Search Textbook

Table of Contents

Chapter 1: Introduction

1.17: An Example of Pseudoscopic Vision

This photo has been arranged so that shadows 1 fall away from the viewer resulting in the possibility of pseudoscopic vision. Shown here is a series of steep-sided creek valleys 2 etched by streams that rise on the Riding Mountain Plateau 3 and deeply dissect the escarpment as they flow northeast. Some disappear into alluvial deposits at the base of the escarpment 4.

It is possible that the viewer sees the valleys as ridges (pseudoscopic vision). Turning the photo upside down can reverse this optical effect. The photo also illustrates the criteria, “tone” and “shape” (shadow) in identification. Very light-toned areas 5 are bedrock outcrops along the valley sides. The dark-toned areas are groups of coniferous trees 6; individual coniferous trees can be identified from their triangular-shaped shadows 7.

Figure 1.17: An Example of Pseudoscopic Vision

Figure 1.17: An Example of Pseudoscopic Vision


Figure 1.17

Vertical air photo: A20374-115

Flight height: 10,020 feet a.s.l.; lens focal length: 152.47 mm

Scale: There is enough height variation here to produce a measurable difference in scale. At the top of the escarpment 1:15,440 (approx.) At the foot of the escarpment 1:17,540 (approx.)

Date: May 14, 1969

Location: Township 20, Range 16 WI

Map sheets: 1:250,000 62J Neepawa

1:50,000 62J/12 Wasagaming

1:125,000 MCR 207 Riding Mountain National Park