Since the staring of GPS, many researchers have investigated its application in aerial photogrammetry. Today, with the full constellation of 24 GPS satellites operational, enabling excellent satellite geometry any time of the day, the need to apply the full potential of GPS for real time aircraft navigation and photogrammetric mapping can be realized. The use of GPS to determine relative positional data for ground control points in a photogrammetric block adjustment is widely accepted and practiced. The camera exposure station coordinates derived by Airborne Kinematic GPS drastically reduces, the number of horizontal and vertical control points needed in aerial triangulation.

In large-scale mapping, the accuracy level of control data required is very high. The lag in time between the camera exposure and the GPS epoch recording in the GPS receiver is critical in deriving accurate coordinates for the exposure station (principal point) coordinates. Due to delay in the electronic transfer of data from camera clicking to GPS receiver in recording the event makes the Lag in time to occur. To meet the high accuracy requirements for the largescale photography and mapping projects the lag in GPS recording time should be derived and applied. In this study, an attempt is made to compute the Lag-time in airborne kinematic GPS derived exposure stations from aerial triangulation.

Aerial triangulation is carried in Digital Photogrammetry work station with conventional method of using ground control points, and the exposure station coordinates are derived. Lag-time is computed by finding difference in coordinates of exposure stations derived from conventional aerial triangulation and from airborne kinematic GPS. The results of this project will help to improve the locational accuracy of GPS derived exposure stations in aerial triangulation.

Aerial photography is carried out in the study area on 1:6000 scale with forward overlap at 60% and lateral overlap at 20% using RMK TOP30/23 camera. During aerial photography the airborne GPS is operated to record the exposure coordinates.

The computer controlled navigation system (CCNS) is loaded with flight planning data from World Wide Mission Planning (WWMP). During aerial photography CCNS takes coordinates of aircraft position from navigation system and navigate pilot for alignment as per flight plan. Based on navigation coordinates, CCNS sends signals to camera for exposure. The camera exposure system is connected to Trimble 4000 SSI dual frequency GPS system on board, which records GPS data continuously at 1.0sec sampling rate. During the camera exposure, camera system sends signal to the onboard GPS system, which record each exposure as an event marker in the GPS data. The GPS data is processed along with ground reference GPS data in differential and the camera exposure station (Principal point) coordinates are derived.

The exposed film is processed using Versamat processor. The total number
of photographs accepted for stereo coverage of entire area is 323 in eleven runs/strips. Each photograph is uniquely numbered with project number, run number and photo number. Photo index is prepared with relative location of different runs and photographs for easy handling.

Ground control survey is conducted such that with out exposure coordinates the aerotriangulation could be carried out with required accuracy as per the preplanning. The data collected and processed in lab using the Leica Geo office software and computed the co-ordinates for 51 GCPs in WGS 84 system.

The aerial negative film is scanned in high precision photogrammetric scanner at 16-micron resolution. Each photograph is stored in separate file of size 230 MB in TIFF format.

Photogrammetry project is created in digital photogrammetry system with software SOCETSET. The scanned images are imported in to the project with support files and pyramid layers are generated. These pyramid layers are useful for quick display of images during the various processes. The flying direction for each run is incorporated as per the flying report given by the aircrew.

Interior orientation is done by AIO –automatic interior orientation for 280 frames and 53 frames are oriented manually. The rate of AIO success depends more on the image quality like contrast and sharpness at the edge of frames where the fiducial marks are registered. The maximum residual accepted in Interior orientation is 0.5 pixels.

The aerotriangulation and block adjustment is carried out with ORIMA – the orientation management software. The Ground control point coordinates are also imported. The APM – automatic tie point measurement is executed. Total 4800 tie points are collected through this program. The connection between the projection centers and tie points are graphically checked. It is observed that the connections between consecutive photographs are sufficiently generated whereas between runs it is not so. This is because of failure of image correlation technique in relief displaced thick vegetation areas like forest. For increasing the stability of block in bundle block adjustment, one hundred tie points are manually measured. Then Common Adjustment Program in Aerotriangulation (CAPA) is executed for bundle block adjustment. The adjusted coordinates for tie points and projection centers are recorded in separate files.

The RMS changes at ground control points are 10 cm in X, 8 cm in Y and 8 cm in Z directions. Figure 2 shows the flow chart of the work. The projection center coordinates derived by aerotriangulation with only ground control points and the exposure coordinates derived from Air borne kinematic GPS are compared and the differences along three directions are computed. The summary of results are tabulated in Table 1 and graphically
represented in Figure 1.