Lab  #1 - Stereoviewing & Scale

 There are 4 "fiducial marks" visible (the black marks on the middles of the edges) there usually 8 (4 along the sides and 4 in the corners) These marks are actually in the film itself, they are used to mount a frame of photography precisely on stereo devises. Most aerial photography is a standard 9" x 9" frame, the fiducial marks are exact markings based on those dimensions. (See Figure 6-5, p142, Jensen Remote Sensing of the Environment)

I. Stereoviewing
 In many instances it is entirely feasible to use single (mono) vertical photographs for the recognition or classification of specific features. Actually, this is the equivalent of using only one eye, a practice referred to as monocular vision.  The all important third dimension of depth perception is provided only when objects are viewed with both eyes.  In this case, the converging lines of sight from each eye are transmitted to the brain, and the result is binocular or stereoscopic vision.  It must be emphasized that the signal from each eye is distinct and separate.  The brain combines the two signals in order to produce stereo vision. 

 When objects greater than 1500 to 2000 feet away are viewed by unaided eyes, the special ability of depth perception is essentially lost.  At such distances, lines of sight from each eye converge very little; the lines of sight are practically parallel when the eyes are focused at infinity. 

See Remote Sensing of the Environment: An Earth Resource Perspective, Jensen, page 137-147

II. Preparing Photographs for Stereo Viewing
 Photographic missions are planned so that prints will overlap about 60 percent of their width in the line of flight and about 30 percent between flight strips.  For effective stereoviewing, prints must be trimmed to the nominal 9 x 9 inch size, preserving the four fiducial marks at the midpoints of each edge.  Then adjacent photographs must be aligned along the the flight line for "true" stereovision and accurate measurements.  See figure below.

 Complete the following steps to locate the principal point (PP), the conjugate principal point (CPP) and the flight line.  Use one each of the following photographs: BTM-6K-29, BTM-6K-34 and BTM-6K-115 (2-24-54)

2. Locate the conjugate principal points (CPPs) on each photograph.  The CPPs, sometime called the corresponding principal points, are transferred using the following steps: 

  A. Tape one photograph to the table with drafting tape.  Image shadows should be oriented toward the observer.  If the shadows fall away from the viewer, there is a tendency to see relief in reverse. 
  B. Move the adjacent photograph along the stabilized photo in the direction of the line of flight until conjugate images on each print are approximately 2 1/4 inches apart (the exact distance will depend on an individual's eye base). 
  C. Place the lens stereoscope over the photographs parallel to the flight line so that the left-hand lens is over the same image as the right-hand lens.  The area directly under the stereoscope lens should then appear as a 3D image. 
  D. Next, fasten down the movable photograph.  While viewing the three-dimensional image, the observer places a pencil lightly on the unmarked print until it appears to fall precisely on the spot chosen for the PP.  This marks the location of the CPP, although a monocular check should be made before the print is  marked. 

3. Repeat Step #2 for all of the photographs.  Each photograph will have one PP and two CPPs except those prints falling at the ends of the flight lines.  These airphotos will have only one CPP.  Mark the adjoining prints with a penciled dot after the PPs and CPPs have been verified. 

4. Locate the flight lines on each print by aligning the PPs and CPPs.  The edges of the aligned circles should be connected with a finely penciled line.  Because of lateral shifting of the aircraft in flight, a straight line will rarely pass through the PP and both CPPs on a given print. 

III. Photo Scale
 Knowledge of the camera focal length and the aircraft altitude makes it possible to determine the photo scale (PS) and the representative fraction (RF) or natural scale of the photo. The photo scale and representative fraction may be calculated as follows: 

 While the foregoing method of deriving photo scale is theoretically sound, it often happens that either camera focal length or flight altitude is unknown to the interpreter.  In such cases, scales may be determined by the ratio of photo distance between two points to map distance (MD) using the map scale (MS) or ground distance (GD) between the same two points. 

 PD and GD are different due to the source the measurement is referring to, Ground Distance (GD) and Map Distance (MP) are used to differentiate a measurement you make from the map source and a real world distance that you calculate (or measure yourself with a measuring tape). When calculating scale, PD and GD (GD is the real world distance) you measure and PD is from the photo, must be in the same units in order to yield a unitless Representative Fraction (RF); the Map Scale Reciprocal (MSR) and the Photo Scale Reciprocal (PSR) are both unitless.

 In applying this technique, the two points selected should be diametrically opposed in such a way that a line connecting them passes near the principal point (PP).  If the points are approximately equidistant from the PP, the effect of photographic tilt upon the scale measurement will be minimized.  Features selected must also be chosen for easy recognition and measurement on map or ground.  Flat terrain is preferred; hilly terrain should be avoided to minimize the effects of relief displacement. 

 Ground distance can be measured with surveying equipment, it may be known in advance, or it can be calculated by multiplying the measured distance on a map by the map scale.  This is the method you will use for this lab and others.

 Look at the Quadrangle print out; the scale bar at the bottom can be used to measure a map distance (MD) and then the same distance on the photo (PD).  Your MD and PD measurments need to be in the same units (m, ft, in or cm) for the calculations.

 A part of an 8.5x11 USGS Quadrangle Map has been scanned, the part of the map that shows our area of interest has been rescaled slightly from the original source map. Use the scale bar and the edge of a piece of paper to measure lengths of roads. (USGS sometimes made mistakes; look for the names of the roads.)

Expressing Scale
 1. Scale ratio is also referred to as the proportional scale.  1:20,000 is read as "one to twenty thousand".
 The scale ratio is always written as one unit on the photo or map to the corresponding number of units on the ground. 
 2. Representative fraction scale (RF): Two other terms refer to the representative fraction scale -- the fractional scale and the RF scale.  It is the scale ratio written in fractional form, (1/20,000)
 3. Equivalent scale: Equivalent scale is also known as the descriptive scale.  For example: one inch equals 5,280 feet (1 inch = 5,280 feet); two inches equals one mile (2 inches = 1 mile); and 100 feet per inch. 
 4. Graphic scale: Also called a bar scale, used on maps and drawings to represent length scale on paper.

Formulae
f = lens focal length
H = flying height above mean terrain (mean ground elevation) 
PS = photo scale; the relationship of photo to ground distances
PSR = photo scale reciprocal; the PS denominator
MS = map scale; the relationship of map to ground distances
MSR = map scale reciprocal; the MS denominator
N.B.: Avery and Berlin use the abbreviation MS to refer to the map scale reciprocal (or as they put it, the map scale denominator).  In this course, MS we will use the definitions given in this handout!
PD = photo distance 
MD = map distance 
GD = ground distance 

Conversions
Distance 
 1 meter = 100 centimeters = 1000 millimeters
 1 foot = 12 inches 
 1 yard = 3 feet
 1 meter = 3.28 feet 
Area 
1 square meter = 10.76 square feet 
1 acre = 43,560 square feet
1 square kilometer = 230.4 acres

If one inch on the photo is equivalent to 1,000 feet on the ground (or 12,000 inches) : RF = 1/12,000; MS = 1/12,000; MSR = 12,000 and the map scale denominator is also equal to 12,000, different words for the same thing

UCSB - IV in 1954

UCSB - IV Goleta USGS Quad

IV. Problems  (please be neat, use a pencil and scratch paper!)

1. General image identification.  Please answer the following questions. 
 A. On what date were the photographs taken? 
 B. What is the airphoto identification information? (flight name, frame number)
 C. At what time of day was the photo taken? (look for shadows)
 D. How many fiducial marks are there on each frame?
 E. What direction is south? (with the date and identification info facing away from you being north, East and West are which edges of the photo?) 
 F. Where is this? 

In the problems that follow, please list the formula to be used in each task, show all of your math work neatly in an organized way and put a box around your final answer.  Label all numbers with their appropriate unit of measurement (e.g. ft, m, ac, in, etc.).  Failure to do so will result in points being lost for each problem!

2. Using the appropriate formula and the map (Goleta 7.5' USGS quad), determine the scale of the airphotos.  Assume that you have no knowledge of the camera focal length. 

3. Once you have determined the nominal scale of the photographs (Problem #2), determine the aircraft altitude given a six inch focal length.  Answer in meters. 

4. If you have no knowledge of the camera focal length, but you knew the aircraft was flying at 10,000 feet, then what camera focal length (in inches) must have been used to obtain the representative fraction that you determined in Problem #2 above?

5. What is the ground distance in meters from Storke Road along El Colegio to the West Entrance of the UCSB campus? 

6. What is the area from Storke Road (mislabeled on the USGS Quad) along El Colego to the intersection of Stadium Road at the edge of campus, and from where Stadium Road intersects El Colego down to the edge of the cliffs and back over to where Storke Road would intersect the cliffs if it extended all the way down? The area you are measuring is roughly a rectangular shape covering all of IV from the edge of campus over to Devereaux. Give your answer in square feet, acres and hectares. Sketch the rectangular area and label your measurements.

7. Scales of vertical photographs change constantly with variations in terrain elevation (topography) because the distance between the aircraft and the ground varies.  Using the appropriate formula and some simple algebraic manipulations, substantiate the previous statement by showing how scale changes with decreasing and increasing elevations from the datum elevation.  The aircraft is flying at 10,000 feet above mean sea level (MSL), the elevation at Point A is 5,200 feet and 2,700 feet at Point B.  The camera focal length is eight inches.  What are the RFs at  Points A and B?  (Hint: Drawing a simple diagram may help.)

8. For a photo with a RF  of 1:8000 determine the following (giving answers to the 2nd decimal place):
  A. The number of meters on the ground per inch on the photo.
  B. The number of acres on the ground per square inch on the photo. (Hint: 43,560 square feet per ac).
  C. The number of square kilometers on the ground per square inch on the photo. (Hint: 247.1 ac per km^2).

9. You want to contract for 1:24000 scale photography, of some property that you are thinking of buying, with a 35 mm camera and a 50 mm focal length lens.  How high should you specify for the  plane to fly above mean ground elevation? Answer in feet.

10. A road segment shown on an aerial photograph can be located on a 1:24000 topographic  map (a standard USGS 7.5' quad).  If the measured distance is 47.6 mm on the map and 94.2 mm on the photograph, what is the scale (RF) of  the photograph?

Extra Credit (not required) 
 - to be turned in as part of your lab 

 1) For a photo with a RF  of 1:8000 determine the following: 
  A. The number of yards on the ground per inch on the photo. 
  B. The number of square kilometers are there on the ground per centimeter square on the photo. 
  C. The number of square miles on the ground per square inch on the photo. 

 2) You want to contract for 1:12,000 scale photography, of some property that you are thinking of buying 
  using a 6 inch focal length lens.  How high should you specify for the plane to fly above mean ground
 elevation? 

 3) A road segment shown on an aerial photograph can be located on a 1:24000 topographic  map (a 
  standard USGS 7.5' quad).  If the measured distance is 52.4 mm on the map and 91.9 mm on the 
  photograph, what is the scale (RF) of  the photograph?