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3.2 TIME/SPACE OFFSET MEASUREMENTS
Coordinate differences between the GPS antenna phase center and the center of the entrance pupil of the camera lens
(entrance nodal point), in a coordinate system parallel to the photo coordinate system, are defined as the "spatial
offsets". These offsets were determined for the systems investigated here by the following procedure. First, the
nominal pitch attitude during flight was re-established on the ground and the aircraft stabilized. The camera was then
leveled and the swing set to zero. A simple laser device, oriented to vertical, was then located below the camera and
adjusted to point at the center of the aperture of the camera lens. This position was marked on the pavement and
subsequently located by GPS methods. This provided the local rectangular horizontal coordinates of the camera
entrance node. For the vertical component, the vertical distance was measured to a tangible point on the camera which
was related in distance to the entrance pupil. This distance was supplied by the camera manufacturer. These
measurements provided some components of the vertical spatial offsets. With the aircraft stabilized, GPS observations
were made by the aircraft system thereby provided coordinates ofthe aircraft antenna phase center. Provided the spatial
offsets in the horizontal were within a few centimeters, the differences between the phase center and the entrance node
were measured and provide the spatial offsets to within a few millimeters in a nominal operational environment.
The offset in time was the difference in time between the event mark generated by the camera and the effective time
of exposure. Modern cameras provide event markers as an electronic pulse at the mid-point of open shutter. Other
cameras can be so equipped. For purposes of this investigation, the effective time of the event mark was measured by
a device placed in the plane of focus and measured the first and last point of light to an accuracy of about ten micro-
seconds.
3.3 OHIO DEPARTMENT OF TRANSPORTATION EXPERIENCES [ODOT]
ODOT has long been interested in calibration and test of their airborne photogrammetric systems. The first test field
was established in the late 1970s and at the time of this investigation consisted of three fields located in Ohio. The first
is for low altitude missions and is flown at 1370 meters above ground, the second is for mid-altitude applications and
is flown at 3000 meters, and the third is for high-altitude applications and is flown at 6100 meters above ground.
The aircraft used by ODOT is a light, twin engine, open ported, Partenavia "Observer" and is shown in Figure 1. The
camera is a Zeiss LMK 15/23 on a stabilized mount. Approximately 40 well-distributed target images appear on each photo.
Figure 1. ODOT Open Photo Port Partenavia Preparing to Fly the Madison Ranges
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 573
