May, 1960 COMMENTS ON THE MAPPING EXPERIMENT 167 
ToPoGRAPHICAL PHOTO-INTERPRETATION OF THE RENFREW TEST AREA 
The participants in the Renfrew experiment had no previous knowledge of the 
landscape, land use, or economic geography of the area in question, and the stereopair 
of aerial photographs was their only source of information in this respect. It would 
have been a real help if some sort of topographic interpretation key could have been 
sent to the participants along with the basic material. 
Mr. Blachut kindly sent me some geographical and geological information about 
the Renfrew test area. This allowed me to present the following topographic inter- 
pretation of the region: 
ALLUVIAL PLAINS. Alluvial plains extend on both sides of the Bonnechere River 
above the town of Renfrew, where the river has almost no gradient. The plain consists 
of marine deposits, with varying amounts of clay. The surface is extremely flat, so 
that contouring is rather difficult and uncertain, although it can eventually be achieved 
by interpolating between spot heights. Some characteristic shallow gullies have cut 
into the plain at right angles to the river. 
ROCK-KNOB UPLANDS. Many rock-knobs have remained above the Pleistocene 
deposits and all the bigger hills in the Renfrew area consist of rocks and show this typical 
form with flat tops, sides rounded by ice, no sharp edges, and steep, partly wooded 
flanks. Contouring is easy, especially where no forest is found. At some places typical 
structures appear, for instance on the eastern side of the biggest rock-knob, and these 
features should be shown in first-order plotting. Second-order plotting might omit 
such details, even if they are characteristic. 
TILL AREAS. During the ice age deposits of till were laid down over most of the 
area. Generally the till surface is flat, but in detail is of considerable variety. At first 
the details seem to be quite irregular, but closer studies reveal rather systematic but 
very shallow drainage-lines, with flats between them. The relative height differences 
are at many places of nearly the same order as the accuracy of contouring, and first- 
order contouring, although extremely difficult, is possible but time-consuming. The 
very detailed system of contour lines in the till area can be seen in Fig. 1. There is a 
land form corresponding to every indentation in the contours. The accuracy of first- 
order instruments is just sufficient to draw contour lines with some reliability. Second- 
and third-order instruments do not permit the evaluation of all forms. It is not possible 
to use a convenient generalization of the contour lines and still present the real character 
of the landscape. In the area of the first small-scale experiment, Vercors 2, no terrain 
was found to be as difficult for contour evaluation as this till area in the Renfrew 
experiment. 
VarLEYs. The main valley is that of the Bonnechere River, which flows from west 
to east. At first, above the town, the valley is shallow. In the town itself there are 
two rock faults, which the river crosses in two distinct steps, and a hydro-electric power 
station has been constructed at each fault. Below the town the bed of the Bonnechere 
River is more deeply incised, with steep slopes of height differences up to 50 metres. 
Contouring is slightly difficult here, but only because the contour lines are very close 
together. There are some tributary rivers flowing from both sides into the Bonnechere 
River. Those on the east side of the town are deeper because of the greater depth of 
the Bonnechere valley into which they flow. The tributary rivers have small but 
characteristic meanders. Plotting these meanders and the many contour lines in the 
deeply-incised slopes, although not difficult, is time-consuming. 
Forest. Forest is an important element, but a hindrance to plotting. Forests within 
the limits of the test area are primarily confined to the river courses and the rock-knob