I was looking at this file description and also this file description for SL2 binary format.

I was curious exactly what data is being used by Reefmaster? Are you only using the position (offsets 104 & 108) and water depth values (offset 60), and then applying the TIN algorithm to the depth values? Or are you actually decoding the sounding packet as well for additional information? The sounding packet size is described by offset 34, but the packet data itself is found at the end of each frame and varies in size.

What about for side-scan bottom imaging. For that, you utilize the SLG files, correct?

Thanks in advance for any answers.

For depth mapping we use the position and the calculated depth value in the file - we do not try to determine the depth ourselves from the sonar data.
For sonar viewer/sidescan mosaic we do of course use the sonar data.
SLG files only contain a single channel, whereas SL2 files can contain multiple channels. Sidescan is only ever recorded to SL2 files.

Thank you so much for the response.

How is the time value parsed from the binary file? It seems to be a weird integer value, that is not UNIX time, eg seconds since epoch 1970. Thanks for any help you can provide.

Time is only stored as milliseconds since the start of the log - there are no absolute times.

If there is no absolute time, how are you cross-referencing to tidal files? Also, in the track list, you show a precise time next to the track name. Where is this found? Is this simply the "file modified" time, or is there something in the header of the SL2 file that describes start time?

If it is the file modified time, and you "touch" the file and lose that date, you would no longer be able to apply tidal information correctly? Also, the modified time would be when the file was closed, so you need to work backward using the milliseconds?

Yes, this is more or less how we do it. There is no recorded start time, so - as a starting point - we subtract the total log length from the file modified time to derive an absolute start time. This often works, but the recommended procedure is to make a note of the starting time of each log (especially if tide offsetting is required) and then manually verify (and update as required) the track start time after import into RM. This will then adjust the absolute times of each data point.

Regarding recording start time (or other absolute time mark), Matt are you sure Navico hasn't added this, perhaps recently?

I have never looked at the raw contents of the SL2 files myself, but this message in the Gofree web upload gives a hint that they actually may now store some sort of absolute time:

"The latest firmware is required to correctly process your sonar logs and marked them with the correct date and times "