HAND-PRODUCED TOBI MOSAICKING

The reasoning for the production of unprocessed TOBI mosaics is twofold: 1) to use the mosaic as a quality control device, 2) to begin familiarization with the data and, if appropriate begin initial interpretation.

The production of the scaled prints for mosaicking follows a sequential order as detailed below. Prior to production the working scale of the final mosaic should be determined and the width of the 6.0km TOBI swath determined for that scale (in this case, a working scale of 1:60,000 meant that the swath width would be 100.0mm).

1.

The MO disk header information, and start and stop times are checked using the program "REPTEST".

2.

The distance between successive 1/2 hour periods along the TOBI track line at the required scale are determined either by the use of software or direct measurement from the track plots. The processing PC has software that requires the input times for each file to be between full 1/2 hours marks, e.g. if a data file begins at 23:16, the first input value would be the distance between the 1/2 marks at 23:00 and 23:30 hours. This is also the case for the end of file. An ascii data file is created for each of the TOBI MO disks (usually 16.25 hours in length), labeled to match the MO disk number. i.e. MO disk No.197 would have an ascii file created called "197.ana"

3.

The raw sidescan data are downloaded from MO disk to the processing PC and scaled at the same time using the programme "ERASDISK" and the ".ana" files for subsequent replaying. A number of parameters are entered such as file name modifier and the offset into the file in minutes, the gamma and gain values (affecting the brightness and contrast of the final print, for this cruise gamma=0.6, gain=1.0), and finally the desired swath width in mm. Scaling is achieved simply by the repetition of lines of sonar data until each 1/2 hour time stamp is the same as in the ".ana" input file. The output from ERASDISK is written to the PC hard disk as special ".scl" files. As the programme reads the data from the MO disk and scales it appropriately, the final sonar image is displayed on the PC monitor as a waterfall display.

4.

The final scaled sonar data are then replayed through a Raytheon thermal line scan recorder using the programme "DISCCRAY".

5.

The TOBI 7.5kHz profiler data is replayed using the programme "PROFRAY". This software reads directly from the MO disk, and uses the encoded depth and altitude data to correct for variation in tow-cable length. Required inputs include the length of time required to be replayed and a "decimation factor" (roughness of the seafloor topography).