LESSON ON PHOTOGRAMMETRIC PROCEDURES PART 3

NUMBER OF PHOTOGRAPHS NECESSARY TO COVER A GIVEN AREA

In the preliminary estimate, the number of photographs required is calculated by dividing the total area to be photographed by the net area covered by a single photograph.

Let       A = total area to be photographed.

            l = length of the photograph in the direction of flight

            w = width of the photograph normal to the direction of flight

            s = scale of photograph =  (i.e., 1 cm = s metres)

            L = net ground distance corresponding to l

            W = net ground distance corresponding to w

            a = net ground area covered by each photograph = L x W

P_t = percentage overlap between successive photographs in the direction of flight (expressed as a ratio)

P_w = side lap (expressed as a ratio).

Since each photograph has a longitudinal lap of  the actual ground length (L) covered by each photograph is given by

                                                                                                  (43)

Similarly, the actual ground with (W) covered by each photograph is given by                                                                                                     (44)

Hence the ground area (a) covered by each photograph

                            (45)

The number of the photographs (N) required is given by      

N = A/a                                                                        (46)

If, however, instead of the total area A, the rectangular dimensions (i.e., length and width) of the ground are given, the number of the photographs required are computed by calculating the number of strips and the number of photographs required in each strip and multiplying the two.

Let       L₁ = dimension of the area parallel to the direction of flight

            L₂ = dimension of the area normal to the direction of flight

            N₁ = number of photographs in each strip

            N₂ = number of strips required

            N = total number of photographs to cover the whole area.

Now net length covered by each photograph = L = (1 – P_l) sl

  Number of photographs in each strip is given by

                                                                 (47)

Similarly, net width covered by each photograph = W = ( 1 – P_w) sw

Hence the number of the strips required are given by

                                                              (48)

Thus, the number of photographs required is

                          (49)