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Chapter 14: Agriculture

14.3: Radar Image of Agricultural Southwestern Manitoba

This image is different from those in figures 14.1 and 14.2 in that it has been created using much longer wavelength radiation in the microwave part of the electromagnetic spectrum. This is a C band (3.75-7.5 cm) radar image obtained by the ERS-1 satellite.[i] Radar is an active sensing system; that is, radiation is emitted from a source, and reflected radiation is recorded. This means, of course, that the wavelength of the radiation can be determined. Radar images have some advantages over photographs and other images using the visible part of the spectrum. In the first place they are not dependent on light and can therefore be obtained at any time of day. Also, if the correct wavelength is used, radar can penetrate cloud and atmospheric pollution. Disadvantages included usually reduced resolution, and the fact that for beginners interpretation is more difficult because there is a tendency to regard the images as photographs.

Some general principles apply to the interpretation of radar images. Rough surfaces reflect a lot of radiation and are therefore light-toned on the image, whereas smooth surfaces reflect little and are therefore dark. Soils and crops with high moisture content have a high reflectivity and are light-toned, whereas dry surfaces reflect little and are dark-toned. Also surfaces that are inclined towards the radiation source reflect a lot and are light-toned, whereas flat surfaces or those inclined away from the radar source reflect little and are dark. Also the shape and orientation of objects will have an effect on their reflectance. A particularly bright response results from a corner reflector in which case adjacent smooth surfaces cause a double reflection that produces a high return and thus a very bright area.

These principles are illustrated on figure 14.3. The Brandon Hills 1 are light-toned because they are wooded, thus presenting a rough surface, whereas Oak Lake 2, Plum Lakes 3, and that part of Whitewater Lake 4 containing water at the time have smooth surfaces and are dark-toned. The light-toned circular features in the east 5 are center-pivot irrigation areas that have high moisture content. Surrounding these are dark-toned areas 6 with non-irrigated crops that are relatively dry and therefore dark-toned. Most of the center-pivot irrigation systems cover a quarter section, but one covers a whole section 7. North of these is a light-toned area—this is Douglas Marsh 8, a wetland with grasses, sedges, cattails and rushes, the route of a former Assiniboine River distributary. It is light-toned because of the roughness of the vegetation surface and its high moisture content.

The main physical features of southwest Manitoba can be identified as a result of the influences mentioned. The Assiniboine Valley 9 and the Little Saskatchewan Valley 10 with rough wooded valley sides are light-toned. Between these two rivers 11 and in a loop of the Assiniboine 12 are two dark-toned dry areas. Southeast of the latter 13 and further south 14 are a former courses of the Assiniboine. In the south the course of the Souris River can be identified. It is noticeable in the southwest corner 15 where the valley floor is light-toned probably because it is grass-covered, and further east 16 the valley can be picked out probably because it lies across the radiation direction. The part of the river below the elbow of capture 17 is light-toned because of wooded valley walls. Southwest of the Souris is the dark-toned Dand Spillway 18 containing a succession of lakes.

East of the irrigated area are medium-light tones of the dunes on the Assiniboine Delta 19, representing a balance between a rough grass-covered surface producing light tones and dry conditions producing dark tones. A similar situation can be seen surrounding Oak Lake—grass-covered surfaces are light-toned 20 whereas flatter dry areas are dark 21.

The extent of agricultural land is indicated by the distribution of rectangular fields in tones ranging from almost black to almost white. The light-toned fields 22 have a rough surface—possibly stubble as this image was obtained on September 17—or wet conditions, or a combination of both. Dark-toned fields 23 are smooth—they do not have a crop covering or they are dry. It is noticeable that there is a greater number of light-toned fields in the south than in the north, possibly because the harvest is more advanced and there are more stubble fields.

Apart from agriculture the only cultural features visible are Brandon 24, light-toned because of many corner reflections and the Trans-Canada Highway 25, dark because it has a smooth surface.

Figure 14.3: Radar Image of Agricultural Southwestern Manitoba

Figure 14.3: Radar Image of Agricultural Southwestern Manitoba

Figure 14.3

ERS-1 Image (Band radar, wave length 3.75-7.5 cm)

Scale: 1:500,000 (approx.)

Date: September 17, 1991

Location of centre point X: Lat 49º 37'N. 100º 13'W (approx.)

Map sheets: 1:250,000 62F Virden

62 G Brandon

62J Neepawa

62K Riding Mountain


[i] The European Space Agency (ESA) launched its first remote sensing satellite ERS-1, on July 17, 1991. The satellite was in a sun synchronous orbit (i.e. it came over places at the same time each day) at an inclination of 98.5º and was at a nominal altitude of 785 km.