Visual interpretation

1. Visual interpretation Chapter 10

2. Visual interpretation in important to GIS development and application. • It is the interpretation of aerial photography by a photo interpreter. • Regarded as somewhat useful but old-fashioned likely to be replaced by digital analyses

3. Visual interpretation is now seen in the new context. • Skills that a photo interpreter uses are applied equally to photographs as well as digital images • So a new name has come into use that of image analyst or image interpreter

4. In addition, remote sensing data has become more detailed, much like photography • Finally, computer science is not evolved to the point where image analysis can be done completely automatically

5. Characteristics of aerial imagery • Visual interpretation is used to extract information from fine resolution imagery at scales of 1:40,000 or larger • And includes almost all aerial photography and fine scale satellite imagery (e.g. IKONOS) • Excludes coarse resolution imagery produced by satellites such as Landsat

6. Characteristics of aerial imagery • GIS requires the following conditions be met • Geometric errors removed • Detailed spatial information is contained • Can provide taxonomic details such as separation of corn from wheat • Spatial relationships can be examined such as vegetation distribution and water bodies • Patterns of change from one day to the next are monitored • Equipment and experienced staff are available • Historical phenomena can be examined such as land-use change

7. Sources of imagery • Two common sources • Archival imagery previously acquired for another purpose. • Imagery acquired specifically for the project at hand

8. Archival imagery • Several sources are available. • USGS • US National Archives and Records Administration • State agencies • Department of Transportation • Department of natural resources. • Such imagery must often be bought

9. Archival imagery • In most cases, computerized databases permit online searches. • Many states update photography on a regular basis. • These are useful in examining change

10. Custom acquisition • Imagery is acquired, according to the user specifications. • The date scale emulsion coverage and other characteristics are determined by the user. • The disadvantages include • high cost • requirement for planning well in advance. • effort devoted to preparing and monitoring the specifications • delays and cancellations due to weather or equipment malfunctions

11. Custom acquisition • Users should prepare a contract or statement of work • Specifies in detail the products and services required from the firm with respect to cost, deadlines, and products to be prepared • Contract is a means for controlling the completeness and quality of the imagery

12. Custom acquisition • The photo interpreter may participate in making decisions about scale, date, time of day, deadlines, choice of film, and coverage

13. Custom acquisition • Photo interpreter should ask: • Will imagery be adequate for the purpose of the project? • Is the film emulsion satisfactory for the task? • Is the season suitable? • Will planned time of day provide appropriate shadowing? • In stereophotography, what will the overlap be?

14. Elements of image interpretation • Image analysis requires explicit recognition of eight elements of image interpretation that form the framework and understanding of an image • Shape • Size • Tone • Texture • Shadow • Site • Association • pattern

15. Shape • The outline of a feature • Important to note that shape depends on perspective • Overhead perspective, introduces scale effect

19. Size • The first to the dimensions of a feature • Relative size determined by comparing the object with familiar nearby features • Absolute size refers to the use of the aerial image to derive measurements

21. Tone • Refers to the average brightness of an area or, in the case of color imagery, to the dominant color of the region • Depends on the nature of the surface in the ankles of observation and illumination. • Smooth surfaces behave like specular reflectors, they tend to reflect radiation in a single direction • These features may appear bright or dark • Rough surfaces behave this diffuse reflectors. • Scatter radiation in all directions. • A peer is medium gray tones

23. Texture • Refers to the variation in tone over a surface or the apparent roughness of the surface as seen in the photo • Created by micro shadows in small irregularities in the surface.

26. Shadow • Refers to large distinctive shadows that revealed the outline of a future as projected onto a flat surface. • Depends on the nature of the object, angle of illumination, perspective, and slope of the ground surface

32. Site • Refers to a futures position with respect to topography and drainage. • Some things occupy a distinctive topographic position because of their function • Sewage treatment facilities at the lowest feasible topographic position. • Power plants located adjacent to water for cooling

37. Association • Association refers to the distinctive spatial interrelationships between features • Schools often associated with athletic fields. • Large parking lots often associated with malls

41. Pattern • Refers to distinctive arrangement of features • Orchards have trees plant can rows • Mobile home parks have rectangular buildings arranged in rows