GSF-reader now working!!

Okay, so I am really excited because my GSF-reader that I coded up in Matlab is now working!! It is not fully-functional yet (I still have to add some sonar-specific readers, and some other optional record types) but it works for one of the sample CARIS-generated GSF files I have.

One of the things holding me up was the fact that I did not realize records were padded with extra bytes to ensure the record size in bytes was divisible by 4. I am not sure why it matters if it is divisible by 4, but apparently it does to GSF.

My next issue to tackle is a Reson sonar-specific quality flag indicator that is written in bits, not bytes, and uses all kinds of bit shifts and masks (joy!). This record is no longer used, but some older GSF versions will include them. Once that is tackled, some of the other GSF files should start working as well.

Also, if I want others to be able to benefit from this work, I should probably eventually convert it over to Python. Having a Python-based GSF reader would be pretty sweet.

There already exists C-code of course that does all this, but I want to have something that generates separate records for the data so that I can play with the ping depths for example, or the backscatter intensities, in a familiar environment. The C-code is really written so that it can be incorporated into other programs. By writing a reader myself, I not only gain a better understanding of the data and how GSF stores them, but I read them into a program where I can readily perform mathematical analysis on them.

Lidar data in Caris pt. 2: waveform view

I have been looking at more lidar data in CARIS today with one of my committee members and am now starting to really figure out how CARIS displays the data. In my previous post I mentioned that the peak with the green line represents the determined depth and the peak with the red line represents an "alternate depth." This "alternate depth" designation in CARIS is somewhat misleading, as this peak actually represents the small amount of energy from the green (532 nm) pulse that is reflected by the water surface. The reason that the peak appears larger, in some instances, than the bottom return is presumably due to the gain settings being amped up so a detection could be made.

Looking at the image below, this does seem to make sense. In this dataset, 3 peaks can clearly be seen. In CARIS, the 1064 nm and 532 nm returns are shown on the same graph, with the first x number of bins coming from the IR return, and the remaining bins coming from the green return. The first unmarked peak represents the IR return from the water surface. The red marked peak is the 532 nm surface return ("alternate depth") and the green marked peak is the 532 nm bottom return. If you look under the lidar tab, the depth for the "alternate depth" is 0.16 m, which becomes negative (out of the water) once a tide value is applied. I am assuming that this depth reading is due to the fact that the green surface return, though nominal, would still have a slight penetration through the water surface. It could also be related to the difference between the IR detected distance to the water surface and the green detected distance to the water surface.

Lidar Data in CARIS HIPS and SIPS

The new version of CARIS HIPS and SIPS (v. 6.1) can now bring in SHOALS and LADS lidar data and waveforms for quality control and to merge it with multibeam data. The import procedure is pretty painless, though there still seem to be a few bugs to work out.

I tested it out with some LADS data I have from Portsmouth Harbor. You can see the selected data points highlighted in yellow in the plan view. The point highlighted in blue is the current "super-selected" point, and it is for this data point that CARIS will display the waveform. If you click either "next" or "previous" in the lefthand lidar menu, the "super-selected" point will move to the next yellow highlighted point and you can view the waveforms for each of the selected soundings.

The waveform box shows the surface and bottom returns for the green waveform. The green line represents the detected bottom depth for the waveform, while the red line represents a possible alternate depth. Unfortunately, there is no way to display axes (neither time nor intensity) or any type of scale on the waveform in order to get a better sense of what is being displayed. This is something CARIS will hopefully correct in later versions or hotfixes.