Abstract:
A method of creating a orthomosaic of a corridor area, the corridor area at least partially described by a corridor path, the method comprising flying an aircraft along a primary flight line approximating the corridor path and capturing a sequence of primary images; flying the aircraft along a secondary flight line substantially parallel to the corridor path and capturing a sequence of secondary images; identifying, in the primary images and secondary images, common features corresponding to common ground points; estimating, via bundle adjustment and from the common ground points, an exterior orientation associated with each primary image and a three-dimensional position associated with each ground point; orthorectifying, using at least some of the exterior orientations and at least some of the three-dimensional ground point positions, at least some of the primary images; and merging the orthorectified primary images to create the orthomosaic.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the efficient and accurate creation of corridor orthomosaics. 
       BACKGROUND OF THE INVENTION 
       [0002]    Accurately georeferenced mosaics of orthophotos, referred to as orthomosaics, are becoming popular alternatives to traditional pictorial maps because they can be created automatically from aerial photos, and because they show actual useful detail on the ground. 
         [0003]    The creation of accurate orthomosaics from aerial photos is well described in the literature. See, for example, Elements of Photogrammetry with Application in GIS, Fourth Edition (Wolf et al.), and the Manual of Photogrammetry, Sixth Edition (American Society for Photogrammetry and Remote Sensing (ASPRS)). 
         [0004]    The creation of an orthomosaic requires the systematic capture of overlapping aerial photos of the area of interest, both to ensure complete coverage of the area of interest, and to ensure that there is sufficient redundancy in the imagery to allow accurate bundle adjustment, orthorectification and alignment of the photos. 
         [0005]    Bundle adjustment is the process by which redundant estimates of ground points and camera poses are refined. Modern bundle adjustment is described in detail in “Bundle Adjustment—A Modern Synthesis” (Triggs et al.). 
         [0006]    Bundle adjustment may operate on the positions of manually-identified ground points, or, increasingly, on the positions of automatically-identified ground features which are automatically matched between overlapping photos. 
         [0007]    Overlapping aerial photos are typically captured by navigating a survey aircraft in a serpentine pattern over the area of interest. The survey aircraft carries an aerial camera system, and the serpentine flight pattern ensures that the photos captured by the camera system overlap both along flight lines within the flight pattern and between adjacent flight lines. 
         [0008]    Corridors containing railway lines, highways, power lines, rivers, canals, coastlines and other narrow meandering features are often of particular interest. However, traditional area-based aerial surveying techniques are sub-optimal for capturing corridors. 
       SUMMARY OF THE INVENTION 
       [0009]    In a first aspect, the present invention provides a method of creating a orthomosaic of a corridor area, the corridor area at least partially described by a corridor path, the method comprising: flying an aircraft along a primary flight line, the primary flight line comprising a sequence of primary flight line segments, each primary flight line segment approximating at least part of the corridor path; capturing, during flight along each primary flight line segment and via an aerial camera system carried by the aircraft, a sequence of primary images, each primary image at least partially overlapping its successor in the sequence; flying the aircraft along a secondary flight line, the secondary flight line comprising a sequence of secondary flight line segments, each secondary flight line segment substantially parallel to at least part of the corridor path; capturing, during flight along each secondary flight line segment and via the aerial camera system carried by the aircraft, a sequence of secondary images, at least some of the secondary images overlapping at least some of the primary images; identifying, in a plurality of the primary images and secondary images, common features corresponding to common ground points; estimating, via bundle adjustment and from the common ground points, an exterior orientation associated with each primary image and a three-dimensional position associated with each ground point; orthorectifying, using at least some of the exterior orientations and at least some of the three-dimensional ground point positions, at least some of the primary images; and merging the orthorectified primary images to create the orthomosaic. 
         [0010]    The aircraft may be flown substantially level along each primary flight line segment, and may be flown along a go-around turn between each primary flight line segment and its successor, the turn having an angle greater than 180 degrees. 
         [0011]    The aircraft may be flown along a turn between each secondary flight line segment and its successor, the turn having an angle less than 90 degrees. 
         [0012]    The aerial camera system may comprise at least one vertical camera for capturing substantially vertical images. 
         [0013]    The aerial camera system may comprise at least one oblique camera for capturing substantially oblique images. 
         [0014]    The primary images and secondary images may comprise both vertical images and oblique images. 
         [0015]    The primary images may comprise vertical images and the secondary images may comprise oblique images. 
         [0016]    The aerial camera system may comprise at least one overview camera for capturing overview images, and a plurality of detail cameras for capturing detail images, each detail image having a higher resolution than the at least one overview image, at least some of the detail images may overlap some of the overview images, and the primary images may comprise both overview images and detail images. 
         [0017]    The secondary images may comprise both overview images and detail images. 
         [0018]    The secondary flight line may be curved and include banked turns. 
     
    
     
       DRAWINGS 
       Figures 
         [0019]      FIG. 1  shows a corridor path of interest. 
           [0020]      FIG. 2  shows the corridor path approximated by a polyline, and the corridor area approximated by a polygon. 
           [0021]      FIG. 3  shows the corridor combined with an intersecting area of interest. 
           [0022]      FIG. 4  shows the serpentine flight pattern of an area-based aerial survey. 
           [0023]      FIG. 5  shows a diagram and equation relating the swath width of an aerial camera system to its angular field of view and altitude above ground level. 
           [0024]      FIG. 6  shows a method for covering the corridor area of interest with a sequence of survey path segments. 
           [0025]      FIG. 7  shows another method for covering the corridor area of interest with a sequence of survey path segments. 
           [0026]      FIG. 8  shows the use of a wider survey path segment to cover the intersecting area of interest. 
           [0027]      FIG. 9  shows the corridor area of interest covered using two sets of flight lines. 
           [0028]      FIG. 10  shows the corridor area of interest covered using a smaller number of flight lines with a wider swath width. 
           [0029]      FIG. 11  shows the use of a primary flight line for capturing primary imagery of the corridor, and a secondary flight line for capturing secondary imagery of the corridor for accuracy purposes. 
           [0030]      FIG. 12  shows the survey aircraft capturing vertical imagery of the corridor while flying level. 
           [0031]      FIG. 13  shows the survey aircraft capturing vertical imagery of the corridor while flying banked. 
           [0032]      FIG. 14  shows the survey aircraft capturing both vertical and oblique imagery of the corridor. 
           [0033]      FIG. 15  shows a flight path with banked turns covering the corridor path. 
           [0034]      FIG. 16  shows a diagram and equations relating the vertical imaging offset of a banked aircraft to its altitude, bank angle, velocity and turn radius. 
           [0035]      FIG. 17  shows a dual-resolution V5-300 HyperCamera aerial camera system. 
           [0036]      FIG. 18  shows the overview field of view and overlapping detail fields of view of a dual-resolution aerial camera system. 
           [0037]      FIG. 19  shows a front elevation of a Cessna 208 aircraft carrying a dual-resolution aerial camera system, and the resultant overview and aggregate detail fields of view. 
           [0038]      FIG. 20  shows a side elevation of a Cessna 208 aircraft carrying a dual-resolution aerial camera system, and the resultant overview and aggregate detail fields of view. 
           [0039]      FIG. 21  shows the overlapping fields of view of three successive shots of a dual-resolution aerial camera system. 
           [0040]      FIG. 22  shows the overlapping fields of view of shots of a dual-resolution aerial camera system in adjacent flight lines. 
           [0041]      FIG. 23  shows a block diagram of a power and control system for an aerial camera system such as a HyperCamera. 
           [0042]      FIG. 24  shows a process flow for efficiently creating an orthomosaic from aerial photos. 
           [0043]      FIG. 25  shows a process flow for efficiently creating an orthomosaic from dual-resolution aerial photos. 
       
    
    
     REFERENCE NUMERALS 
       [0000]    
       
         
           
               100  Corridor path. 
               102  Polyline approximating corridor path. 
               104  Polyline vertex. 
               106  Polygon approximating corridor shape. 
               108  Intersecting area of interest. 
               110  Area-based area of interest. 
               112  Flight line. 
               114  Turn-around between successive flight lines. 
               120  Survey path segment. 
               122  Survey path segment swath. 
               124  Wider survey path segment swath for intersecting area of interest. 
               126  Go-around between successive flight line segments. 
               128  Flight line segment. 
               130  Primary flight line segment. 
               132  Secondary flight line segment. 
               134  Gentle turn between successive flight line segments. 
               136  Aggregate swath. 
               140  Vertical imaging field of view. 
               142  Oblique imaging field of view. 
               150  Ground. 
               152  Corridor centreline on ground. 
               154  Curved flight path. 
               156  Bank angle. 
               158  Bank offset. 
               160  Detail field of view. 
               164  Longitudinal detail field of view. 
               170  Overview field of view. 
               172  Lateral overview field of view. 
               174  Longitudinal overview field of view. 
               180  Aggregate detail field of view. 
               182  Lateral aggregate detail field of view. 
               212  Camera hole in floor of aircraft. 
               220  Direction of flight. 
               230  Aerial survey aircraft. 
               300  Computer. 
               302  Pilot display. 
               304  Inertial Measurement Unit (IMU). 
               306  Global Navigation Satellite System (GNSS) receiver. 
               308  Analog-to-digital converters (ADCs). 
               310  Camera control unit (CCU). 
               320  Battery unit. 
               322  Aircraft auxiliary power. 
               324  Ground power unit (GPU). 
               326  DC-DC converter(s). 
               330  Angular motion compensation (AMC) unit(s). 
               340  Camera(s). 
               350  Aerial camera system. 
               352  Dual-resolution aerial camera system. 
               400  Detail photos. 
               402  Overview photos. 
               404  Orthomosaic. 
               410  Match features step. 
               412  Solve for pose &amp; positions step. 
               414  Orthorectify photos step. 
               416  Blend orthophotos step. 
           
         
       
     
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0099]    A corridor area of interest (simply “corridor” hereafter) typically consists of a strip of land along an arbitrary path  100 , as shown in  FIG. 1 . The corridor may follow a physical structure, such as a railway line or river. The length of the corridor is typically much longer than the width of the corridor. 
         [0100]    The width of the corridor may, in general, vary along the path, but for many corridors a fixed width applies. For illustrative purposes in this specification only fixed-width corridors are generally shown. 
         [0101]    The corridor may be continuous or discontinuous, and the corridor may comprise multiple smaller paths such as loops or forks. For illustrative purposes in this specification only continuous unforked corridors are shown. 
         [0102]    As shown in  FIG. 2 , the corridor path may be approximated by a polyline  102 , consisting of a sequence of straight-line segments between successive vertices  104 . The polyline is constructed so that the maximum perpendicular distance from the path to the polyline is within a defined tolerance. For a desired corridor width the polyline vertices  104  can be offset perpendicular to the path to obtain the vertices of a polygon  106  that encloses the corridor. By adjusting the tolerance, and hence the number of vertices, the polyline and polygon can approximate the corridor path and corridor area with arbitrary precision. The desired corridor width is typically expanded to accommodate the polyline tolerance, i.e. to ensure that the polygon  106  encloses the desired corridor area. 
         [0103]    Rather than being defined explicitly via a path, a corridor may also be defined directly via one or more shapes (e.g. polygons). 
         [0104]    The corridor may intersect with an area of interest  108  that is not fully enclosed by the corridor boundary, such as a town adjacent to a highway. The corridor survey area may then be defined by the union of the corridor boundary and the boundary of the area of interest, as shown in  FIG. 3 . 
         [0105]    When capturing a traditional extended survey area  110 , as shown in  FIG. 4 , the survey aircraft typically follows a serpentine flight pattern. The flight plan consists of a number of parallel flight lines  112 , separated by a lateral offset. Each flight line specifies a start and end location and altitude. The aircraft travels in a straight line from the start point to the end point. At the end of a flight line the aircraft performs a 180-degree turn  114  to return along a laterally spaced parallel path specified by the next flight line&#39;s start and end locations. 
         [0106]    The location and number of flight lines are calculated from a number of parameters, including the survey boundary, flight altitude, ground elevation, camera system field of view, and the desired forward overlap and side overlap. 
         [0107]    Traditional aerial survey flight planning is well described in the literature. See for example U.S. Pat. No. 6,711,475 (Murphy), the contents of which are incorporated herein by reference. 
         [0108]    A further factor in calculating the location and number of flight lines is a contingency distance at the survey boundary. The contingency distance is added to the survey boundary to enlarge the area of capture. This allows for non-uniform imagery capture due to turbulence or changes in aircraft pitch, yaw or roll near the survey edge and ensures complete coverage within the survey area. 
         [0109]      FIG. 5  shows a survey aircraft  230  carrying an aerial camera system  350 . The diagram and equation relate the swath width (w)  258  of the aerial camera system  350 , i.e. where its field of view intersects the ground  150 , to its angular field of view (beta)  250  and altitude above ground level (a)  254 . 
         [0110]    For corridor capture, flight plan generation combines the flight line calculation method for traditional survey capture with an additional process that fits multiple survey path segments to the corridor survey area using a cost minimisation process. 
         [0111]    Firstly, traditional survey flight planning calculations are used to determine the minimum number of parallel flight lines required to capture the width of the corridor survey. The flight planning calculations include a contingency distance added to the corridor width to ensure complete imagery coverage. For high altitude surveys or narrow corridor paths, it may be possible to capture the full corridor width with a single flight line, i.e. if the corridor width is contained within the swath width  258  of the camera system. For lower altitude surveys or wider corridors, multiple flight lines may be required to capture the full corridor width. 
         [0112]    The flight planning process calculates an aggregate swath width for a set of adjacent flight lines, based on the swath width  258  of the camera system and the required lateral overlap. The contingency added to the corridor survey width is subtracted from the aggregate swath width to determine the usable swath width for corridor flight planning. 
         [0113]    Secondly, the corridor survey area is subdivided into multiple linear survey path segments. The width of each path segment is determined by the aggregate swath width. The length and orientation of each path segment is determined by the direction and variation in the corridor path. For corridor paths with extended straight sections, e.g. railway lines, the survey may be able to be subdivided into a small number of long path segments. For meandering corridor survey paths, e.g. rivers, the survey may require subdivision into a larger number of short path segments. The path segments may be oriented in any direction, and may intersect at arbitrary angles. 
         [0114]    It is desirable to minimise the number of path segments flown by the aircraft, as the survey aircraft may need to perform a go-around turn to travel from the end of one path segment to the beginning of the next path segment. 
         [0115]    Many methods exists for subdividing the survey into path segments. A simple method, illustrated in  FIG. 6 , starts at one end of the corridor and creates a path segment  120  in the direction of the corridor path. The path segment is terminated when the survey boundary is no longer contained within the usable swath  122 . At this point, a new path segment  120  is started at this point in the direction of the corridor path and the process is repeated until the end of the corridor is reached. 
         [0116]    Another method, illustrated in  FIG. 7 , uses cost minimisation to reduce the number of path segments  120 . By starting path segments  120  at a point perpendicularly offset from the corridor path  100  and varying the path segment orientation, it is typically possible to achieve a longer path segment compared to the simple method described above. This minimisation process finds the optimal path start position and orientation for each path segment. 
         [0117]    Another method uses cost minimisation to maximise the length of path segments  120 . This method finds the longest possible path segment for the corridor path. The method then finds the next longest path segment. The process continues until the entire corridor is contained within the path segments. 
         [0118]    Another method derives the path segments  120  directly from the straight-line segments of the corridor polyline  102 . 
         [0119]    To ensure complete coverage of the corridor survey area  106 , it is necessary to adjust the start and end point of each path segment due to the intersection of path segments at arbitrary angles and the contingency applied to survey boundaries. The length of each path segment is increased by a contingency value to increase overlap between adjacent path segments and to ensure areas near the corridor survey edges are fully captured by imagery at path segment intersections. 
         [0120]    The number of flight lines per path segment is not restricted to the minimum number of flight lines calculated by the flight planning calculations. Increasing the number of flight lines increases the usable swath width, which generally decreases the number of path segments required to capture the survey imagery. 
         [0121]    Increasing the number of flight lines is also of benefit where the path segmentation creates a large number of short path segments, e.g. when capturing a meandering river. 
         [0122]    Increasing the number of flight lines is also of benefit when the width of the corridor survey path is not constant, allowing thin corridor segments to be captured with fewer flight lines, and wider corridor segments to be captured with a larger number of flight lines. 
         [0123]    Increasing the number of flight lines is also of benefit when capturing a corridor survey path that is combined with an intersecting area of interest  108 , e.g. town adjacent to a highway, as illustrated in  FIG. 8  where a wider path segment  124  is used to capture the intersecting area of interest  108 . 
         [0124]    A flight plan is generated to plot a path for the aircraft to navigate so that all flight lines of all path segments are captured. 
         [0125]      FIG. 9  shows two parallel sets of flight line segments  128  used to cover the corridor area  106 . Here the path segments  120  (not shown) are derived directly from the straight-line segments of the corridor path polyline  102 . 
         [0126]    Firstly, the flight plan creation process creates an ordered list of flight line segments  128 . Where the path segments  120  contain multiple flight lines, the flight line segments  128  within each path segment  120  may be flown sequentially to complete each segment  120 . Alternately, the corridor length may be flown multiple times where the flight plan specifies one flight line segment  128  per path segment  120  in one direction along the corridor, followed by a return path flying a second flight line segment  128  in each path segment  120 , and so on until the complete corridor width is captured. 
         [0127]    Secondly, the path between consecutive flight line segments  128  is created. This specifies the path the pilot should follow to travel from the end of one flight line segment  128  to the start of the next flight line segment  128 . The pilot may also be allowed to navigate between successive flight line segments  128  freely. As consecutive flight line segments  128  may intersect at an arbitrary angle, the required turn may be achieved with a small change in bearing, or may be achieved with a go-around  126  where the aircraft makes anything up to a 360-degree turn to align itself with the next flight line segment  128 . Consecutive flight line segments  128  may intersect or may be separated by a distance. 
         [0128]    The swath width  258  of an aerial camera system  350  increases with increasing field of view angle  250 , and with increasing altitude  254 .  FIG. 10  shows two smaller parallel sets of flight line segments  128  used to cover the corridor area  106 , assuming higher-altitude operation and/or wider-angle imaging than in  FIG. 9 , i.e. with a wider aggregate swath width  136 . 
         [0129]    To create a complete aerial orthomosaic for a survey area, every point within the survey boundary must be captured by the camera system. This is generally achieved through the use of overlap which allows for variation in the aircraft&#39;s yaw, pitch and roll between adjacent captured images. 
         [0130]    Overlap is also used to improve the alignment of the orthomosaic with existing orthomosaics or survey ground features. The alignment is improved by imaging the same point on the ground from multiple angles, allowing the position and orientation of the camera system to be calculated with greater accuracy. 
         [0131]    In general, increasing the overlap in a particular direction improves the orthomosaic alignment in the same direction. Increasing the forward overlap improves alignment in the direction of the flight path. Increasing the side overlap improves the alignment in the direction perpendicular to the flight path. 
         [0132]    In the case of a corridor captured with a single flight line per path segment, overlap exists in the forward direction only. The absence of side overlap may cause misalignment of the orthomosaic with ground features. Misalignment error vectors are generally in a direction perpendicular to the corridor path in this case. 
         [0133]    In general, a minimum of two or more parallel flight line segments  128  per survey path segment  120  should be captured to enable generation of orthomosaics with accurate alignment to ground features. 
         [0134]    For corridor survey paths with a narrow width, capture of two parallel flight line segments  128  per path segment  120  may result in the capture of a significant area outside of the corridor survey boundary. 
         [0135]    The following method optimises the process of capturing imagery of a narrow corridor survey path with accurate alignment. The method uses two flight lines with different planning characteristics, referred to as primary and secondary flight lines. 
         [0136]    Primary flight lines are captured for the purpose of orthomosaic generation and require complete coverage of the corridor survey path. 
         [0137]    Secondary flight lines are captured for the purpose of side overlap with the primary flight lines and are flown parallel to and offset laterally from the primary flight lines. Continuous side overlap is not required to achieve orthomosaic alignment. Alignment is achieved if each secondary flight line overlaps a majority of the length of its parallel primary flight line. 
         [0138]    A flight plan for the primary and secondary flight lines is generated by firstly planning the corridor survey path with a single flight line configuration. The single primary flight line will generally follow the corridor path  100  (or corridor path polyline  102 ). Secondly, secondary flight line segments parallel to and offset laterally from the primary flight line segments are added to the flight plan. 
         [0139]    As the secondary flight lines do not require complete coverage, the flight plan can allow for “free flying” the length of the corridor over the secondary flight line segments, where the aircraft turns directly from one flight line segment to the next. This enables the secondary flight lines to be captured without the go-arounds  126  that would be required to capture the complete length of every secondary flight line segment. The secondary flight line can also be flown along a curved flight path with banked turns as discussed further below. 
         [0140]      FIG. 11  shows a primary flight line, comprising a sequence of primary flight line segments  130 , used to cover the corridor area  106 , and a secondary flight line, comprising a sequence of secondary flight line segments  132 , used to provide overlap for accuracy purposes. The primary flight line segments  130  are typically joined by go-arounds  126 . The secondary flight line segments  132  are typically joined by turns 134 which may be flown freely. 
         [0141]    When using a dual-resolution aerial camera system  352 , as discussed in more detail below, the camera system may be configured to only capture overview imagery along the secondary flight line, as the overview imagery provides maximum overlap between flight lines 
         [0142]    The secondary flight line may be captured a significant time after the primary flight line, e.g. days or weeks later if convenient. This allows for the capture of long corridors while ferrying aircraft between locations. 
         [0143]    When using a dual-resolution aerial camera system  352 , the secondary flight line can be captured with both overview and detail cameras, allowing orthomosaics to be generated within the field of view of the secondary flight line. This creates a wider orthomosaic in areas of secondary flight line overlap but a narrower orthomosaic at corridor path segment intersections. 
         [0144]    A further method for capturing corridors is available to aircraft that contain a camera system with a wide lateral field of view, e.g. achieved through multiple cameras capturing vertical imagery, left oblique imagery and/or right oblique imagery. Aerial camera systems that capture both vertical and oblique imagery are described in U.S. Pat. Nos. 8,497,905 and 8,675,068 (Nixon), the contents of which are herein incorporated by cross-reference. 
         [0145]    The flight plan plots a flight path along the flight lines using turns between consecutive flight line segments  128 , without the use of go-arounds  126  to turn from one flight line to the next. The aircraft follows the flight path directly from one segment to the next, banking the aircraft to perform the turns. The flight line segments are planned such that the banking angle is less than the limit of the field of view of the oblique cameras. 
         [0146]    The orthomosaic generation process uses the imagery closest to the nadir point. When flying horizontally, the vertical pointing camera is nadir, as shown in  FIG. 12 . When the aircraft is banking, the left or right oblique imagery is closest to nadir and is used for orthomosaic generation, as shown in  FIG. 13 . 
         [0147]    A Digital Elevation Model (DEM) is a common by-product of the orthomosaic generation process. A DEM may be created by calculating the elevation of every point within the survey area. The elevation at a point may be calculated by locating the point in multiple images that contain the point. If the point is present in three or more images, its elevation may be triangulated using the interior and exterior orientation of the cameras. 
         [0148]    The point elevations can be calculated with greater accuracy when each point is captured in a large number of images from different angles. This is achieved through the use of forward and side overlap. 
         [0149]    In the case of corridor capture with primary and secondary flight lines, only part of the survey area may contain images captured with side overlap. 
         [0150]    A method to increase the imagery area containing side overlap is to capture oblique images from the secondary flight lines. The oblique images are captured by an imaging system directed at the centre of the primary flight line imagery, as shown in  FIG. 14 . 
         [0151]    Additionally, the imagery captured from the secondary flight line may be used to generate an oblique orthomosaic of the corridor survey area. 
         [0152]    A further method for capturing a corridor uses a curved flight path  154  based on the corridor path  100 , as shown in  FIG. 15 . 
         [0153]    The curved flight path can be offset towards the center of curvature of the corridor path  100  at any given point, to account for the offset (d)  158  induced by the bank of the aircraft. As shown in the diagram and equations in  FIG. 16 , the offset (d)  158  is related to the altitude above ground level (a)  254 , bank angle (theta)  156 , aircraft velocity (v), bank radius (r), and gravity (g). Initially assuming the bank radius (r) is the radius of the corridor path  100  (at any given point), the final bank radius (r) and bank angle (theta)  156  can be arrived at iteratively. 
         [0154]    Even when flying a curved flight path  154 , if the corridor contains a sharp turn the pilot can perform a go-around  126  as usual. 
         [0155]    Any suitable aerial camera system  350  may be utilised for corridor capture. 
         [0156]    Sufficient redundancy for accurate bundle adjustment typically dictates the choice a longitudinal (forward) overlap of at least 60%, i.e. between successive photos along a flight line, and a lateral (side) overlap of at least 40%, i.e. between photos on adjacent flight lines. This is often referred to as 60/40 overlap. 
         [0157]    The chosen overlap determines both the required flying time and the number of photos captured (and subsequently processed). High overlap is therefore expensive, both in terms of flying time and processing time, and practical choices of overlap represent a compromise between cost and orthomosaic accuracy. 
         [0158]    The use of a dual-resolution or multi-resolution camera systems  352  provides a powerful way to reduce overlap without compromising accuracy. The capture and processing of multi-resolution aerial photos is described in U.S. Pat. Nos. 8,497,905 and 8,675,068 (Nixon), the contents of which are herein incorporated by cross-reference. Multi-resolution sets of photos allow orthomosaic accuracy to be derived from the overlap between lower-resolution overview photos, while orthomosaic detail is derived from higher-resolution detail photos. 
         [0159]    U.S. Pat. Nos. 8,497,905 and 8,675,068 (Nixon), describe a family of external camera pods attachable to a small aircraft comprising multi-resolution vertical and oblique aerial imaging systems. U.S. patent application Ser. No. 14/310,523 (Tarlinton) and Ser. No. 14/478,380 (Lapstun), the contents of which are incorporated herein by reference, describe the HyperCamera™ family of multi-resolution aerial camera systems suitable for deployment in aircraft that have a standard camera hole. 
         [0160]      FIG. 17  shows a dual-resolution V5-300 HyperCamera aerial camera system  352  which comprises one wide-angle overview camera and five narrow-angle detail camera, deployable in the cockpit or cabin of most survey aircraft that have a standard (e.g. 20-inch diameter) camera hole  212 . 
         [0161]      FIG. 18  shows the projection of the three-dimensional fields of view  160  and  170  of the detail cameras and overview camera of the HyperCamera unit onto a ground plane. It shows how the detail field of views  160  overlap in a direction perpendicular to the direction of flight  220 . 
         [0162]      FIG. 19  shows a front elevation of the Cessna 208 survey aircraft  230  carrying a dual-resolution aerial camera system, and shows the lateral overview field of view  172  of the camera system  352 , and the aggregate lateral detail field of view  182  of the camera system  352 . The aggregate lateral detail field of view  182  is the aggregate of the five individual overlapping lateral detail fields of view  162 . 
         [0163]      FIG. 20  shows a side elevation of the Cessna 208 survey aircraft  230  carrying a HyperCamera, and shows the longitudinal overview field of view  174  of the camera system  352 , and the longitudinal detail field of view  164  of the camera system  352 . 
         [0164]      FIG. 17  shows the overlapping overview fields of view  170  and aggregate detail fields of view  180  of three successive shots in the direction of flight  220 . The aggregate detail field of view  180  is the aggregate of the five individual overlapping detail fields of view  160 . At the camera firing rate illustrated in the figure (i.e. as implied by the longitudinal overlap), the aggregate detail fields of view  180  overlap by about 20% longitudinally, while the overview fields of view  170  overlap by about 85% longitudinally. 
         [0165]      FIG. 18  shows the overlapping overview fields of view  170  and aggregate detail fields of view  180  of two shots from adjacent flight lines, i.e. flown in opposite directions  220 . At the flight-line spacing illustrated in the figure, the aggregate detail fields of view  180  overlap by between 20% and 25% laterally, while the overview fields of view  170  overlap by about 40% laterally. 
         [0166]    As already noted, traditional single-resolution aerial surveys are typically operated with 60/40 overlap, i.e. 60% forward (or longitudinal) overlap, and 40% side (or lateral) overlap. With the multi-resolution HyperCamera operated as shown in  FIGS. 21 and 22 , overview photos are captured with better than 85/40 overlap, and detail photos are typically captured with only 20/20 overlap or less. 
         [0167]      FIG. 20  shows a block diagram of a power and control system for an aerial camera system  350 , such as a dual-resolution HyperCamera system  352 . The camera(s)  340  are controlled by a computer  300  via a set of analog-to-digital converters  308  (ADCs). 
         [0168]    The computer  300  uses one or more Global Navigation Satellite System (GNSS) receiver  304  to monitor the position and speed of the survey aircraft  230  in real time. The GNSS receiver(s) may be compatible with a variety of space-based satellite navigation systems, including the Global Positioning System (GPS), GLONASS, Galileo and BeiDou. 
         [0169]    The computer  300  provides precisely-timed firing signals to the camera(s)  340  via the ADC(s)  308 , to trigger camera exposure, according to a stored flight plan and the real-time position and speed of the aircraft. If a camera(s)  340  incorporate an auto-focus mechanism then the computer  300  also provides a focus signal to each such camera to trigger auto-focus prior to exposure. 
         [0170]    The computer  300  may fire the camera(s)  340  at the same rate. Alternatively, the computer  300  may fire the overview camera(s) of a dual-resolution system at a different rate to the detail cameras, i.e. either a higher rate or lower rate, to achieve a different overlap between successive overview photos, i.e. either a higher overlap or a lower overlap, independent of the overlap between successive detail photos. The computer  300  may fire the cameras simultaneously, or it may stagger the timing of the firing, e.g. to achieve a different alignment of photos longitudinally, or to reduce peak power consumption. 
         [0171]    The flight plan describes each flight line making up the survey, and the nominal camera firing rate along each flight line required to ensure that the necessary overlap is maintained between successive shots. The firing rate is sensitive to the elevation of the terrain below the aircraft, i.e. the higher the terrain the higher the firing rate needs to be. It is adjusted by the computer  300  according to the actual ground speed of the aircraft, which may vary from its nominal speed due to wind and the pilot&#39;s operation of the aircraft. 
         [0172]    The computer  300  also uses the flight plan and real-time GNSS position to guide the pilot along each flight line via a pilot display  302 . 
         [0173]    As shown in  FIG. 20 , the position data from the GNSS receiver is optionally augmented with orientation information from an inertial measurement unit  306  (IMU). This allows the computer  300  to provide enhanced feedback to the pilot on how closely the pilot is following the flight plan. In the absence of the IMU  306  the GNSS receiver connects directly to the computer  300 . 
         [0174]    The computer stores the GNSS position (and optionally IMU orientation, if the IMU  306  is present) of each shot. This is used during subsequent processing of the photos to produce an accurate orthomosaic. 
         [0175]    One or more optional angular motion compensation (AMC) units  330 , responsive to the orientation reported by the IMU  306 , correct the orientation of the cameras so that they maintain a consistent pointing direction over time, despite the aircraft rolling, pitching or yawing during flight. This ensures that the captured photos can be used to create a photomosaic without gaps, while allowing the overlap between successive shots and between adjacent flight lines to be minimised. 
         [0176]    The AMC  330  may consist of a platform with two or three axes of rotation (i.e. roll and pitch; or roll, pitch and yaw) upon which the camera(s)  340  are mounted. Commercially-available AMC platforms include the PAV series from Leica Geosystems. 
         [0177]    Alternatively, the AMC  330  may comprise one or more beam-steering mechanisms in the optical path of each camera (or group of cameras), whereby the pointing direction of the cameras is corrected by beam-steering. 
         [0178]    Angular motion compensation becomes increasingly important as the flying altitude is increased and/or the ground sampling distance (GSD) is decreased. 
         [0179]    Motion blur due to the forward motion of the aircraft is equal to the speed of the aircraft multiplied by the exposure time of the camera. Once motion blur becomes a significant fraction of (or exceeds) the GSD it becomes useful to provide a forward motion compensation (FMC) mechanism to reduce or eliminate motion blur. FMC can be provided in a number of ways, including translating or rotating the optical axis of the camera (by moving the image sensor, or an intermediate mirror, or the camera itself), or by time delayed integration (TDI) of adjacent lines of pixels in the image sensor. FMC can be provided via an AMC unit. 
         [0180]    Each camera  340  may store its shots locally, e.g. in removable flash memory. This eliminates the need for centralised storage in the camera system, and the need for a high-bandwidth data communication channel between the cameras and the centralised storage. Alternatively the camera system may incorporate centralised storage (not shown). 
         [0181]    The GNSS position of each shot may be delivered to each camera  340 , to allow the camera to tag each photo with its GNSS position. 
         [0182]    The cameras  340  are powered by a battery unit  320 . The battery unit  320  provides a voltage higher than the voltage required by all connected components, e.g. between 24V and 28V, and the voltage requirement of each connected component is provided via a DC-DC converter  326 . For example, a Nikon D800 camera requires less than 10V. Additional DC-DC converters  326  also provide appropriate voltages to power the computer  300 , the pilot display  302 , the GNSS receiver  304 , the IMU  306 , and the AMC(s)  330 . For clarity these power connections are omitted in  FIG. 23 . 
         [0183]    The battery unit  320  contains two 12V or 14V batteries or a single 24V or 28V battery. It contains a charging circuit that allows it to be trickle-charged from an aircraft with a suitable auxiliary power source  322 , allowing it to remain charged at all times. It may also be charged on the ground from a ground power unit  324  (GPU). 
         [0184]    The ADCs  308  and DC-DC converters  326  may be housed in a camera control unit  310  (CCU). This may additionally include a USB interface to allow the computer  300  to control the ADCs. 
         [0185]    The DC-DC converters  326  that provide power to the cameras  340  may be located in the CCU  310  or closer to the cameras in the distribution boxes  150 . 
         [0186]    Photos captured by the camera system  350  are intended to be seamlessly stitched into an orthomosaic, and  FIG. 24  shows a process flow for efficiently creating an orthomosaic from detail photos  400  captured by one or more detail cameras  340 . 
         [0187]    If the camera system  350  is a dual-resolution (or multi-resolution) camera system  352  then the process flow, as shown in  FIG. 25 , also uses overview photos  402  from one or more overview cameras  340 . 
         [0188]    The process consists of four main steps: (1) features are automatically detected in each of the photos  400  (and optionally  402 ) and matched between photos (step  410 ); bundle adjustment is used to iteratively refine initial estimates of the real-world three-dimensional position of each feature, as well as the camera pose (three-dimensional position and orientation) and camera calibration (focal length and radial distortion etc.) associated with each photo (at step  412 ); each detail photo  400  is orthorectified according to its camera pose and terrain elevation data (at step  414 ); and the orthorectified photos (orthophotos) are blended to form the final orthomosaic  404  (at step  416 ). 
         [0189]    In a single-resolution system the accuracy of the orthomosaic  404  derives from the conventional high overlap between detail photos  400 , and the detail in the orthomosaic  404  also derives from the detail photos  400 . 
         [0190]    In a dual-resolution system the accuracy of the orthomosaic  404  derives from the high overlap between lower-resolution overview photos  402 , while detail in the orthomosaic  404  derives from the higher-resolution detail photos  400 . 
         [0191]    The orthomosaic is typically stored as an image pyramid, i.e. within which different (binary) zoom levels are pre-computed for fast access at any zoom level. Lower zoom levels in the pyramid are generated from higher zoom levels by low-pass filtering and subsampling, thus the entire pyramid may be generated from the detail-resolution orthomosaic. As an alternative, lower zoom levels may be generated from an orthomosaic created from the overview photos  402 , in which case the overview photos  402  are also orthorectified and blended as described above for the detail photos  400 . 
         [0192]    An initial estimate of the camera pose of each photo, subsequently refined by the bundle adjustment process (at step  412 ), is derived from the GNSS position of each photo, as well as its IMU-derived orientation, if available. 
         [0193]    The terrain data used to orthorectify (at step  414 ) the detail photos  400  may be based on 3D feature positions obtained from bundle adjustment (at step  412 ), or may be terrain data sourced from elsewhere (such as from a LiDAR aerial survey). 
         [0194]    Automatically detected ground features may be augmented with manually-identified ground points, each of which may have an accurate surveyed real-world position (and is then referred to as a ground control point). 
         [0195]    The present invention has been described with reference to a number of preferred embodiments. It will be appreciated by someone of ordinary skill in the art that a number of alternative embodiments of the present invention exist, and that the scope of the invention is only limited by the attached claims.