Austin Photo Interpretometer User manual

INTRODUCTION

This booklet constitutes the basic hand-
book of operation for the Austin Photo
Interpretometer, manufactured by Ad-
vertising Displays, Inc., Covington, Ky.
IMPORTANT: Before attemPting to
operate the instrument, study this hand-
book carefuily. Do not lose or destroy it
as it acts as a protective pad to hold the
instrument in place, and also acts as a
ready-reference booklet.

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                   Section  I

               DESCRIPTION

The Austin Photo Interpretometer is a combination
instrument which is an invaluable aid for the photo
interpreter. It  consists of a stereoscope together
with a base having scales for making very close
measurements. In effect, it operates as a stereo-
scope, a height finder, and a measuring device for
making linear photographic measurements.
The stereoscope is fitted with folding legs and is ad-
justable for inter-ocular separation. It is used with-
out the base for stereoscopic examination of the'
photographs. Figure 1 illustrates the instrument.
The stereoscope  is affixed to the base by inserting the
legs into four holes. See figure 2. When set up in
this manner, the Photo Interpretometer can be used
as an instrument for making linear measurements
or as a Height Finder.
The base of the instrument contains two moveable
transparent plates, (see figure 3). The right-hand
plate will hereinafter be referred to as the "X" slide.
This slide contains two horizontal scales, one with
graduations which equal .005 feet and the other with
graduations which equal one millimeter. This plate
also contains a small dot with a long pointer above.

A vernier screw assembly on the right-hand side of
the instrument is fitted with a knurl (K-1, figure 3).
Rotation of this knurl causes the "X" slide to move
backward or forward in a horizontal direction and
also measures the amount of movement of the "X"
slide. The vernier has two dials (D-1 and D-2,
figure 3). Vernier dial D-l measures increments of
hundred-thousandths par:ts of feet graduated .00005
feet. Vernier dial D-2 measures increments of hun-
dredths of millimeters, graduated .02 mm.
On the left-hand side of the instrument another
transparent plate is movable in a direction at an
angle of 90" to the movement of the "X" slide. Here-
inafter this plate will be referred to as the "Y"
slide. This slide contains a series of dots spaced .005
feet apart, numbered zero to 50. This slide is used
only when the instrument is used as a Height Finder.

Section  ll
         PREPARATION FOR USE
For stereoscopic examination, it is necessary that
the photographs be properly mounted. To do this
establish the center point of each photograph by
drawing intersecting Iines from the collimating
marks. Find and prick the point on the right photo-
graph 'rvhich corresponds to, or is homologous with
the photographic detail at the center of the left
photograph.
In the like manner, find and prick the point on the
left photograph which is homologous u'ith the center
point of the right photograph. Lay the two photo-
graphs side by side and position so that a straight-
edge rvill join all four pricked points. Rule a fine
line, broken at the points. This line establishes the
stereoscopic base line of the pair. (See figure 4).

       MOUNTING PHOTOGRAPHS
Temporarily, fasten the left photograph to mount-
ing surface, by means of scotch tape, along the left
edge. Lay the right photograph on top of, and

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 partially overlapping the left. With the aid of a
 straightedge, shift position of right photograph
 until homologous points measure approximately 2l+
 inches apart.
 The stereoscopic base line of both photographs must
 now exactly coincide with eaeh other and with the
 straightedge which is used in positioning the photo_
 graphs. The distance of separation of homologous
 points (ZYe inches) is desirable; however, exact
 distance of separation of these points is a matter
of personal preference.
 Experienced interpreters can readily adjust un-
marked photographs by orienting them while using
the Photo Interpretometer.  It is well, however, to
bear in mind that most accurate results are obtained
by using carefully marked prints.
After prints have been temporarily adjusted, make
necessary corrections and fasten both to the mount_
ing surface. The pair should be taped along the right
and left edges only so that either print can be lifted
and placed dorvn on top of the other in order to
completely examine the entire stereoscopic overlap.
(See figure 4-A).

I                                       i                         Seclion  lll          '
                   OPERATION
        Moking Lineqr Meosuremenls With the
                 Photo Interprelomeler
       Place the Instrument over the photograph with the
       stereoscope in postion. While looking through the
        stereoscope with the left eye closed, rotate knurl
       (K-1) in a clockwise direction until it stops. The
,      vernier dials will then read zero.
        Shift position of the instrument until the zero mark
      of horizontal scale (S-1, figure 3) is on the left                                i
                                                                                                                                                                                                                               I  ,    edge of the object to be measured. Should the total

                                                                                                                                                                                                                                                                                                                              ,lI      length of object exactly coincide with any gradua-
                                                                                                                                               1I       tion on this scale, then there is no need to use the
        vernier. For example, the roof of a building meas-
       ures exactly .025 feet.
       However, should the object being measured fall in
      between two graduation marks then it would be
       necessary to use the vernier to determine the exact
      measurement. This is done by adding the vernier
       measurement to that made with the'horizontal scale.
       In the example cited above, the roof of the building
      measured exactly .025 feet, but had the same roof
      measured instead more than .025 but less than .030
       feet, proceed as follows, to determine the exact
       measurement:

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 Sel vernier ol zero. Place the ze-ro graduation of the
 .005 foot scale (S-1) on the left edge of the roof.
 While looking through the right eye piece and hold-
ing the instrument firmly to the photograph with
the left hand, gradually rotate vernier knurl (K-1)
 in a counterclockwise direction until graduation 2b
 is in line with the right edge of the roof. The vernier
 reading (D-1) must be added to the horizontal scale
reading (S-1) to determine the total measurement.
Figure 5 is an outline drawing made from an aerial
photograph in order to clearly demonstrate this
point. In actual practice, such a line drawing would
not be available, but instead, the operator would
be working with an actual photograph.
On this drawing note that the zero graduation of
.005 foot scale (S-1) is exactly at the left edge
of the building roof. The right edge of this roof
measures somewhere between graduation 25 and
30. The vernier indicator is set on zero.
Now refer to figure 5-A, and notice that the .,X,'
slide has been moved by rotating vernier knurl
(K-l)  in a counterclockwise direction, and that
graduation 25 now is exactly at the right-hand
side of the roof.

The whole scale has shifted as the zero mark on the
.005 foot scale (S-1) no longer coincides with the

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left side of the building roof. The vernier dial (D-1)
reads 435 (.00435 feet), which represents the
amount of movement of the "X" slide. The figures,
.025 feet read on horizontal scale (S-1) plus .00435
feet, read on the vernier dial (D-1), total .02935
feet whieh is the measurement of the roof.
The same procedure must be followed in making
metric measurements, excepting of course that the
metric vernier dial (D-2) should be used with the
metric horizontal scale (S-2).
It is often desirable to measure very small objects
such as the gauge of a railroad track, the width of
a sidewalk or road, the length of an automobile,
airplane, small boat or ship, etc. If these objects are
smaller than .005 feet in case the decimal feet meas-
urement is being made, or smaller than one milli-
meter if metric measurements are used, it is desir-
able to operate the instrument as follows:
Set vernier at zero and place pointer on left edge
of object to be measured. Rotate vernier knurl until
pointer is on right edge of object. Vernier dial read-
ing is the total measurement of object.
Refer to figure 5, and note that the vernier
is set at zero and that the pointer near the center of
the "X" slide rests on one rail of a railroad track.
Now refer to figure 5-A, and note that the vernier

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      has been rotated causing the ,,X" slide to shift until
      the pointer is resting on the opposite rail of the
        track. Vernier reads 4Bb (.0048b feet), which repre_
       sents the total movement of the ,,X,' slide and is
     an accurate measurement of the photograph of the
       railroad track.

     As the gauges of railroad tracks in different parts
       of the world are usually known, they afford a good
     means of computing scale. If the gauge of a rail_
       road track, the width of a sidewalk, the wing spread
       of an airplane or other object of known size is meas_
 {tl     ured with the Photo Interpretometer, then it is easy
       to determine the scale of the photograph by dividing ll
      the known size of the object by the measurement
$    made with the Photo Interpretometer. The result
       is a multiplying factor for all other measurements
     made on the same photograph to determine actual
         size.
       IMPORTANT: For greatest accuracy, it is
           advisable to complete all measurements by
           final clockwise adjustment of the vernier.
          This compensates for any slight amount of
          "play" in the vernier screw.
     The Photo Interpretometer is designed for making
     more accurate measurements than are practical with
       straight scales not having any vernier adjustment.

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I     Section lV
   USING THE PHOTO.INTERPREIOTYIETER
         A5 A HEIGHT FINDER
The two horizontal scales (S-1 and S-2) on'the
"X" movement slide are not used r'r'hen the instru-
ment is used as a Height Finder.
The series of dots on the "Y" slide are used for
measuring parallax. As explained before, the dots
are spaced .005 feet apart.
Read thele dots as follows: 5 :  .005 ft., 10 :  .010
ft., 15 :  .015 ft., etc., etc.
Refer to figure 6, which is a view showing what the
operator would see when looking through both eye
pieces of the instrument. It  represents a stereo-
scopically fused image, although, it is impossible to
show any depth in a printed illustration; therefore,
this view is intended to demonstrate the operation
of the instrument as a Height Finder although no
relief is apparent.                                                a
This illustration shows the series of red dots which
are numbered 0, 5, 10, etc., to 50, reading from right
to left. Note that the pointer is above the dot marked
20. lf, instead of an illustration, this were actually
a stereoscopically fused image, this dot (20) would
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        appear to rest directly on,the ground near the base
       of the building.
       The vemier dial (D-1) reads 137 (.00137 feet). The
         pointer over 20 means .020 feet, this plus the read-
        ing of .00137 feet on the vernier dial makes a total
        of .02137 feet, which represents the gauge reading
         of the spot being measured
     Now refer to figure 6-.4', and note that the instru-I       ment has been moved so that a dot with the pointer
 I      above rests on top of the building. The pointer is
     now above dot numbered 15 which means .015
          feet, the vernier now reads 465 (.00465 feet). These
      two figures total .01965 feet, which is the gauge I        reading for the top of the building. Subtract one
J      reading from the other, and the difference is .00172
         feet. This is the difference in gauge readings be-
       tween the two elevations (that is, the ground at the
       base of the building and the top of the building
           itself ). This difference in gauge readings is used to
        determine the actual height of the building in for-
 I      mulae as explained later.
                    PREPARATIONI
        Place the instrument over a sheet of white paper.
       While looking through the eye pieces, move the "Y"
         slide either upward or downward until the dot with
       the pointer above, on the "X" slide is fused with

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one or another of the dots on the "Y" slide. To do
this, hold the instrument firmly with the right hand,
take hold of the "Y" slide at the top and bottom
edges with the thumb and forefinger of the left
hand. This slide can be moved easily and will re-
main in position once it has been properly adjusted.
This is done to bring the dots on the "Y" slide into
horizontal alignment with the single dot on the
"X" slide. When this is properly done, the dots will
merge stereoscopically and the instrument is then
ploperly adjusted for the operator.
It has been stated in the preceding paragraph that
the pointer will point to one or another of the dots
on the "Y" slide. The inter-ocular adjustment of the
stereoscope and also the inter.-pupillary separation
of the individual will determine which dot on the
"Y" slide rvill stereoscopically merge with the single
dot on the "X" slide
With some individuals it may be that the single dot
on the "X" slide will jump from one dot to another
on the "Y" slide
  IMPORTANT: It is absolutely necessary to
   have stereoscopic fusion of the dots to get
  .any results when measuring parallax.
   Hereinafter, reference to "the dot" will be un-
   derstood to mean a stereoscopically fused
   image of the single dot on the "X" slide with
   one or another of the dots on the "Y" slide.

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              OPERATION
Set vernier dial (D-1) in mid-position, reading 250.
Now place the instrument over a pair of properly
mounted photographs and orient it  until proper
fusion produces a stereoscopic image. If the "Y"
slide has been properly adjusted, then the dots will
also fuse when stereoscopic fusion of the photo-
graphs has been effected.
After this has been done, it will be noted that the
pointer is above one of the dots on the "Y" slide,
and this dot will appear to be in a plane a little bit
above or below the ground" or it may appear. to be
exactly on the ground at this point.
By rotating knurl (K-l) this dot, with pointer above,
will appear to descend if vernier knurl (K-1) is
rotated in a counterclockwise direction. If the knurl
(K-1) is rotated in a clockwise direction, this dot
will apparently rise.
Rotate vernier knurl in either a clockwise or counter-
clock-wise direction, (depending upon whether or not
the dot appears to be above or below the terrain)
until the dot appears to rest exactly in the same
plane as the spot being measured. Then take a gauge
reading by noting the position of the pointer and
adding that to the reading of the ver.nier dial (D-l).
Example: The pointer is over figure 20 on the "Y"
slide. This means .020 feet. The vernier reads 1.2b,
this means .00125 feet. The two added together

(.02t25. feet) represent the gauge reading for the spot being measured
 Next, take a gauge reading as described above for
 the next spot to be measured and subtract one from
 the other-this gives the difference in gauge read-
 ings between the two spots and will be used in
 formulae for determining the height.
 This procedure is followed in all other cases where
 it is desirable to take gauge readings at different
 points.
   IMPORTANT: In making gauge readings for
   height alu;ays start with vernier dial set at
    mid-position reading 250.
Please note that only decimal feet readings have
been used in determining heights. The metric scale
and vernier play no part in these operations.
    DETERMINING HEIGHTS BY HAVING
      TWO POINTS OF KNOWN
         ETEVATIONAT DIFFERENCE
Take gauge readings at two points of known eleva-
tional difference such as the top and bottom of a
building or other object of known height, subtract
the lower reading from the higher, and the result
represents the difference in gauge readings.

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 Next divide the known actual elevational difference
 by the difference in gauge readings, the result will
 be a multiplying factor for all subsequent differences
 in gauge readings for that pair of photographs.

For example, gauge readings have been made on a
 building that is known to be 50 feet high. First
reading .01045 feet, second .00920 feet. Difference
 in gauge reading is .00125 feet, divided into 50 feet
known elevation, equals 40,000, which is the multi-
 plying factor for all subsequent differences in gauge
 readings.
   PROCEDURE IN DETERMINING HEIGHT
  OF OBJECTS IN AERIAT PHOTOGRAPHS
 BY     OF PARAIIAX
      '\,IEANS               'IAEASUREMENTS
FORMULAE FOR HEIGHT-Where no known
difference in elevation can be found on the photo-
graph, the height of an object may be calculated
from the parallax measurement together with the
scale of photographs and focal length of camera
lens. The height is determined by the following
formula provided the prints are contact:
   ho  :       f xP
       RF (W+P)
Where: ho -  Height of object.       P  :  Parallax distance as measured by

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means of the Photo Interpretometer.
        f-w-  FocalDistancelengthbetweenof cameracenterslens.of photos.
               This factor is determined by plotting
                center of one photo on the other and
             measuring photo distance between
                 centers.
     RF-   Scale of photos.

All measurements must be in the same units. The
Photo Interpretometer measures parallax in decimal
parts of feet. Other factors such as focal length and
distance between centers of pictures must also be
determined in feet for substitution in the formula.
The RF should be calculated as accurately as pos-
sible by whatever means are available. This may be
determined by measuring on the photo objects of
known size and using the formula: RF :  photo
measurement divided by ground measurement in the
same units. RF may be determined by comparison
with maps of known scale.

                Distance on photo
RF of photo :            X RF of map                Distance on map
The RF may be calculated by using the formula:
   RF:     f;-t:l.

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where H is elevation of aircraft above ground, or
elevation of aircraft above sea level minus elevation
of ground above sea level. Altitude of aircraft ginen
in marginal inf ormation is generally an appronima-
tion and results obtained bg using this f,gure usill
be correspondingly approrimate. However, when H
is known (in feet) the formula may be used in the
following form:
   (2)ho-HXP
       w+P

Once the height of an object on the photographs  is
determined and its parallax has been measured, the
height of other objects on the photos can be deter-
mined by direct proportions as follows:
    (3) ho :  ho'      ho'-  ho X P'
      PP'             P
Where ho' and P' are height and parallax measure-
ment respectively of second object.

Example:
It is desired to determine the height of a structure
on a stero pair. The parallax as measured by the
interpretometer is .00155 foot. The marginal infor-
mation on the prints indicates that the focal length
of the camera lens is 6 inches  (.5 foot).

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Enlargements: In the event that the photos used
are not contact prints, the marginal information
will lead to elroneous results unless properly inter-
preted. The effect of enlarging a print is similar
to that of using a camera with a longer focal length,
but with the same height of aircraft or camera.
In measuring parallax on blow-ups it is essential
that the height of aircraft remain the constant fac-
tor and that "f" be considered as varying with the
scale or amount of enlargement.  If the height of
the aircraft above glound is known with reasonable
accuracy, the formula for height of object may be
used in the following form:
  (2\ho:HXP
      w+P
Here again, the height of afucraft given in the
marginal information of the photo is probably in-
accurate, and if "f" fot the blow-up can be calcu-
lated from the amount of enlargement (that is if
the enlargement is three times then "f" for the en-
largement will be three times that for the contact
print), then the height of the object in the photo
can be determined by the formula:
  (1)ho:   fXP
         RF (W+P)
where RF of the enlargement has been determined
by scaling objects of known size or by comparison
with maps.

Section V
               GENERAT
The foregoing sections have been devoted to the
general principles and operation of the instrument.
Careful study of this handbook should enable
even the most inexperienced operator to get good
results. Practice with a good pair of stereoscopic
photographs. It will not take long to realize that
the Austin Photo Interpretometer can be very help-
ful in photo interpretation. Handle the instrument
as you would any piece of optical equipment.
After use, it should be carefully packed in the case.
To do this, the instrument should be "knocked down"
and the base should be packed in the box first. Then
this booklet should be placed on top of the base in
the left-hand side of the box. The stereoscope
should be folded and placed on top of the booklet
and the lid carefully closed and locked.
                  Section Vl
             MAINTENANCE
The Photo Interpretometer is so constructed that a
detailed description herein, of disassembly and re-
assembly, is not necessary.
Occasionally, it may be necessary to set the vernier
dials. To do this rotate the knurl (K-1) in a clock-

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wise direction until it stops. While holding this at
stopped position with the right hand, rotate the
vernier dials with the left hand until zero gradua-
tion on both dials are in line with indicator above.
These dials are friction held. and will rotate as de-
 scribed.
The vernier dial (D-1) is marked 0 at one end and
000 at the other. The vernier dial (D-2) is marked
0 at one end and 00 at the other. When referring
to zero in this booklet, it is understood that the
meaning is 0.
00 and 000 simply indicate the end of the metric
and decimal feet vernier seales.
IMPORTANT: The "X"  slide must move freely
when the vernier knurl is rotated. If the clips which
hold this slide are too tight they will cause the slide
to "drag" or "bind."
To adjust the "X" slide proceed as follows: Set ver-
nier dial (D-1) at 250. Loosen the four screws which
hold the clips. Rotate vernier knurl to make sure
that the slide moves freely.
Reset vernier at 250. Set the clips close to the glass
and tighten the screws. The clips should be set elose
enough to prevent excessive side play, but must not
be too tight. If they are, the accuracy of the instru-
ment will be affected.

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