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,
a. mm in 20 cm?
b. cm in 3.75 m?
c. m in 2.5 km?
d. cm in 6 km?
e. mm in 8 km?
Similar Triangles & Photography
Reminder
25 cm
8 cm
10 cm
x cm
Find the value of x.
Enlargement Factor =
=
25
10
5
2
x=
5
2
× 8 cm
x = 20 cm
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.
a
b
8 cm
10 m
7 cm
3m
5m
12 cm
xm
x cm
Answers:
a i
3
2
ii 10.5 cm
b i
3
5
ii 6 m
How a Lens Works
Lens
Object
Height
Image
Height
Distance to object
Focal Length
Example
Lens
Object
Height
Image
Height
x
14 mm
Distance to object
Focal Length
10 m
35 mm
Enlargement Factor =
=
=
10 m
35 mm
10000 mm
x=
2000
7
x = 4000 mm
35 mm
2000
7
× 14 mm
x=4m
Satellite
Photography
Image: NASA
Similar Triangles
in Space
Focal length
Image
size
of camera
Lens
Height above
ground
(Altitude)
Object size
Example
90 mm
20 mm
EF =
Lens
=
135 km
=
135 km
90 mm
135 000 000 mm
90 mm
1 500 000
x = 1500000 × 20 mm
x
x =
30 000 000 mm
x =
30 km
Invergordon Academy Playing Fields
Aerial Photo
Altitude = 400 m
Focal length = 152 mm
Is the 100 metre track
marked correctly?
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