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Manitoba from the Air
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Description
Air Photo Interpretation Criteria, Tone and Shadow, Applied to an Area in the Upper Part of Elm River Southeast of Portage la Prairie

This very large-scale photo (1:3,000) shows the upper part of Elm River, a palaeochannel (former channel) of the Assiniboine River, across an alluvial fan south of Portage la Prairie[i]. In this area the creek is really a series of small lakes 1 rather than a continuous flow to the east. In the northwest tonal variations from very light 2 to dark 3 indicate meander scrolls left behind as Elm River migrated to the southeast. At this large scale, rounded shadow shape 4 is a good indicator of deciduous trees including those growing along the river course 5 and those planted in a shelterbelt around a farm 6. Also shadows of poles 7 carrying a power line can be seen along a light-toned north/south road 8.

The notation in the top right hand corner of the photo indicates that photo A20774-21 is the first in a line flown from east to west that includes photos 21-33. The photographs were taken on July 7, 1968 from a height of 2,400 feet above ground level[ii] using a camera having a lens with focal length 152.24 mm.

Notes

[i] W. F. Rannie, “The Portage la Prairie ‘Floodplain Fan’,” in Alluvial Fans: A Field Approach. eds. A. H. Rachocki and M. Church. Chichester, England: John Wiley and Sons, 1990, 179-193.

[ii] This is a mistake. Using the given data for flight height and lens focal length results in a scale of 1:4,800. However, when the photo is compared with the relevant 1:50,000 topographic map a scale of 1:3,000 is obtained, which is what it would be if a flight height of 2,400 feet above sea level were used in the calculation of the scale.

Figure 1.11

Air Photo Interpretation Criteria, Tone and Shadow, Applied to an Area in the Upper Part of Elm River Southeast of Portage la Prairie