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## Teacher’s Notes: Digital Image Sources

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**Teacher’s Notes: Digital Image Sources**Digital images from space can be found at the following NASA web site: http://eol.jsc.nasa.gov/sseop/clickmap/ Click the map for images of that area. You can include in your search results information such as Altitude and Focal Length of the camera used by selecting FCLT and ALT at the bottom of the page. Images can be used freely if attributed to NASA. For added impact Google Earth (http://earth.google.com/) can be used to zoom from space to Tower Bridge, the Channel, etc. Depending on your location you may find better resolution images of your town on Microsoft’s Live Local site: http://local.live.com/ Information on aerial photography: http://landmap.ac.uk/ipc/ccrs/chapter2/chapter2_7_e.html**Starter**• How many, • mm in 20 cm? • cm in 3.75 m? • m in 2.5 km? • cm in 6 km? • mm in 8 km?**25**10 5 2 × 8 cm 5 2 = x = 20 cm Reminder 25 cm 8 cm 10 cm x cm Find the value of x. Enlargement Factor = x =**10 m**8 cm 7 cm 3 m 5 m 12 cm x m x cm Answers: a i ii 10.5 cm b i ii 6 m Exercise In each case, the pair of triangles are similar. Equal angles are marked. Find: i the relevant scale factor (enlargement or reduction) ii the missing length. b a**How a Lens Works**Lens Object Height Image Height Focal Length Distance to object**Lens**Object Height x Image Height 14 mm Focal Length 35 mm Distance to object 10 m Example**10 m**35 mm 2000 7 × 14 mm 10000 mm 35 mm = x = 4 m 2000 7 = Enlargement Factor = x = x = 4000 mm**Satellite Photography**Image: NASA**Similar Triangles in Space**Image size Focal length of camera Lens Height above ground (Altitude) Object size**135 000 000 mm**90 mm = 135 km 90 mm = 1 500 000 1500000 × 20 mm x = 30 km Example 90 mm 20 mm EF = Lens 135 km x = x = 30 000 000 mm x**Is the 100 metre track marked correctly?**Invergordon Academy Playing Fields Aerial Photo Altitude = 400 m Focal length = 152 mm**Tower Bridge London**Aerial Photo Altitude = 2500 ft (762 m) Focal length = 152 mm Calculate the span of the bridge.**Calculate the shortest distance across the Channel.**(marked) Image: NASA English Channel Photo taken onboard Space Shuttle Altitude = 126 miles (203 km) Focal length = 90 mm**Actual Distances**• Tower Bridge – 244 m • English Channel – 20 miles or 32 km