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GPS CONTROLLED TRIANGULATION 
OF SINGLE FLIGHT LINES 
Ayman Habib, Kurt Novak 
Department of Geodetic Science and Surveying 
The Ohio State University 
Columbus, Ohio, 43210-1247 
Abstract 
Aerial triangulation controlled by GPS observations in the aircraft has been established as a precise method 
of photogrammetric point determination without the need of ground control. New developments of kinematic 
differential GPS processing yield accurate exposure locations instantaneously. If the GPS observations are available 
for blocks of aerial photos, the aerial triangulation can be carried out without any ground control points. 
Unfortunately, this method cannot be applied for single flight lines, since the GPS observations do not recover the roll 
angle of the aircraft. Therefore, ground control is mandatory for GPS controlled strip triangulation. 
This paper investigates GPS controlled strip triangulation using known, linear features on the ground that 
are approximately parallel to the flight line. Point to point correspondence between the linear feature on the ground 
and in the image is not necessary. This described technique models the linear feature in the images by low order 
polynomials and forces the known line on the ground onto this function. Thus, the roll angle can be determined. In 
this paper, we investigate the effects of different GPS measurement accuracies both in the air and on the ground on 
the results. Experiments using simulated and real data are presented. We also show that this new technique is useful 
for mapping railroads. Our tests verify the practical feasibility of GPS controlled strip triangulation with known linear 
objects on the ground. 
KEY WORDS: GPS controlled aerotriangulation, Strip triangulation. 
1. Introduction 
The implementation of the NAVSTAR Global 
Positioning System (GPS) changed the way in which 
photogrammetric images are collected and aerial blocks 
are controlled. There are many different applications 
for GPS in photogrammetry (Ackermann, 1992-a), such 
as: 
Precise photo flight navigation, 
Pin-point photography, 
Positioning of the perspective center for 
aerial triangulation, and 
GPS positioning of ground control points. 
For the first and second applications real-time 
differential GPS based on C/A-code or P-code pseudo- 
range observations is required. Accuracies of 1 to 5 
meters are sufficient in this case. The trajectory of the 
plane can be calculated continuously by observing at 
least four satellites in the aircraft. Pseudo range 
measurements are corrected by observations transmitted 
from a base station receiver. The computed position of 
the airplane can be displayed on a flight navigation 
System, The comparison of the airplane's actual position 
With the planned flight line yields corrections which are 
presented to the pilot or directly fed into the auto pilot 
System, These flight navigation systems permit the 
precise execution of a flight plan and the automatic 
exposure of aerial cameras. 
The other two applications rely on more precise 
positioning, but do not require real-time updates. This 
means that both pseudo ranges and phases must be 
observed, in order to compute the position of the GPS 
antennas to within a few centimeters. The last 
application (control point survey with GPS) pertains to 
geodesy, and will not be discussed here. 
The combination of GPS and photogrammetric 
measurements have been widely used for aerial 
triangulation to reduce or even eliminate ground control 
points. Lapine (1992) reports that aerial triangulation 
can be done without any ground control provided that 
the satellite signals are not blocked during the flight 
mission. However, there are a few problems that need 
attention in GPS aerotriangulation. Among these are the 
calibration of the GPS antenna offset, the camera time 
offset, the initial phase ambiguity, signal interruptions, 
and datum problems (Ackermann, 1992-b). 
The GPS observations made during the photo 
flight in the aircraft refer to the phase center of the GPS 
antenna. For aerial triangulation, however, the position 
of the camera's perspective center is needed. The offset 
between the GPS antenna and the perspective center has 
to be determined through a calibration procedure before 
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