Geography Reference
In-Depth Information
Material and Methods
UAV Photogrammetry
The field of UAV photogrammetry deals with methods and technologies for the
acquisition of measurements, maps, digital terrain models and other products from
photographic images.
A pilot may not be physically present in the aircraft. UAV photogrammetry
combines the advantage of the vertical aerial view of aerial photogrammetry and
the advantages of close distances and high image detail of ground photogrammetry
(Aber
2010
). The basic principle of photogrammetric measurements is the
geometric-mathematical reconstruction of the direction of the photographic rays
in the image. UAV photogrammetry is a novel measurement tool that works on the
same principle and which can be used to obtain geographic data for new applica-
tions. Additional information about obtaining data with UAV models can be found
in Aber (
2010
) and Mi
ˇ
ijovsk
´
and V
´
vra (
2012
).
To properly implement aerotriangulation, it is necessary to know the elements of
the external and internal orientations. The exterior orientation elements include
the X, Y, and Z coordinates of the camera on the platform and the three angles of
camera tilt (
ω
,
ˆ
, and
ʺ
). These coordinates and angles are relative to the ground
coordinate system. The X, Y, and Z coordinates are provided by very precise DGPS
systems. The tilts of the camera can easily be measured using a three level system,
preferably an INS (Inertial Navigation System) or IMU device (Inertial Measure-
ment Unit). In most cases, the size, weight, and price do not allow the use of UAS in
the models, and it is necessary to perform the aerotriangulation without knowing
the exterior orientation elements. The most commonly used method is to determine
the elements of internal orientation using GCPs (Ground Control Points) with
known X, Y, and Z coordinates.
The position accuracy of the GCPs is the most important factor that affects the
accuracy of the final aerotriangulation. Measurements using a standard GPS device
or subtracting the coordinates from a map can be sufficient when working with
small scale images. However, for high accuracy, Small Format Aerial Photography
(SFAP), a total station, or GPS device with dual-frequency correction data must be
used to focus the GCPs.
The elements of internal orientation can be determined either in special labora-
tories or by self-calibration. While aerial cameras are usually calibrated by the
manufacturer, the calibration protocols for small cameras used in SFAP applica-
tions are not known.
In this study, the camera calibration was performed with the PhotoModeler and
iWitness software. Several types of calibration were performed, including both a
single calibration grid sheet and a multi-grid calibration sheet (Table
1
).
