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    Basics of Photogrammetry
    Before describing the operation of the V-STARS system, a brief introduction to photogrammetry is provided for those who are unfamiliar with the technology.
    .
    Photogrammetry, as its name implies, is a 3-dimensional coordinate measuring technique that uses photographs as the fundamental medium for metrology (or measurement).
    3 ( ).
    The fundamental principle used by photogrammetry is triangulation.
    .
    By taking photographs from at least two different locations, so-called "lines of sight" can be developed from each camera to points on the object.
    " " .
    These lines of sight (sometimes called rays owing to their optical nature) are mathematically intersected to produce the 3-dimensional coordinates of the points of interest.
    ( ) 3 .
    Triangulation is also the principle used by theodolites for coordinate measurement.
    theodolites .
    If you are familiar with these instruments, you will find many similarities (and some differences) between photogrammetry and theodolites.
    ) theodolites.
    Even closer to home, triangulation is also the way your two eyes work together to gauge distance (called depth perception).
    ( ).
    This primer is separated into two parts.
    .
    Photography describes the photographic principles involved in photogrammetry, while Metrology describes the techniques for producing 3-dimensional coordinates from two-dimensional photographs.
    3 .
    Photography
    Photography - The First Part of Photogrammetry
    --
    Taking photographs is, of course, essential for making a photogrammetric measurement.
    .
    To obtain the high accuracy, reliability and automation the system is capable of, photographs must be of the highest quality.
    .
    Fortunately, because of the design of the system, photography with V-STARS is actually simpler than film photography.
    .
    The three main considerations for good photography are:
    :

    1.
    1.
    Field of View
    2.
    2.
    Focusing
    3.
    3.
    Exposure

    Field of View
    The camera's field of view defines how much it sees and is a function of the focal length of the lens and the size (often called the format) of the digital sensor.
    ( ) .
    For a given lens, a larger format sensor has a larger field of view.
    .
    Similarly, for a given size sensor, a shorter focal length lens has a wider field of view.
    .
    The relationship between format size, lens focal length and field of view is shown below:
    :

    The standard lenses available with V-STARS are so-called medium angle lenses and have about 50 wide fields of view.
    50 .
    The wider the field of view, the more you see from a given location.
    .
    For a medium angle lens, a convenient rule of thumb is that you will generally need to get back as far from the object as the size of the object.
    .
    For example, you will get about three meters (ten feet) back to see a three-meter (ten foot) object.
    ( ) ( (.
    In general, there is a tradeoff between the field of view of a lens and accuracy.
    .
    Although wider-angle lenses need less room around the object, they also tend to be less accurate.
    .
    (The reasons for this are beyond the scope of this introduction.) Thus, you generally want to use the longest focal length lens you can.
    ( .) .
    The medium angle lenses provided with V-STARS represent a good compromise between field of view and accuracy.
    .
    Focusing
    One consideration for normal photography is, of course, focusing the lens so the image is sharp.
    .
    The range of acceptable sharpness is called the depth of focus.
    .
    The depth of focus of a lens is a function of many factors, including: the focal length of the lens, the format size, the distance from the camera to the object, the size of the object, and the f-number of the lens.
    : .
    As you can appreciate from all the factors listed above, the depth of focus can be a complex function.
    .
    V-STARS has been designed so that images will be in acceptable focus for points between 0.5 meters (20 inches) and 60 meters (200 feet) from the camera.
    0.5 (20 ) 60 (200 ) .
    Fixing the focus effectively eliminates the depth of focus problem.
    .
    Exposure
    Camera Exposure
    For photogrammetry purposes, it is desirable to set the targets bright and the background dim.
    .
    When retro-reflective targeting is used, the target and background exposures are almost completely independent of each other.
    .
    The target exposure is completely determined by the flash power while the background exposure is determined by the ambient illumination. The amount of background exposure is controlled by the shutter time.
    . .
    Eliminating the background exposure makes the targets easier to find and measure.
    .
    However, if there is no background image whatsoever, trying to figure out which target is which can be difficult.
    .
    Usually, a compromise is reached and the background exposure is set so the object is dim enough to not interfere with target measurement, but still bright enough that it can be seen when enhanced.
    .
    Background Exposure
    The shutter time is used to control the background exposure.
    .
    When the camera is off-line, the shutter time is selected using the mode switches that are located on the top of the camera next to the display.
    .
    The available shutter times on an INCA2 range from 8 milliseconds to 8 seconds.
    INCA2 8 8 .
    The INCA2 camera has an AUTO Exposure feature that can be used to automatically set the shutter speed.
    INCA2 .
    The default setting is to use the AUTO Exposure.
    .
    If AUTO Exposure is selected, the shutter exposure is set automatically the first time you take a picture on a job.
    .
    Target Exposure
    The flash power setting for the target exposure depends on the distance from the camera to the targets, and the target size.
    .
    The following diagram gives recommended flash power settings at varying distances.
    .
    If you are shooting the object in sections, use the size of the sections.
    .
    The tables assume the recommended target size (which is also listed) is used.
    ( ) .
    If the targets are smaller than this, you may want to increase the flash power setting one step to help compensate.
    .
    The tables assume the default lens f-number of F11 is used with an INCA.
    F11 .
    It is important to check the lens and make sure it is set to f11, the default setting for the lens.
    F11 .

    Metrology
    Metrology - The Second Part of Photogrammetry
    --
    Photography in its broadest sense is a process that converts the real 3-dimensional world into flat 2-dimensional images. The camera is the device that makes this transformation or mapping from 3 dimensions to 2 dimensions.
    3 2 . 3 2 .
    Unfortunately, we cannot map the 3-dimensional world onto two dimensions completely so some information is lost (primarily the depth).
    (3) ( ).

    Photogrammetry in its broadest sense reverses the photographic process described above.
    .
    It converts or maps the flat 2-dimensional images back into the real 3-dimensional world.
    2 3 .
    However, since information is lost in the photographic process, we cannot reconstruct the 3-dimensional world completely with just one photograph.
    3 .
    As a minimum, we require two different photographs to reconstruct the 3-dimensional world.
    3 .
    If this process was perfect, the two photographs are more than enough information to perfectly reconstruct the 3-dimensional world they represent.
    3 .
    Unfortunately, the photography and measuring process is not perfect so the reconstruction of the 3-dimensional world is also imperfect.
    3 .
    However, we can take more photographs and use the extra information in them to improve the process.
    .
    The 3-dimensional coordinates we produce from the measurements of multiple photographs are the end result of photogrammetry.
    3 .
    Photogrammetry uses the basic principle of Triangulation, whereby intersecting lines in space are used to compute the location of a point in all three dimensions.
    .
    However, in order to triangulate a set of points one must also know the camera position and aiming angles (together called the orientation) for all the pictures in the set.
    ( .
    A process called Resection does this.
    .
    Finally, because the V-STARS camera is a precision measuring instrument, it must be calibrated so its errors can be defined and removed.
    .
    One of the most powerful features of V-STARS is its ability to produce this camera calibration as a byproduct of the measurement in a process called Self-calibration.
    .
    Although each of these techniques is best described separately, they are actually all performed simultaneously in a process called the Bundle Adjustment.
    .
    Triangulation
    Triangulation is the principle used by both photogrammetry and theodolites to produce 3-dimensional point measurements.
    theodolites 3 .
    By mathematically intersecting converging lines in space, the precise location of the point can be determined.
    .
    However, unlike theodolites, photogrammetry can measure multiple points at a time with virtually no limit on the number of simultaneously triangulated points.
    theodolites .

    In the case of theodolites, two angles are measured to generate a line from each theodolite.
    theodolites .
    In the case of photogrammetry, it is the two-dimensional (x, y) location of the target on the image that is measured to produce this line.
    ( ) .
    By taking pictures from at least two different locations and measuring the same target in each picture a "line of sight" is developed from each camera location to the target.
    " " .
    If the camera location and aiming direction are known (we describe how this is done in Resection), the lines can be mathematically intersected to produce the XYZ coordinates of each targeted point.
    ( ) XYZ .
    Resection
    Resection is the procedure used to determine the final position and aiming (called the orientation) of the camera when a picture is taken. Typically all the points that are seen and known in XYZ in the image are used to determine this orientation.
    ( ) . XYZ .
    V-STARS uses the AutoStart or SuperStart operation to get the preliminary camera orientation. This orientation is based on the AutoBar or any known coded targets.
    SuperStart AutoBar .
    For a strong resection, you should have at least twelve well-distributed points in each photograph. If your measurement does not have this many points, or they are not well distributed, it is recommendable to add points.
    . .
    Points that are added to strengthen the solution are called "fill-in" points.
    "".
    If the XYZ coordinates of the points on the object are known (we describe in Triangulation how this is done), we can compute the camera's orientation.
    XYZ ( ) .
    It is important to realize that both the position and aiming direction of the camera are needed.
    .
    It is not sufficient to know only the camera's position since the camera could be located in the same place but be aimed in any direction.
    .
    Consequently, we must know the camera's position which is defined by three coordinates, and where it is aimed which is defined by three angles.
    .
    Thus, although three values are needed to define a target point (three coordinates for its position), we need six values to define a picture (three coordinates for
    ( ) )
    position, and three angles for the aiming direction).
    ).
    Self-Calibration
    Although the cameras and lenses used in the V-STARS system are of the highest quality, they must still be precisely calibrated to remove errors that are present in the system.
    .
    Some of these error terms can be described in terms of their physical cause while others are more empirically derived.
    .
    In any case, all of these error terms are automatically solved for by V-STARS along with the XYZ coordinates of the target points and the orientation (position and aiming angles) of each picture in a process called the Bundle Adjustment.
    XYZ ) ) .
    This ability to calibrate the camera as a byproduct of the measurement is called Self-calibration and it means the camera will be calibrated at the time of measurement, and under the environmental conditions that exist (temperature, humidity, etc.) at the time of measurement.
    .. ( ) .
    This is far superior to relying on an old and possibly outdated laboratory calibration that may have been done under dramatically different conditions than existed at the time of measurement.
    .
    There are certain requirements that must be met in order to self-calibrate a camera, but they are usually easy to do.
    .
    First, the measurement must have what is called roll diversity. This usually means you must take some photographs with the camera horizontal and some photographs with the camera vertical.
    .
    Although you will get better results if you take about half of your shots one-way and half the other, this is not critical.
    .
    What is critical is that you must have at least one picture that is rolled approximately 90 differently than the others.
    90 .
    If you do not, you cannot self-calibrate the camera.
    .
    Instead, you will have to rely on a pre-existing calibration that is less reliable and less accurate.
    .
    A second requirement is that you must measure a minimum number of photographs taken from a minimum number of different locations.
    .
    You should measure at least six photographs if the object is two-dimensional (the object is essentially flat) or four photographs if the object is three-dimensional.
    ( ) .
    Also, the photographs should be taken from at least three different locations.
    .
    Since most jobs will take at least this many photographs there is usually no reason not to self-calibrate the camera.
    .
    In fact, we strongly recommend that you always take enough photographs to self-calibrate the camera because it is so quick and easy to take and measure extra photographs.
    .
    A final requirement is that you must have a minimum number of well-distributed points on each photograph and for the entire measurement.
    .
    Specifically, you should have at least twelve well-distributed points on each photograph, and at least twenty points for the entire measurement.
    .
    Well-distributed means the points are distributed fairly evenly throughout the photograph.
    .
    It is much better for example to have twelve points distributed evenly throughout the picture than to have fifty clustered together in one small area. If you do not happen to need this many points for the measurement or they are not well distributed, we recommend you add points to the measurement. As you will see, it is very quick and easy to add extra points to the measurement so feel free to do so.
    . . .
    Bundle Adjustment
    The Bundle Adjustment is the program that processes the photographic measurements to produce the final XYZ coordinates of all the measured points.
    XYZ .
    In order to do this, it must Triangulate the target points, Resect the pictures and Self-calibrate the camera.
    .
    The Bundle Adjustment program is called STAR, which stands for Self-Calibration, Triangulation and Resection.
    .
    The real power of the bundle adjustment is that it is able to do all three of these things simultaneously.
    .
    If you review the descriptions of Triangulation and Resection, it appears there is a problem.
    .
    In order to triangulate the measured points, we must know the orientation of the pictures.
    .
    However, in order to orient the pictures, we must know the coordinates of the measured points.
    .
    How do we get started here- The answer is the bundle adjustment has the capability to figure them both out simultaneously and to self-calibrate the camera as well!
    !
    This is where the name bundle adjustment comes from because it bundles all these things together and solves them all at the same time.
    .
    The Bundle Adjustment does need a little help though.
    .
    It must have the preliminary orientation for each photograph in order to get started.
    .
    This preliminary orientation is accomplished with the AutoStart or SuperStart procedures that are done for every photograph.
    SuperStart .
    When STAR is finished it then produces the following:
    :
    1.
    1.
    XYZ coordinates (and accuracy estimates) for each point
    XYZ ( )
    2.
    2.
    The XYZ coordinates and 3 aiming angles (and accuracy estimates) for each picture.
    XYZ 3) ) .
    3.
    3.
    The camera calibration parameters (and their accuracy estimates).
    ( ).
    Measuring Accuracy
    V-STARS in the single camera mode provides accuracies comparable to those achieved by other large volume, high accuracy coordinate measurement systems such as Digital Theodolites, Co-ordinate Measuring Machines (CMMs), and Laser Trackers.
    Theodolites (CMMs) .
    Typical accuracies are 25 to 50 microns (0.001" to 0.002") on a 3-meter (ten foot) object for the INCA2 and 50 to 100 microns (0.002" to 0.004") on a 3-meter (ten foot) object for the E3 system.
    25 50 (0.001 "0.002") 3 ( ) INCA2 50 100 (0.002 "0.004") 3 ( ) E3 .
    However, the accuracy of a photogrammetric measurement can vary significantly since accuracy depends on several inter-related factors.
    .
    The most important are:
    :
    1.
    1.
    The resolution (and quality) of the camera you are using,
    ()
    2.
    2.
    The size of the object you're measuring,
    3.
    3.
    The number of photographs you're taking, and
    4.
    4.
    The geometric layout of the pictures relative to the object and to each other.
    .
    The diagram below illustrates the effects of the four factors and their influence on accuracy.
    .

    The diagram represents a pyramid with the four factors at the base of the pyramid and high accuracy at the top of the pyramid.
    .
    To get higher accuracy ( a higher pyramid) you need more of the items shown on the lines of the pyramid (higher resolution, smaller size, more photos, and wider, but not too wide, geometry).
    ( ) ( ).
    See Appendix A. - "How Accurate is V-STARS?"
    -- " ҿ"
    Scaling Photogrammetry
    Photogrammetric measurements are inherently dimensionless.
    .
    An example of this is shown below.
    .
    The picture of the first car could be a picture of a full-size car or of a matchbox model; there is no way to tell. However, if we know the size of something that is also in the picture, we can now say something about the size of the car.
    .
    (Theodolites are another inherently dimensionless technology).
    ( Theodolites ).


    To scale a photogrammetric measurement, we must have at least one known distance. If we know the actual coordinates beforehand of some targeted points, we can compute the distances between these points and use these to scale the measurement.
    . .
    Another possibility is to use a fixture with targets on it and measure this along with the object.
    .
    The distance between the targets on the bar is known and can be used to scale the measurement. Such fixtures are commonly called scale bars.
    .
    See also Attaching the Scale Bar (s), and the questions in Appendix A regarding scale.
    () .
    Multiple Scale Distances
    Whenever possible, you should use more than one distance to scale the measurement.
    .
    V-STARS combines the individual scale measurements to provide higher scale accuracy.
    .
    More importantly, this allows you to find scale errors.
    .
    This is important because when a single scale distance is used and it is in error, the entire measurement will be incorrectly scaled. On the other hand, if you have multiple scale distances, scale errors can be detected and removed.
    . .
    With two known distances, if one is in error you will be able to detect a scale error but usually you cannot tell which one is in error.
    .
    (Sometimes, though, you can tell by inspecting the scale points).
    ( ).
    With three known scale distances, you can usually detect if one of them is in error and remove it.
    .
    When scale bars are used, one good technique is to use a bar that has more than two targets.
    .
    Another technique is to use more than one scale bar.
    .
    Alternatively, you can use both techniques.
    .
    It is up to you, but, whenever possible, you should use multiple scale distances.
    .
    Long Scale Distances
    The scale distance(s) should be as long as practical because any inaccuracy in the scale distance(s) is magnified by the proportion of the size of the object to the scale distance.
    () () .
    For example, if a one meter (40") scale distance is used on a 10 meter (400") object, and the scale distance has 0.1 mm (0.004") of error, then the object will have ten times this error, or 1mm (0.040").
    (40 ") 10 (400") 0.1 (0.004 ") 1mm (0.040 ").
    In some cases, a measurement may not need to be precisely scaled.
    .
    For example, some surface or shape measurements do not require accurate scale.
    .
    In this case, you can use nominal distances to provide scale or you can use the AutoBar for scale. However, the AutoBar is too small to accurately scale a measurement.
    AutoBar . AutoBar .
    Measuring
    Measuring with V-STARS
    No matter what kind of measurement you are doing, measuring with V-STARS usually consists of the following steps.
    .
    1.
    1.
    Planning the Measurement
    2.
    2.
    Targeting the Object
    3.
    3.
    Taking Pictures
    4.
    4.
    Measuring Pictures
    5.
    5.
    Processing Pictures (to get 3-dimensional coordinates)
    (3) )
    6.
    6.
    Analyzing the Results (manipulating the results to help check and visualize the results)
    ( )
    The list above is a general guide. However, every measurement project is different, and therefore the content and even sometimes the order of the steps given above may be different depending on the project requirements and sometimes operator preference. For example, on some projects, you will take all pictures first (to minimize time on-site typically) and then measure them, while on others you will measure each picture after it is taken.
    ( ) .
    On other projects, you will take and measure some pictures, and then process them to get preliminary results so you can make measuring the remaining pictures easier.
    .
    Still, all the steps listed above are carried out in some fashion on every project.
    .
    Each of these steps is described in detail in the following chapters.
    .

    Planning the Measurement
    Proper planning is essential for making a successful measurement.
    .
    This is especially true if the measurement is complex or if it is the first time, you have done this particular type of measurement.
    .
    To plan properly, you must have information about the measurement.
    .
    Use the following list of questions to help you plan the measurement.
    .
    Questions You Should Ask: Remember "V-STARS"
    : " "
    V isibility - Can the points of interest on the object be seen?
    isibility --
    Remember that V-STARS is a "line of sight" technology based on triangulation.
    " " .
    That means the points must be seen from at least two different locations to be measured.
    .
    For higher accuracy, the points should be seen from very different locations and from more than just two locations.
    .
    (See Measuring Accuracy for more details on how geometry and the number of photos affect accuracy).
    ( ).
    Also, remember that V-STARS never measures the points of interest directly.
    .
    Instead, V-STARS measures retro-reflective targets that are placed on, or in a known relationship to, the points of interest.
    .
    If a point of interest cannot be seen directly, often some form of offset target can be devised to measure the point indirectly.
    .
    S ize and Shape - What is the size and shape of the object?
    ize --
    The size and shape (convex, concave, single-sided, multiple sided, etc.) will determine how complex the measurement will be, how much room you will need around the object, and the level of accuracy you can obtain.
    ( ) .
    The size and shape will also determine what type and size of targets will be used.
    .
    T argeting - Can the points of interest on the object be targeted?
    argeting -- ݿ
    If they cannot, you must use another method (V-STARS in the multiple camera mode using the touch probe for example, See the V-STARS/M manual for details). Targeting the object to obtain the measurements you desire can often be one of the most challenging and time consuming aspects of a project. See Targeting for more information.
    ( / ). . .
    A ccuracy - What level of accuracy is desired or required?
    ccuracy -- ȿ
    Notice the terms desired and required are both used.
    .
    It is important to distinguish what level of accuracy is wanted and what level of accuracy is acceptable.
    .
    Taking more pictures can increase photogrammetric accuracy significantly.
    .
    It is important to realize that this accuracy improvement will reach a diminishing point of return.
    .
    This tradeoff must be considered when deciding how many photographs to take.
    .
    See Measuring Accuracy for more details on the factors (including the number of photographs) that affect accuracy.
    ( ) .
    It is also important to define the level of accuracy in a clear and unambiguous way.
    .
    There are many ways of specifying accuracy.
    .
    For example, is the accuracy specified an absolute range (meaning no value should be outside the entire range) or is it an RMS value (meaning, on average, 67% of the values will be within plus or minus the accuracy specification).
    ( ) ( 67 ٪ ).
    R oom - How much room is there around the object?
    oom --
    This question relates to visibility again.
    .
    The amount of room around the object will determine if the project is even feasible, and if it is, will determine to some extent the number of photographs you take.
    .
    Remember that for the standard medium angle lens provide with V-STARS, the rule of thumb is you will see about as much of the object as your distance back from the object. For example, if you are 3 meters from the object you can see about 3 meters of the object.
    3 3 .
    If there is not enough room to see the entire object in the photograph, you can still measure the object by photographing it in overlapping sections (this technique is called mosaicing) but this makes the measurement more complicated.
    ( mosaicing) .
    S cale - Will scale be used?
    cale -- ޿
    If so, how will it be applied to the object?
    ڿ
    Although this may seem a bit trivial at first, figuring out where to put the scale bar(s) so they do not block targets or are themselves blocked can be one of the more challenging aspects of a measurement.
    () .
    Add in the fact that it is desirable to have the scale bar be about the length of the object you are measuring, and that it must be rigidly attached to the object, and that if scale is important then we recommend using multiple scale distances, and this seemingly trivial task can sometimes be downright daunting.
    .
    See Scaling Photogrammetry, and the questions on scale in Appendix A.
    .
    Defining a Coordinate System
    All coordinate measurement systems must use some working coordinate system.
    .
    V-STARS automatically defines a local coordinate system that uses the coordinate system defined by the first picture you measure. If the AutoBar is used, the V-STARS measurement is in the coordinate system defined by the AutoBar.
    AutoBar AutoBar.
    This is shown in the adjacent image.
    .
    If an AutoBar is not used, V-STARS uses the coordinates system on the driver file you have selected.
    AutoBar .
    In either case, all the images and measurements are then defined in the local coordinate system.
    .
    Typically, this local system is not usually the final coordinate system desired by the user.
    .
    Your plan must include a way to define the user coordinate system you desire.
    .
    In some cases, the local coordinate system defined by V-STARS is sufficient.
    .
    In other cases, you must transform this local system into the desired user coordinate system.
    .
    This transformation can be automatically done for you by V-STARS using design data, or you can use the WINTRANS program provided with V-STARS.
    WINTRANS .
    Often, the user coordinate system is defined by a subset of the measured points that have coordinates in the user's desired coordinate system.
    .
    These points may consist of precisely made tooling targets that are located in bushed holes, or they may be defined by features on the measured object (such as part edges, or hole locations or intersections of lines, planes, etc.) that are targeted in some way.
    ( ) .
    In any case, it is important that the points representing the user-defined coordinate system be targeted precisely or else the accuracy of the transformation will be degraded.
    .
    In fact, the accuracy of placing the targets precisely on the user coordinate system's defining features often is the determining factor in overall measurement accuracy.
    .
    Fortunately, many different Target Types are available to help with this.
    .
    See the WINTRANS manual for details on performing coordinate transformations.
    WINTRANS .
    Coordinate systems are also called axis systems since the coordinate system is often defined by aligning certain points to the coordinate axes.
    .
    In this document, coordinate system and axis system are used interchangeably and mean the same thing.
    .
    Measurements
    Types of Measurements
    Photogrammetry is a versatile, powerful, and flexible measuring technology. Measurements have been done on land, sea (and undersea), and air, and even in outer space on objects smaller than a football to larger than a football field.
    . () .
    Photogrammetry is widely used in the aerospace, antenna, shipbuilding, construction, and automotive industries for a wide variety of measurement tasks.
    .
    Although every photogrammetric project is somewhat different, we have separated them into broad categories to help describe general approaches for performing a successful measurement.
    .
    Measurements can be classified as initial or repeat, and as completely overlapping or partially overlapping.
    .
    The two categories are not mutually exclusive; initial measurements can be completely overlapping or partially overlapping, and so can repeat measurements.
    .
    In general, a completely overlapping, repeat measurement is the easiest type of measurement while an initial, partially overlapping measurement is the most difficult.
    .
    Initial and Repeat Measurements
    A repeat measurement is one in which approximate coordinates are available for all (or nearly all) of the target points, while for an initial measurement there are no approximate coordinates available.
    ( ) .
    In general, these coordinates are available from an earlier measurement of the object (hence the name repeat measurement), but they can also be from a set of design coordinates. All that matters is that they are accurate enough to allow the software to correctly measure all the targets on each photograph.
    ( ) .
    Then, after each photograph is oriented (using the AutoStart or SuperStart procedure), we can use a process called driveback to quickly and automatically find and measure all the visible points.
    ( SuperStart) driveback .
    To use driveback, the coordinates should be much more accurate than the closest target spacing.
    driveback .
    Therefore, if targets are 100mm (4 inches) apart, the coordinates' accuracy should be much better than this, say better than 25 mm (1 inch).
    100mm (4 ) ' 25 (1 ).
    The better the approximations are, the faster and easier the measurement will be.
    .
    If no approximate coordinates are available, you can use the AutoBar provided with the system to do an initial measurement.
    AutoBar .
    With the AutoMeasure command, initial measurements are now nearly as fast and easy as repeat measurements.
    AutoMeasure .
    Completely or Partially Overlapping Measurements
    A completely overlapping measurement is one in which the entire object is seen in every photograph, while in a partially overlapping measurement, the object must be photographed in sections (because of space limitations or accuracy requirements or because of the complexity of the object). Partially overlapping measurements must have sufficient common coverage to hold (or "tie") the entire measurement together as a unified whole.
    ( ). ( "") .
    The need for common coverage among photographs is described with the help of the figures below.
    .

    In the first figure, we have two completely independent measurements of two flat panels.
    .
    Each of the panels is very accurately measured, but since there is no common coverage, we can say nothing about the relationship between the two panels.
    .
    For example, we cannot even say how far the panels are from each other or how they are oriented to each other.
    .
    If we now measure the panels with enough partial overlap that a line of points is seen in common between the two panels, we have now connected the two panels together but not with sufficient overlap to completely determine the relationship between the two panels.
    .

    The common line only acts like a hinge, the two panels are connected there but they could be at any angle to each other that this "hinge" connection allows.
    "" .
    Therefore, the overlap must be more than just a line of points; it must be at least two-dimensional.
    .
    If we now add a third point in common between the two measurements that is away from the line (so the three points form a triangle), the "hinge" is now locked in place and the relationship between the two panels is established.
    ( ) "" .
    Therefore, as a minimum, there must be three points, forming a triangle that is seen in common between the two sets of photography.
    .
    Of course, by adding more points and more overlap the tie between the two panels is more strongly established.
    . The strongest tie between the two panels is
    '1' '1' '2'
    1 2

    /




    1. arssa2@yahoo.com

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      arssa2
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    photographic sets ǡ :

    35

    35 24 36 ( 2436). ɡ 1/500 1/1000 ( 1/12000 ). . .

    viewfinders . . 35 single lens reflex. .

    . ǡ auto exposure auto focus .

    fish eye lenses ultra-wide angle lenses zoom tele photo lenses 1000 2000 macro micro UV photography.

    medium format

    120 220 : 4.56 66 67 69.

    . .

    :

    SLR ǡ 45 pentaprism ȡ focusin screen .

    twin lens reflex ɡ ޡ ֡ parallax .

    viewfinder: .

    Large Format

    ( ɡ ) 1012.5 12.517.5 17.525. ɡ wide angle lenses depth of field shift tilt swing raise fall.

    .



    still pictures ɡ :

    : ɡ 811 ɡ . .

    : 120 220 612 617 .

    : . editing .

    : ɡ flash . .

    : ǡ () ӡ .

    : 35 ɡ endoscopes.



    '1' '1' '2'
    1 2

    /




    1. arssa2@yahoo.com

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    .
    20 150 .

    . .
    :
    1-
    2-
    :
    .
    5-95.
    .
    :
    .
    .
    :
    -
    -
    - .

    / :
    1) .
    2) .
    3) .
    4) .
    5) .

    __________________
    '1' '1' '2'
    1 2

    /




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      Mohamed Hashem
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    ..
    ..
    ..

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    ..

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    40%
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      abdelgaid
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