Abstract:
A computerized system for displaying and making measurements based upon captured oblique images. The system includes an image-capturing device capturing oblique aerial images at image-capturing events and issuing image-data signals corresponding to captured images; at least one geo-locating device issuing a corresponding at least one geo-locating signal indicative at least in part of a geo-location of said image-capturing device during each image capturing event; a computer system receiving and storing said image-data signals and said at least one geo-locating signal; and the computer system executing image and data acquiring software reading the image-data signals and the at least one geo-locating signal, and associating each of the image-data signal with a corresponding at least one geo-locating signal for each image-capturing event.

Description:
INCORPORATION BY REFERENCE 
       [0001]    This application is a divisional of U.S. Ser. No. 13/534,907, filed Jun. 27, 2012, which is a continuation of U.S. Ser. No. 13/217,885, filed Aug. 25, 2011, which issued as U.S. Pat. No. 8,233,666, which is a continuation of U.S. Ser. No. 12/950,643, Filed Nov. 19, 2010, which issued as U.S. Pat. No. 8,068,643, which is a continuation of U.S. Ser. No. 12/853,616, Filed Aug. 10, 2010, which issued as U.S. Pat. No. 7,995,799, which is a continuation of U.S. Ser. No. 12/186,889, filed Aug. 6, 2008, which issued as U.S. Pat. No. 7,787,659, on Aug. 31, 2010, which is a continuation of U.S. Ser. No. 10/701,839, filed on Nov. 5, 2003, which issued as U.S. Pat. No. 7,424,133, on Sep. 9, 2008, which claims priority to the provisional patent application identified by U.S. Ser. No. 60/425,275, filed Nov. 8, 2002, of which the entire content of each is hereby expressly incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to photogrammetry. More particularly, the present invention relates to a method and apparatus for capturing oblique images and for measuring the objects and distances between the objects depicted therein. 
       BACKGROUND 
       [0003]    Photogrammetry is the science of making measurements of and between objects depicted within photographs, especially aerial photographs. Generally, photogrammetry involves taking images of terrestrial features and deriving data therefrom, such as, for example, data indicating relative distances between and sizes of objects within the images. Photogrammetry may also involve coupling the photographs with other data, such as data representative of latitude and longitude. In effect, the image is overlaid and conformed to a particular spatial coordinate system. 
         [0004]    Conventional photogrammetry involves the capture and/or acquisition of orthogonal images. The image-capturing device, such as a camera or sensor, is carried by a vehicle or platform, such as an airplane or satellite, and is aimed at a nadir point that is directly below and/or vertically downward from that platform. The point or pixel in the image that corresponds to the nadir point is the only point/pixel that is truly orthogonal to the image-capturing device. All other points or pixels in the image are actually oblique relative to the image-capturing device. As the points or pixels become increasingly distant from the nadir point they become increasingly oblique relative to the image-capturing device and the ground sample distance (i.e., the surface area corresponding to or covered by each pixel) also increases. Such obliqueness in an orthogonal image causes features in the image to be distorted, especially images relatively distant from the nadir point. 
         [0005]    Such distortion is removed, or compensated for, by the process of ortho-rectification which, in essence, removes the obliqueness from the orthogonal image by fitting or warping each pixel of an orthogonal image onto an orthometric grid or coordinate system. The process of ortho-rectification creates an image wherein all pixels have the same ground sample distance and are oriented to the north. Thus, any point on an ortho-rectified image can be located using an X, Y coordinate system and, so long as the image scale is known, the length and width of terrestrial features as well as the relative distance between those features can be calculated. 
         [0006]    Although the process of ortho-rectification compensates to a degree for oblique distortions in an orthogonal image, it introduces other undesirable distortions and/or inaccuracies in the ortho-rectified orthogonal image. Objects depicted in ortho-rectified orthogonal images may be difficult to recognize and/or identify since most observers are not accustomed to viewing objects, particularly terrestrial features, from above. To an untrained observer an ortho-rectified image has a number of distortions. Roads that are actually straight appear curved and buildings may appear to tilt. Further, ortho-rectified images contain substantially no information as to the height of terrestrial features. The interpretation and analysis of orthogonal and/or ortho-rectified orthogonal images is typically performed by highly-trained analysts whom have undergone years of specialized training and experience in order to identify objects and terrestrial features in such images. 
         [0007]    Thus, although orthogonal and ortho-rectified images are useful in photogrammetry, they lack information as to the height of features depicted therein and require highly-trained analysts to interpret detail from what the images depict. 
         [0008]    Oblique images are images that are captured with the image-capturing device aimed or pointed generally to the side of and downward from the platform that carries the image-capturing device. Oblique images, unlike orthogonal images, display the sides of terrestrial features, such as houses, buildings and/or mountains, as well as the tops thereof. Thus, viewing an oblique image is more natural and intuitive than viewing an orthogonal or ortho-rectified image, and even casual observers are able to recognize and interpret terrestrial features and other objects depicted in oblique images. Each pixel in the foreground of an oblique image corresponds to a relatively small area of the surface or object depicted (Le., each foreground pixel has a relatively small ground sample distance) whereas each pixel in the background corresponds to a relatively large area of the surface or object depicted (i.e., each background pixel has a relatively large ground sample distance). Oblique images capture a generally trapezoidal area or view of the subject surface or object, with the foreground of the trapezoid having a substantially smaller ground sample distance (i.e., a higher resolution) than the background of the trapezoid. 
         [0009]    Oblique images are considered to be of little or no use in photogrammetry. The conventional approach of forcing the variously-sized foreground and background pixels of an oblique image into a uniform size to thereby warp the image onto a coordinate system dramatically distorts the oblique image and thereby renders identification of objects and the taking of measurements of objects depicted therein a laborious and inaccurate task. Correcting for terrain displacement within an oblique image by using an elevation model further distorts the images thereby increasing the difficulty with which measurements can be made and reducing the accuracy of any such measurements. 
         [0010]    Thus, although oblique images are considered as being of little or no use in photogrammetry, they are easily interpreted and contain information as to the height of features depicted therein. Therefore, what is needed in the art is a method and apparatus for photogrammetry that enable geo-location and accurate measurements within oblique images. 
         [0011]    Moreover, what is needed in the art is a method and apparatus for photogrammetry that enable the measurement of heights and relative heights of objects within an image. Furthermore, what is needed in the art is a method and apparatus for photogrammetry that utilizes more intuitive and natural images. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a method and apparatus for capturing, displaying, and making measurements of objects and distances between objects depicted within oblique images. The present invention comprises, in one form thereof, a computerized system for displaying, geolocating, and taking measurements from captured oblique images. The system includes a data file accessible by the computer system. The data file includes a plurality of image files corresponding to a plurality of captured oblique images, and positional data corresponding to the images. Image display and analysis software is executed by the system for reading the data file and displaying at least a portion of the captured oblique images. The software retrieves the positional data for one or more user-selected points on the displayed image, and calculates a separation distance between any two or more selected points. The separation distance calculation is user-selectable to determine various parameters including linear distance between, area encompassed within, relative elevation of, and height difference between selected points. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become apparent and be more completely understood by reference to the following description of one embodiment of the invention when read in conjunction with the accompanying drawings, wherein: 
           [0014]      FIG. 1  illustrates one embodiment of a platform or vehicle carrying an image-capturing system of the present invention, and shows exemplary orthogonal and oblique images taken thereby; 
           [0015]      FIG. 2  is a diagrammatic view of the image-capturing system of  FIG. 1 ; 
           [0016]      FIG. 3  is a block diagram of the image-capturing computer system of  FIG. 2 ; 
           [0017]      FIG. 4  is a representation of an exemplary output data file of the image-capturing system of  FIG. 1 ; 
           [0018]      FIG. 5  is a block diagram of one embodiment of an image display and measurement computer system of the present invention for displaying and taking measurements of and between objects depicted in the images captured by the image-capturing system of  FIG. 1 ; 
           [0019]      FIG. 6  depicts an exemplary image displayed on the system of  FIG. 5 , and illustrates one embodiment of the method of the present invention for the measurement of and between objects depicted in such an image; 
           [0020]      FIGS. 7 and 8  illustrate one embodiment of a method for capturing oblique images of the present invention; 
           [0021]      FIGS. 9 and 10  illustrate a second embodiment of a method for capturing oblique images of the present invention. 
       
    
    
       [0022]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Referring now to the drawings, and particularly to  FIG. 1 , one embodiment of an apparatus for capturing and geolocating oblique images of the present invention is shown. Apparatus  10  includes a platform or vehicle  20  that carries image-capturing and geolocating system  30 . 
         [0024]    Platform  20 , such as, for example, an airplane, space shuttle, rocket, satellite, or any other suitable vehicle, carries image-capturing system  30  over a predefined area of and at one or more predetermined altitudes above surface  31 , such as, for example, the earth&#39;s surface or any other surface of interest. As such, platform  20  is capable of controlled movement or flight, either manned or unmanned, along a predefined flight path or course through, for example, the earth&#39;s atmosphere or outer space. Image capturing platform  20  includes a system for generating and regulating power (not shown) that includes, for example, one or more generators, fuel cells, solar panels, and/or batteries, for powering image-capturing system  30 . 
         [0025]    Image-capturing and geo-locating system  30 , as best shown in  FIG. 2 , includes image capturing devices  32   a  and  32   b , a global positioning system (GPS) receiver  34 , an inertial navigation unit (INU)  36 , clock  38 , gyroscope  40 , compass  42  and altimeter  44 , each of which are interconnected with image-capturing computer system  46 . 
         [0026]    Image-capturing devices  32   a  and  32   b , such as, for example, conventional cameras, digital cameras, digital sensors, charge-coupled devices, or other suitable image-capturing devices, are capable of capturing images photographically or electronically. Image-capturing devices  32   a  and  32   b  have known or determinable characteristics including focal length, sensor size and aspect ratio, radial and other distortion terms, principal point offset, pixel pitch, and alignment. Image-capturing devices  32   a  and  32   b  acquire images and issue image data signals (IDS)  48   a  and  48   b , respectively, corresponding to the particular images or photographs taken and which are stored in image-capturing computer system  46 , as will be more particularly described hereinafter. 
         [0027]    As best shown in  FIG. 1 , image-capturing devices  32   a  and  32   b  have respective central axes A 1  and A 2 , and are mounted to platform  20  such that axes A 1  and A 2  are each at an angle of declination Ø relative to a horizontal plane P. Declination angle Ø is virtually any oblique angle, but is preferably from approximately 20° (twenty degrees) to approximately 60° (sixty degrees) and is most preferably from approximately 40° (forty degrees) to approximately 50° (fifty degrees). 
         [0028]    GPS receiver  34  receives global positioning system signals  52  that are transmitted by one or more global positioning system satellites  54 . The GPS signals  52 , in known fashion, enable the precise location of platform  20  relative to surface  31  to be determined. GPS receiver  34  decodes GPS signals  52  and issues location signals/data  56 , that are dependent at least in part upon GPS signals  52  and which are indicative of the precise location of platform  20  relative to surface  31 . Location signals/data  56  corresponding to each image captured by image-capturing devices  32   a  and  32   b  are received and stored by image-capturing computer system  46 . 
         [0029]    INU  36  is a conventional inertial navigation unit that is coupled to and detects changes in the velocity, including translational and rotational velocity, of image-capturing devices  32   a  and  32   b  and/or platform  20 . INU  36  issues velocity signals/data  58  indicative of such velocities and/or changes therein to image-capturing computer system  46 , which stores velocity signals/data  58  corresponding to each image captured by image-capturing devices  32   a  and  32   b  are received and stored by image-capturing computer system  46 . 
         [0030]    Clock  38  keeps a precise time measurement (time of validity) that is used to synchronize events within image-capturing and geo-locating system  30 . Clock  38  provides time data/clock signal  62  that is indicative of the precise time that an image is taken by image-capturing devices  32   a  and  32   b . Time data  62  is also provided to and stored by image-capturing computer system  46 . Alternatively, clock  38  is integral with image-capturing computer system  46 , such as, for example, a clock software program. 
         [0031]    Gyroscope  40  is a conventional gyroscope as commonly found on airplanes and/or within commercial navigation systems for airplanes. Gyroscope  40  provides signals including pitch signal  64 , roll signal  66  and yaw signal  68 , which are respectively indicative of pitch, roll and yaw of platform  20 . Pitch signal  64 , roll signal  66  and yaw signal  68  corresponding to each image captured by mage-capturing devices  32   a  and  32   b  are received and stored by image-capturing computer system  46 . 
         [0032]    Compass  42 , such as, for example, a conventional electronic compass, indicates the heading of platform  20 . Compass  42  issues heading signal/data  72  that is indicative of the heading of platform  20 . Image-capturing computer system  46  receives and stores the heading signals/data  72  that correspond to each image captured by image-capturing devices  32   a  and  32   b.    
         [0033]    Altimeter  44  indicates the altitude of platform  20 . Altimeter  44  issues altitude signal/data  74 , and image-capturing computer system  46  receives and stores the altitude signal/data  74  that correspond to each image captured by image-capturing devices  32   a  and  32   b.    
         [0034]    As best shown in  FIG. 3 , image-capturing computer system  46 , such as, for example, a conventional laptop personal computer, includes memory  82 , input devices  84   a  and  84   b , display device  86 , and input and output (I/O) ports  88 . Image-capturing computer system  46  executes image and data acquiring software  90 , which is stored in memory  82 . Memory  82  also stores data used and/or calculated by image-capturing computer system  46  during the operation thereof, and includes, for example, non-volatile read-only memory, random access memory, hard disk memory, removable memory cards and/or other suitable memory storage devices and/or media. Input devices  84   a  and  84   b , such as, for example, a mouse, keyboard, joystick, or other such input devices, enable the input of data and interaction of a user with software being executed by image-capturing computer system  46 . Display device  86 , such as, for example, a liquid crystal display or cathode ray tube, displays information to the user of image-capturing computer system  46 . I/O ports  88 , such as, for example, serial and parallel data input and output ports, enable the input and/or output of data to and from image-capturing computer system  46 . 
         [0035]    Each of the above-described data signals is connected to image-capturing computer system  46 . More particularly, image data signals  48 , location signals  56 , velocity signals  58 , time data signal  62 , pitch, roll and yaw signals  64 ,  66  and  68 , respectively, heading signal  72  and altitude signal  74  are received via I/O ports  88  by and stored within memory  82  of image-capturing computer system  46 . 
         [0036]    In use, image-capturing computer system  46  executes image and data acquiring software  90 , which, in general, controls the reading, manipulation, and storing of the above-described data signals. More particularly, image and data acquiring software  90  reads image data signals  48   a  and  48   b  and stores them within memory  82 . Each of the location signals  56 , velocity signals  58 , time data signal  62 , pitch, roll and yaw signals  64 ,  66  and  68 , respectively, heading signal  72  and altitude signal  74  that represent the conditions existing at the instant an image is acquired or captured by image-capturing devices  32   a  and  32   b  and which correspond to the particular image data signals  48   a  and  48   b  representing the captured images are received by image-capturing computer system  46  via I/O ports  88 . Image-capturing computer system  46  executing image and data acquiring software  90  issues image-capture signal  92  to image-capturing devices  32   a  and  32   b  to thereby cause those devices to acquire or capture an image at predetermined locations and/or at predetermined intervals which are dependent at least in part upon the velocity of platform  20 . 
         [0037]    Image and data acquiring software  90  decodes as necessary and stores the aforementioned signals within memory  82 , and associates the data signals with the corresponding image signals  48   a  and  48   b . Thus, the altitude, orientation in terms of roll, pitch, and yaw, and the location of image-capturing devices  32   a  and  32   b  relative to surface  31 , i.e., longitude and latitude, for every image captured by image-capturing devices  32   a  and  32   b  is known. 
         [0038]    Platform  20  is piloted or otherwise guided through an image-capturing path that passes over a particular area of surface  31 , such as, for-example, a predefined area of the surface of the earth or of another planet. Preferably, the image-capturing path of platform  20  is at right angles to at least one of the boundaries of the area of interest. The number of times platform  20  and/or image-capturing devices  32   a ,  32   b  pass over the area of interest is dependent at least in part upon the size of the area and the amount of detail desired in the captured images. The particular details of the image-capturing path of platform  20  are described more particularly hereinafter. 
         [0039]    As platform  20  passes over the area of interest a number of oblique images are captured by image-capturing devices  32   a  and  32   b . As will be understood by those of ordinary skill in the art, images are captured or acquired by image-capturing devices  32   a  and  32   b  at predetermined image capture intervals which are dependent at least in part upon the velocity of platform  20 . 
         [0040]    Image data signals  48   a  and  48   b  corresponding to each image acquired are received by and stored within memory  82  of image-capturing computer system  46  via I/O ports  88 . Similarly, the data signals (i.e., image data signals  48 , location signals  56 , velocity signals  58 , time data signal  62 , pitch, roll and yaw signals  64 ,  66  and  68 , respectively, heading signal  72  and altitude signal  74 ) corresponding to each captured image are received and stored within memory  82  of image-capturing computer system  46  via I/O ports  88 . Thus, the location of image-capturing device  32   a  and  32   b  relative to surface  32  at the precise moment each image is captured is recorded within memory  82  and associated with the corresponding captured image. 
         [0041]    As best shown in  FIG. 1 , the location of image-capturing devices  32   a  and  32   b  relative to the earth corresponds to the nadir point N of orthogonal image  102 . Thus, the exact geo-location of the nadir point N of orthogonal image  102  is indicated by location signals  56 , velocity signals  58 , time data signal  62 , pitch, roll and yaw signals  64 , 66  and  68 , respectively, heading signal  72  and altitude signal  74 . Once the nadir point N of orthogonal image  102  is known, the geo-location of any other pixel or point within image  102  is determinable in known manner. 
         [0042]    When image-capturing devices  32   a  and  32   b  are capturing oblique images, such as oblique images  104   a  and  104   b  ( FIG. 1 ), the location of image-capturing devices  32   a  and  32   b  relative to surface  31  is similarly indicated by location signals  56 , velocity signals  58 , time data signal  62 , pitch, roll and yaw signals  64 , 66  and  68 , respectively, heading signal  72 , altitude signal  74  and the known angle of declination 0 of the primary axes A 1  and A 2  of image-capturing devices  32   a  and  32   b , respectively. 
         [0043]    It should be particularly noted that a calibration process enables image and data acquiring software  90  to incorporate correction factors and/or correct for any error inherent in or due to image-capturing device  32 , such as, for example, error due to calibrated focal length, sensor size, radial distortion, principal point offset, and alignment. 
         [0044]    Image and data acquiring software  90  creates and stores in memory  82  one or more output image and data files  120 . More particularly, image and data acquiring software  90  converts image data signals  48   a ,  48   b  and the orientation data signals (i.e., image data signals  48 , location signals  56 , velocity signals  58 , time data signal  62 , pitch, roll and yaw signals  64 ,  66  and  68 , respectively, heading signal  72  and altitude signal  74 ) into computer-readable output image and data files  120 . As best shown in  FIG. 4 , output image and data file  120  contains a plurality of captured image files I 1 , I 2 , . . . , I n  corresponding to captured oblique images, and the positional data C PD1 , C PD2 , . . . , C PDn  corresponding thereto. 
         [0045]    Image files I 1 , I 2 , . . . , I n  of the image and data file  120  are stored in virtually any computer-readable image or graphics file format, such as, for example, JPEG, TIFF, GIF, BMP, or PDF file formats, and are cross-referenced with the positional data C PD1 , C PD2 , . . . , C PDn  which is also stored as computer-readable data. Alternatively, positional data C PD1 , C PD2 , . . . , C PDn  is embedded within the corresponding image files I 1 , I 2 , . . . , I n  in known manner. Image data files  120  are then processed, either by image and data acquiring software  90  or by post-processing, to correct for errors, such as, for example, errors due to flight path deviations and other errors known to one of ordinary skill in the art. Thereafter, image data files  120  are ready for use to display and make measurements of and between the objects depicted within the captured images, including measurements of the heights of such objects. 
         [0046]    Referring now to  FIG. 5 , image display and measurement computer system  130 , such as, for example, a conventional desktop personal computer or a mobile computer terminal in a police car, includes memory  132 , input devices  134   a  and  134   b , display device  136 , and network connection  138 . Image-capturing computer system  130  executes image display and analysis software  140 , which is stored in memory  132 . Memory  132  includes, for example, non-volatile read-only memory, random access memory, hard disk memory, removable memory cards and/or other suitable memory storage devices and/or media. Input devices  134   a  and  134   b , such as, for example, a mouse, keyboard, joystick, or other such input devices, enable the input of data and interaction of a user with image display and analysis software  140  being executed by image display and measurement computer system  130 . Display device  136 , such as, for example, a liquid crystal display or cathode ray tube, displays information to the user of image display and measurement computer system  130 . Network connection  138  connects image display and measurement computer system  130  to a network (not shown), such as, for example, a local-area network, wide-area network, the Internet and/or the World Wide Web. 
         [0047]    In use, and referring now to  FIG. 6 , image display and measurement computer system  130  executing image display and analysis software  140  accesses one or more 10 output image and data files  120  that have been read into memory  132 , such as, for example, via network connection  138 , a floppy disk drive, removable memory card or other suitable means. One or more of the captured images I 1 , I n , . . . , I n  of output image and data files  120  is thereafter displayed as displayed oblique image  142  under the control of image display and analysis software  140 . At approximately the same time, one or more data portions C PD1 , C PD2 , . . . , C PDn  corresponding to displayed oblique image  142  are read into a readily-accessible portion of memory  132 . 
         [0048]    It should be particularly noted that displayed oblique image  142  is displayed substantially as captured, i.e., displayed image  142  is not warped or fitted to any coordinate system nor is displayed image  142  ortho-rectified. Rather than warping displayed image  142  to a coordinate system in order to enable measurement of objects depicted therein, image display and analysis software  140 , in general, determines the geo-locations of selected pixels only as needed, or “on the fly”, by referencing data portions C PD1 , C PD2 , . . . , C PDn  of output image and data files  120  and calculating the position and/or geo-location of those selected pixels using one or more projection equations as is more particularly described hereinafter. 
         [0049]    Generally, a user of display and measurement computer system  130  takes measurements of and between objects depicted in displayed oblique image  142  by selecting one of several available measuring modes provided within image display and analysis software  140 . The user selects the desired measurement mode by accessing, for example, a series of pull-down menus or toolbars M, or via keyboard commands. The measuring modes provided by image display and analysis software  140  include, for example, a distance mode that enables measurement of the distance between two or more selected points, an area mode that enables measurement of the area encompassed by several selected and interconnected points, a height mode that enables measurement of the height between two or more selected points, and an elevation mode that enables the measurement of the change in elevation of one selected point relative to one or more other selected points. 
         [0050]    After selecting the desired measurement mode, the user of image display and analysis software  140  selects with one of input devices  134   a ,  134   b  a starting point or starting pixel  152  and an ending point or pixel  154  on displayed image  142 , and image display and analysis software  140  automatically calculates and displays the quantity sought, such as, for example, the distance between starting pixel  152  and ending pixel  154 . 
         [0051]    When the user selects starting point/pixel  152 , the geo-location of the point corresponding thereto on surface  31  is calculated by image display and analysis software  140  which executes one or more projection equations using the data portions C PD1 , C PD2 , . . . , C PDn  of output image and data files  120  that correspond to the particular image being displayed. The longitude and latitude of the point on surface  31  corresponding to pixel  152  are then displayed by image display and analysis software  140  on display  136 , such as, for example, by superimposing the longitude and latitude on displayed image  142  adjacent the selected point/pixel or in pop-up display box elsewhere on display  136 . The same process is repeated by the user for the selection of the end pixel/point  154 , and by image display and analysis software  140  for the retrieval and display of the longitude and latitude information. 
         [0052]    The calculation of the distance between starting and ending points/pixels  152 ,  154 , respectively, is accomplished by determining the geo-location of each selected pixel  152 ,  154  “on the fly”. The data portions C PD1 , C PD2 , . . . , C PDn  of output image and data file  120  corresponding to the displayed image are retrieved, and the geo-location of the point on surface  31  corresponding to each selected pixel are then determined. The difference between the geo-locations corresponding to the selected pixels determines the distance between the pixels. 
         [0053]    As an example of how the geo-location of a given point or pixel within displayed oblique image  142  is determined, we will assume that displayed image  142  corresponds to orthogonal image  104   a  ( FIG. 1 ). The user of image display and analysis software  140  selects pixel  154  which, for simplicity, corresponds to center C ( FIG. 1 ) of oblique image  104   a . As shown in  FIG. 1 , line  106  extends along horizontal plane G from a point  108  thereon that is directly below image-capturing device  32   a  to the center C of the near border or edge  108  of oblique image  104   a . An extension of primary axis A 1  intersects with center C. Angle Ø is the angle formed between line  106  the extension of primary axis A 1 . Thus, a triangle (not referenced) is formed having vertices at image-capturing device  32   a , point  108  and center C, and having sides  106 , the extension of primary axis A 1  and vertical (dashed) line  110  between point  108  and image-capturing device  32   a.    
         [0054]    Ground plane G is a substantially horizontal, flat or non-sloping ground plane (and which typically will have an elevation that reflects the average elevation of the terrain), and therefore the above-described triangle includes a right angle between side/line  110  and sideline  106 . Since angle Ø and the altitude of image-capturing device  32  (i.e., the length of side  110 ) are known, the hypotenuse (i.e., the length of the extension of primary axis A 1 ) and remaining other side of the right triangle are calculated by simple geometry. Further, since the exact position of image-capturing device  32   a  is known at the time the image corresponding to displayed image  142  was captured, the latitude and longitude of point  108  are also known. Knowing the length of side  106 , calculated as described above, enables the exact geo-location of pixel  154  corresponding to center C of oblique image  104   a  to be determined by image display and analysis software  140 . Once the geo-location of the point corresponding to pixel  154  is known, the geo-location of any other pixel in displayed oblique image  142  is determinable using the known camera characteristics, such as, for example, focal length, sensor size and aspect ratio, radial and other distortion terms, etc. The distance between the two or more points corresponding to two or more selected pixels within displayed image  142  is calculated by image display and analysis software  140  by determining the difference between the geo-locations of the selected pixels using known algorithms, such as, for example, the Gauss formula and/or the vanishing point formula, dependent upon the selected measuring mode. The measurement of objects depicted or appearing in displayed image  142  is conducted by a substantially similar procedure to the procedure described above for measuring distances between selected pixels. For example, the lengths, widths and heights of objects, such as, for example, buildings, rivers, roads, and virtually any other geographic or man-made structure, appearing within displayed image  142  are measured by selecting the appropriate/desired measurement mode and selecting starting and ending pixels. 
         [0055]    It should be particularly noted that in the distance measuring mode of image display and analysis software  140  the distance between the starting and ending points/pixels  152 ,  154 , respectively, is determinable along virtually any path, such as, for example, a “straight-line” path P1 or a path P2 that involves the selection of intermediate points/pixels and one or more “straight-line” segments interconnected therewith. 
         [0056]    It should also be particularly noted that the distance measuring mode of image display and analysis software  140  determines the distance between selected pixels according to a “walk the earth” method. The “walk the earth method” creates a series of interconnected line segments, represented collectively by paths P1 and P2, that extend between the selected pixels/points and which lie upon or conform to the planar faces of a series of interconnected facets that define a tessellated ground plane. The tessellated ground plane, as will be more particularly described hereinafter, closely follows or recreates the terrain of surface  31 , and therefore paths P1 and P2 also closely follow the terrain of surface  31 . By measuring the distance along the terrain simulated by the tessellated ground plane, the “walk the earth” method provides for a more accurate and useful measurement of the distance between selected points than the conventional approach, which warps the image onto a flat earth or average elevation plane system and measures the distance between selected points along the flat earth or plane and substantially ignores variations in terrain between the points. 
         [0057]    For example, a contractor preparing to bid on a contract for paving a roadway over uneven or hilly terrain can determine the approximate amount or area of roadway involved using image display and analysis software  140  and the “walk the earth” measurement method provided thereby. The contractor can obtain the approximate amount or area of roadway from his or her own office without having to send a surveying crew to the site to obtain the measurements necessary. 
         [0058]    In contrast to the “walk the earth” method provided by the present invention, the “flat earth” or average elevation distance calculating approaches include inherent inaccuracies when measuring distances between points and/or objects disposed on uneven terrain and when measuring the sizes and/or heights of objects similarly disposed. Even a modest slope or grade in the surface being captured results in a difference in the elevation of the nadir point relative to virtually any other point of interest thereon. Thus, referring again to  FIG. 1 , the triangle formed by line  106 , the extension of primary axis A 1  and the vertical (dashed) line  110  between point  108  and image-capturing device  32   a  may not be a right triangle. If such is the case, any geometric calculations assuming that triangle to be a right triangle would contain errors, and such calculations would be reduced to approximations due to even a relatively slight gradient or slope between the points of interest. 
         [0059]    For example, if surface  31  slopes upward between nadir point N and center C at the near or bottom edge  108  of oblique image  104  then second line  110  intersects surface  31  before the point at which such intersection would occur on a level or non-sloping surface  31 . If center C is fifteen feet higher than nadir point N and with a declination angle Ø equal to 40° (forty degrees), the calculated location of center C would be off by approximately 17.8 feet without correction for the change in elevation between the points. 
         [0060]    As generally discussed above, in order to compensate at least in part for changes in elevation and the resultant inaccuracies in the measurement of and between objects within image  142 , image display and analysis software  140  references, as necessary, points within displayed image  142  and on surface  31  to a pre-calculated tessellated or faceted ground plane generally designated  160  in  FIG. 6 . Tessellated ground plane  160  includes a plurality of individual facets  162   a ,  162   b ,  162   c , etc., each of which are interconnected to each other and are defined by four vertices (not referenced, but shown as points) having respective elevations. Adjacent pairs of facets  162   a ,  162   b ,  162   c , etc., share two vertices. Each facet  162   a ,  162   b ,  162   c , etc., has a respective pitch and slope. Tessellated ground plane  160  is created based upon various data and resources, such as, for example, topographical maps, and/or digital raster graphics, survey data, and various other sources. 
         [0061]    Generally, the geo-location of a point of interest on displayed image  142  is calculated by determining which of facets  162   a ,  162   b ,  162   c , etc., correspond to that point of interest. Thus, the location of the point of interest is calculated based on the characteristics, i.e., elevation, pitch and slope, of facets  162   a ,  162   b ,  162   c , etc., rather than based upon a flat or average-elevation ground plane. Error is introduced only in so far as the topography of surface  31  and the location of the point of interest thereon deviate from the planar surface of the facet  162   a ,  162   b ,  162   c , etc, within which the point of interest lies. That error is reducible through a bilinear interpolation of the elevation of the point of interest within a particular one of facets  162   a ,  162   b ,  162   c , etc., and using that interpolated elevation in the location calculation performed by image display and analysis software  140 . 
         [0062]    To use tessellated ground plane  160 , image display and analysis software  140  employs a modified ray-tracing algorithm to find the intersection of the ray projected from the image-capturing device  32   a  or  32   b  towards surface  31  and tessellated ground plane  160 . The algorithm determines not only which of facets  162   a ,  162   b ,  162   c , etc., is intersected by the ray, but also where within the facet the intersection occurs. By use of bi-linear interpolation, a fairly precise ground location can be determined. For the reverse projection, tessellated ground plane  160  is used to find the ground elevation value for the input ground location also using bi-linear interpolation. The elevation and location are then used to project backwards through a model of the image-capturing device  32   a  or  32   b  to determine which of the pixels within displayed image  142  corresponds to the given location. 
         [0063]    More particularly, and as an example, image display and analysis software  140  performs and/or calculates the geo-location of point  164  by superimposing and/or fitting tessellated ground plane  160  to at least a portion  166 , such as, for example, a hill, of surface  31 . It should be noted that only a small portion of tessellated ground plane  160  and facets  162   a ,  162   b ,  162   c , etc., thereof is shown along the profile of portion  166  of surface  31 . As discussed above, each of facets  162   a ,  162   b ,  162   c , etc., are defined by four vertices, each of which have respective elevations, and each of the facets have respective pitches and slopes. The specific position of point  164  upon the plane/surface of the facet  162   a ,  162   b ,  162   c , etc., within which point  164  (or its projection) lies is determined as described above. 
         [0064]    Tessellated ground plane  160  is preferably created outside the operation of image display and measurement computer system  130  and image display and analysis software  140 . Rather, tessellated ground plane  160  takes the form of a relatively simple data table or look-up table  168  stored within memory  132  of and/or accessible to image display and measurement computer system  130 . The computing resources required to calculate the locations of all the vertices of the many facets of a typical ground plane do not necessarily have to reside within image display and measurement computer system  130 . Thus, image display and measurement computer system  130  is compatible for use with and executable by a conventional personal computer without requiring additional computing resources. 
         [0065]    Calculating tessellated ground plane  160  outside of image display and measurement computer system  130  enables virtually any level of detail to be incorporated into tessellated ground plane  160 , i.e., the size and/or area covered by or corresponding to each of facets  162   a ,  162   b ,  162   c , etc., can be as large or as small as desired, without significantly increasing the calculation time, slowing the operation of, nor significantly increasing the resources required by image display and measurement computer system  130  and/or image display and analysis software  140 . Display and measurement computer system  130  can therefore be a relatively basic and uncomplicated computer system. 
         [0066]    The size of facets  162   a ,  162   b ,  162   c , etc., are uniform in size throughout a particular displayed image  142 . For example, if displayed image  142  corresponds to an area that is approximately 750 feet wide in the foreground by approximately 900 feet deep, the image can be broken into facets that are approximately 50 square feet, thus yielding about 15 facets in width and 18 facets in depth. Alternatively, the size of facets  162   a ,  162   b ,  162   c , etc., are uniform in terms of the number of pixels contained therein, i.e., each facet is the same number of pixels wide and the same number of pixels deep. Facets in the foreground of displayed image  142 , where the pixel density is greatest, would therefore be dimensionally smaller than facets in the background of displayed image  142  where pixel density is lowest. Since it is desirable to take most measurements in the foreground of a displayed image where pixel density is greatest, creating facets that are uniform in terms of the number of pixels they contain has the advantage of providing more accurate measurements in the foreground of displayed image  142  relative to facets that are dimensionally uniform. 
         [0067]    Another advantage of using pixels as a basis for defining the dimensions of facets  162   a ,  162   b ,  162   c , etc., is that the location calculation (pixel location to ground location) is relatively simple. A user operates image display and measurement computer system  130  to select a pixel within a given facet, image display and analysis software  140  looks up the data for the facet corresponding to the selected pixel, the elevation of the selected pixel is calculated as discussed above, and that elevation is used within the location calculation. 
         [0068]    Generally, the method of capturing oblique images of the present invention divides an area of interest, such as, for example, a county, into sectors of generally uniform size, such as, for example, sectors that are approximately one square mile in area. This is done to facilitate the creation of a flight plan to capture oblique images covering every inch of the area of interest, and to organize and name the sectors and/or images thereof for easy reference, storage and retrieval (a process known in the art as “sectorization”). Because the edges of any geographic area of interest, such as a county, rarely falls on even square mile boundaries, the method of capturing oblique images of the present invention provides more sectors than there are square miles in the area of interest—how many more depends largely on the length of the county borders as well as how straight or jagged they are. Typically, you can expect one extra sector for every two to three miles of border. So if a county or other area of interest is roughly 20 miles by 35 miles, or 700 square miles, the area will be divided into approximately from 740 to 780 sectors. 
         [0069]    The method of capturing oblique images of the present invention, in general, captures the oblique images from at least two compass directions, and provides full coverage of the area of interest from at least those two compass directions. Referring now to  FIGS. 7 and 8 , a first embodiment of a method for capturing oblique images of the present invention is shown. For sake of clarity,  FIGS. 7 and 8  is based on a system having only one image-capturing device. However, it is to be understood that two or more image-capturing devices can be used. 
         [0070]    The image-capturing device captures one or more oblique images during each pass over area  200 . The image-capturing device, as discussed above, is aimed at an angle over area  200  to capture oblique images thereof. Area  200  is traversed in a back-and-forth pattern, similar to the way a lawn is mowed, by the image-carrying device and/or the platform to ensure double coverage of area  200 . 
         [0071]    More particularly, area  200  is traversed by image-carrying device  32  and/or platform  20  following a first path  202  to thereby capture oblique images of portions  202   a ,  202   b , and  202   c  of area  200 . Area  200  is then traversed by image-carrying device  32  and/or platform  20  following a second path  204  that is parallel and spaced apart from, and in an opposite direction to, i.e., 180° (one-hundred and eighty degrees) from, first path  202 , to thereby capture oblique images of portions  204   a ,  204   b ,  204   c  of area  200 . By comparing  FIGS. 7 and 8 , it is seen that a portion  207  ( FIG. 8 ) of area  200  is covered by images  202   a - c  captured from a first direction or perspective, and by images  204   a - c  captured from a second direction or perspective. As such, the middle portion of area  200  is 100% (one-hundred percent) double covered. The above-described pattern of traversing or passing over area  200  along opposing paths that are parallel to paths  202  and  204  is repeated until the entirety of area  200  is completely covered by at least one oblique image captured from paths that are parallel to, spaced apart from each other as dictated by the size of area  200 , and in the same direction as paths  202  and  204  to thereby one-hundred percent double cover area  200  from those perspectives/directions. 
         [0072]    If desired, and for enhanced detail, area  200  is covered by two additional opposing and parallel third and fourth paths  206  and  208 , respectively, that are perpendicular to paths  202  and  204  as shown in  FIGS. 9 and 10 . Area  200  is therefore traversed by image-carrying device  32  and/or platform  20  following third path  206  to capture oblique images of portions  206   a ,  206   b  and  206   c  of area  200 , and is then traversed along fourth path  208  that is parallel, spaced apart from, and opposite to third path  206  to capture oblique images of portions  208   a ,  208   b  and  208   c  of area  200 . This pattern of traversing or passing over area  200  along opposing paths that are parallel to paths  206  and  208  is similarly repeated until the entirety of area  200  is completely covered by at least one oblique image captured from paths that are parallel to, spaced apart from as dictated by the size of area  200 , and in the same direction as paths  206  and  208  to thereby one-hundred percent double cover area  200  from those directions/perspectives. 
         [0073]    As described above, image-carrying device  32  and/or platform  20 , traverses or passes over area  200  along a predetermined path. However, it is to be understood that image-carrying device and/or platform  20  do not necessarily pass or traverse directly over area  200  but rather may pass or traverse an area adjacent, proximate to, or even somewhat removed from, area  200  in order to ensure that the portion of area  200  that is being imaged falls within the image-capture field of the image-capturing device. Path  202 , as shown in  FIG. 7 , is such a path that does not pass directly over area  200  but yet captures oblique images thereof. 
         [0074]    The present invention is capable of capturing images at various levels of resolution or ground sample distances. A first level of detail, hereinafter referred to as a community level, has a ground sample distance of, for example, approximately two-feet per pixel. For orthogonal community-level images, the ground sample distance remains substantially constant throughout the image. Orthogonal community-level images are captured with sufficient overlap to provide stereo pair coverage. For oblique community-level images, the ground sample distance varies from, for example, approximately one-foot per pixel in the foreground of the image to approximately two-feet per pixel in the mid-ground of the image, and to approximately four-feet per pixel in the background of the image. Oblique community-level images are captured with sufficient overlap such that each area of interest is typically covered by at least two oblique images from each compass direction captured. Approximately ten oblique community-level images are captured per sector. 
         [0075]    A second level of detail, hereinafter referred to as a neighborhood level, is significantly more detailed than the community-level images. Neighborhood-level images have a ground sample distance of, for example, approximately six-inches per pixel. For orthogonal neighborhood-level images, the ground sample distance remains substantially constant. Oblique neighborhood-level images have a ground sample distance of, for example, from approximately four-inches per pixel in the foreground of the image to approximately six-inches per pixel in the mid-ground of the image, and to approximately ten-inches per pixel in the background of the image. Oblique neighborhood-level images are captured with sufficient overlap such that each area of interest is typically covered by at least two oblique images from each compass direction captured, and such that opposing compass directions provide 100% overlap with each other. Approximately one hundred (100) oblique area images are captured per sector. 
         [0076]    It should be particularly noted that capturing oblique community and/or neighborhood-level images from all four compass directions ensures that every point in the image will appear in the foreground or lower portion of at least one of the captured oblique images, where ground sample distance is lowest and image detail is greatest. 
         [0077]    In the embodiment shown, image-capturing and geo-locating system  30  includes a gyroscope, compass and altimeter. However, it is to be understood that the image-capturing and geo-locating system of the present invention can be alternately configured, such as, for example, to derive and/or calculate altitude, pitch, roll and yaw, and compass heading from the GPS and INU signals/data, thereby rendering one or more of the gyroscope, compass and altimeter unnecessary. In fact, in the embodiment shown, image-capturing devices are at an equal angle of declination relative to a horizontal plane. However, it is to be understood that the declination angles of the image-capturing devices do not have to be equal. 
         [0078]    In the embodiment shown, image-capturing computer system executes image and data acquiring software’ that issues a common or single image-capture signal to the image-capturing devices to thereby cause those devices to acquire or capture an image. However, it is to be understood that the present invention can be alternately configured to separately cause the image-capturing devices to capture images at different instants and/or at different intervals. 
         [0079]    In the embodiment shown, the method of the present invention captures oblique images to provide double coverage of an area of interest from paths/perspectives that are substantially opposite to each other, i.e., 180° (one-hundred and eighty degrees) relative to each other. However, it is to be understood that the method of the present invention can be alternately configured to provide double coverage from paths/perspectives that are generally and/or substantially perpendicular relative to each other. 
         [0080]    While the present invention has been described as having a preferred design, the invention can be further modified within the spirit and scope of this disclosure. This disclosure is therefore intended to encompass any equivalents to the structures and elements disclosed herein. Further, this disclosure is intended to encompass any variations, uses, or adaptations of the present invention that use the general principles disclosed herein. Moreover, this disclosure is intended to encompass any departures from the subject matter disclosed that come within the known or customary practice in the pertinent art and which fall within the limits of the appended claims.