This page shows sample images from the photogrammetric processing of a small section of an agricultural land survey we completed early in 2016. This is only a small part of the complete survey, which covers around 7 ha. The brief required a ground sampling distance of 1cm, which mandated a high image density. We used a Sony full-frame (35mm) digital camera with a 36MP sensor with a Zeiss lens, and flew our UAV at a height of approximately 75m above ground level (AGL). Image overlap was 85% along-track and 65% across-track. We captured a total of 284 images during a single flight lasting 6 minutes. Ground Control Points were deliberately excluded from the photogrammetric processing in order to test the accuracy of Pix4D's statistical corrections to the airborne GNSS positioning.

 

This map shows the flight track covering the area in a pre-programmed grid pattern designed to achieve the required image overlap. Each of the dots represents a point at which the camera acquired an image (uncorrected low-accuracy GNSS positions).

Map

 

Location and generation of the initial tie points completed. Pix4D has worked from the recorded GNSS camera positions (blue dots) and computed corrected camera positions (green dots). The image thumbnails (green rectangles) don’t represent the true image overlaps.

Flight track and tie points - nadir

An oblique view of the tie-point cloud. This cloud is relatively sparse, containing only sufficient 3D points to allow Pix4D to correctly align the individual images into a single ortho-photo. Pix4D has located tie-points for an average of 100,000 ground features per image.

Flight track and point cloud - oblique

Pix4D has now "densified" the point cloud to produce a regularly-spaced 3D digital surface model.

Point cloud

This is the final orthomosaic overlaid onto Google Earth. There were no Ground Control Points used in this survey, so there is a very slight misalignment between the point cloud and the true surface near the corners of the cloud. The overall alignment and scaling achieved through statistical correction of the GNSS data is nonetheless very impressive.

Point cloud in GE

The finished digital surface model with contour colouring, superimposed onto Google Earth.

DSM in GE

The contour map of the site, produced automatically by Pix4D from the DSM, with a contour interval of 1m and a horizontal resolution of 10cm.

Contours