconnaissance, surveying, archaeology, forest- and town-planning, and other scientific and technical purposes. These photographs are usually taken in a sequence, with each photograph overlapping part of the photographs taken before and after. This allows stereoscopic viewing, whereby three-dimensional views of the land surface can be obtained. Stereoscopic aerial photography, such as that used by Helen Leach in Palliser Bay, can- page 19not, unfortunately, be adequately displayed in a book intended for wide public readership. It requires the assistance of a small but expensive pair of stereoscopic glasses to achieve basic results in archaeology in the field.
In contrast, oblique photography (taken at an oblique angle to the ground surface) has more uses in illustration. It is more successful, as a single photograph, in showing the shape or relief of the surface of the ground. Usually taken closer to the subject, it has relatively limited use for conventional land survey and map-making. However, it is still possible to take accurate measurements from such photographs, so they can also be used for analytical purposes.
The difference between vertical and oblique photographs is illustrated in a unique photograph taken by Air Maps (New Zealand), Ltd, of the Kuaotunu Peninsula, Coromandel, and the setting of its wide variety of historical features. In the foreground are the goldfields of Bald Spur and Waitaia Ridge (more or less a vertical view), 30 and in the distance an oblique view of Opito Beach and Sarah's Gully, where, in the late 1950s, Jack Golson first archaeologically documented the early Māori settlement of the North Island. This settlement consisted of a small whānau or hapū settlement at the foot of a ridge in the inner part of a small open bay. The site revealed storage pits (demonstrating horticulture), dated to the early thirteenth century. 31
To see the relief of the earth or archaeological features on the surface without the benefit of a stereoscope, it is essential to use the advantages of shade and oblique light in defining the site. 32 In an oblique view, with the camera oriented towards the sun, early or late in the day or in winter is generally best; the surface features of the site will cast long shadows and can be seen readily. Disturbances in the surface of the ground throw a shadow much longer than the feature is high. A pattern to features which cover very large areas may be revealed that cannot be seen walking about on the ground under any lighting conditions. Aspect—whether the slope of the site faces towards or away from the sun—is very important.
However, there are some potential difficulties. The sun should not shine into the lens either directly or indirectly from water surfaces—a potential problem with a wide-angle lens. There may also be problems with haze, water vapour, dust or smoke gathering and reflecting light. The closer the camera is to the feature, the less significant these problems will be. Other angles produce different results. With the sun behind the observer and at a low angle in the sky (i.e., with the camera viewpoint oriented with the sunlight), very spectacular and strong colour and shadow effects can be gained. The light will be bright on the lit faces, and the camera will pick up the very long, very dark shadows cast behind the features of interest (provided the angle of view is high enough). However, with the sun behind the observer, archaeological features are usually strongly and evenly lit, obscuring their own shadows, and tend to be indiscernible. My own practice has developed to take both medium-angle (at about 45° to the horizontal) and high-angle (near 90°) obliques, the latter at a higher altitude or with a wide-angle lens. High-angle oblique views require uncomfortably tight banks (steep turns) by the aeroplane to get a view unencumbered by its wheels.
To this point, we have been discussing archaeological sites that show on photographs because they have surface relief, i.e., there is a curve or depression in the surface of the land. Not all sites have such relief features. However, sites without relief can still show from the air either when they are exposed on bare surface-soil (when ploughed, for example), 33 or, as is more common, when they lie under the vegetated ground-surface. 34 The former, soil marks, show as dark or light marks in the soil. The latter, crop marks, may have been caused by hard surfaces, such as paving or the lines of a foundation or path: crops or grass will not thrive on such places. The site may have incorporated a ditch or a drain, which filled with soil more fertile, deeper or moister than the surrounding earth, promoting greener grass or better crops. The contrast may be accentuated in spring as conditions slowly become dry. Specialised film, such as infrared which is sensitive to variations in the light reflected from leaves of high or low water content, may enhance the contrast. Crop marks of course also show on sites with surface relief, where there are stony soils or where the slope directly faces the midday sun, as it does on north-facing banks. In these situations, lush spring or autumn growth may be burnt brown by the heat of the sun.
Kuaotunu Peninsula from the west, illustrating vertical and oblique effects
The photograph has been taken with a wide-angle lens. The view is vertical (immediately below the camera viewpoint) in the foreground. An oblique view oriented to the east shows from the middle distance. Several gold-mines were in operation from 1889 on the Waitaia Ridge (across centre) and Bald Spur (bottom right). Left centre is Otama Beach. In the far distance are Opito Bay and the localities of the sites of Skippers Ridge and Sarah's Gully. Mt Tahanga, source of widely used adze-stone, is the rounded knob on the far right of the photograph.
Sarah's Gully, scene of the first North Island archaeology in the 1950s, where Jack Golson first documented the early Polynesian settlement of the North Island. On the far ridge (above the rocky shoreline) is the Sarah's Gully pā. Golson's site, in which early forms of storage pits were discovered, was on the near ridge and the slopes just above Cross Creek (foreground). Later excavations were on middens on the far beach frontage, and on the sandy area on the near side of Cross Creek. The earliest radiocarbon dates for these sites indicate settlement in the early thirteenth century.
Crop marks and soil marks
Left. Soil marks of a ploughed-out pā on the Waikohu River, near Pūhā, East Coast. The pā lies on the left of the river on the opposite bank to the prominent house, centre. There were two ditches and banks, described by Elsdon Best in The Pa Maori as 'bulky earthworks'. These enclosed the point formed by the sinous stream marked by willows (coming in from the left) and the main Waikohu River. From top to bottom the pattern is: light grey line, subsoil of the outer lip of the ditch; broad light grey line, subsoil fill of the main bank; and dark patches, the topsoil and probable ovens protected from the plough by the main bank. The river is 30 m across.
Right. Crop marks outline the double ditch and bank of a pā on the high terrace at Nukutaurua, Māhia Peninsula. Fragments of the ditches and banks only survive outside of the fenceline running up and down right of centre on the photograph. Traces of the outer ditch lie at right angles to the large surviving fragment, and can be seen as a dark line enclosing the point. A pattern of later ploughing can also be detected.
These photographs were taken in the course of circling the pā in the late afternoon, with sunlight coming from a low angle to the north-west. The first view (top) is taken from the east, with the main features lit from behind and showing clearly. The second view (bottom) is taken from the west, with features lit from the front (i.e., from behind the camera) and showing less well. However, the photograph is taken from a fairly steep oblique angle so that the shadows created by some features, and the hill itself, can still be seen. The site is about 100 m long.
So far, it might appear that aerial photography is a tool without parallel in archaeological sciences, and that it may as well displace all other methods of surveying and observing sites. That is not at all true, because the following important limitations apply. First, although aerial photographs can sometimes show with exact clarity and better than a ground view the sequence in which parts of a site were laid down over large areas, that sequence is as a general rule established or confirmed by archaeological excavation or the close examination of sections that are cut through the site. Second, excavation is also necessary to determine the plan of buried archaeological features, although these do sometimes show in aerial photographs. Third, and self-evidently, archaeological excavation is essential to take samples for further analysis in laboratories.
To be seen effectively, then, New Zealand sites require strong oblique lighting conditions, typical of higher latitudes, winter, or early morning and late afternoon. Unfortunately, for perfectly valid reasons, it is typical practice in general aerial survey to take the photographs in summer and near midday, and archaeological features cannot be detected because of the lack of clear shadows and sometimes the effects of thistles or other seasonal vegetation growth. The problem is exacerbated in the far north where shadows almost disappear in summer at midday (because of the low latitude, 35°S).
The height at which the photograph is taken is very important in determining the size of the objects which can be seen on the ground: too far away and the site is imperceptible, too close and only the detail of a large site is visible. Archaeological sites vary in size: from a single spot where an artefact may have been found to a pā site that extends for more than a kilometre along a ridge. The plan of an individual storage pit may be as small as 2 by 1 m. 37 Forty years ago, flying went as low as 1,500 m for aerial photographic survey purposes, and the negatives were at a scale (depending on the lens) of as much as 1:10,000. At this scale 1 cm on the photograph equals 100 m measured on the ground. A ditch and bank 8 m across would measure just under 1 mm on the negative, giving ample scope to detect and document archaeological sites. Now, heights are triple that, with scales of 1:25,000 being typical. Archaeological features are small enough to become indiscernible at this scale. In this book, most of the oblique photographs were taken at heights of from 250 to 500 m so that detail as small as individual thistles is clear.
The final problem is destruction of sites and photographs. The physical record of Māori settlement in New Zealand has been lost very rapidly in the last two decades because of land development. The increasing rarity of these sites makes them valuable. Although there is reason to be gloomy and indeed alarmist about this disappearing heritage, there are factors which reduce the loss. First, sites constructed in the earth itself are robust and will survive fire, if not bulldozing or erosion. An example of loss caused by river erosion is illustrated here in the site Marama Tāwhana on the Uawa River, Tolaga Bay. Second, and most important from the perspective of this book, the sites have long been recorded by aerial photography. Where destruction has occurred, earlier aerial photographs are the more valuable. These photographs lie in archives which have not been much researched but from which there is much to be learnt.
To sum up: aerial photography preserves what may otherwise be physically destroyed, illustrates with great power what is there to be seen, and can act as a source of carefully interpreted knowledge in its own right, a powerful tool of particular value in reconnaissance, mapping page 26 and illustration of historic landscapes and sites. The potential for aerial photographs to reveal information about the patterns of sites and human settlement is enormous, 38 yet very little exploited. The positioning of sites in the landscape can reveal much about how people used the surrounding land, and about the relationships between the groups of people that lived on the sites. Even less explored is the potential of aerial photography to reveal pattern within individual settlements, 39 although the pattern within garden areas has been closely analysed. 40 It is not the primary technique of archaeology; excavation is essential to the closer understanding of the function of particular site types in a system of settlement and the sequence in which the site was used over time (the function and arrangement of a site may change over time).
Marama Tawhana, on the Uawa River, Tolaga Bay
These two short lengths of ditch once defended a pā on the river point extending as much as 150 m out to the left (east) of the photograph. The inner bank of the pā (on the left) has been heavily eroded by recent floods such as those caused by Cyclone Bola. Even in the 1940s, however, aerial photographs show that there was only a small area about 5 m across and 20 m long surviving within (left of) the ditch and bank defences. The river point has built up downstream (top right, to the south-west) while the upstream bank has eroded, abruptly terminating the defences. The site is about 350 years old.