I just started my Reefmaster trial and had a couple questions:

1. I have a Lowrance Mark-4 HDI w/ HDI Skimmer Transducer – for the purpose of just logging depth and bottom composition data, is this a good combo? Any suggestions for a better combo (reasonable price) that would provide better results in recording the raw sonar logs?
2. I have access to a survey grade RTK GPS unit (Trimble R8) and was planning on doing live logging with the NMEA streams on a laptop. Is there anyway to also get the bottom composition using this process? Would I also have to store a raw log on the device and combine that data to the map later? Would it be better to have a fishfinder that takes the NMEA in from the GPS unit and then record the raw sonar log so that the bottom composition would have better horizontal accuracy?
3. I have several bodies of water I am planning on mapping, from a small 80 acre to a large 1,500 acre reservoir. I am planning on setting up an 80’ x 80’ grid and traveling around 5 knots. Is this a good strategy for collecting the above data? I want the best balance of time / quality data.

Thanks for any help - I have been researching this quite a bit and can not wait to start mapping.

Not sure that the R8 will fundamentally benefit you other than to give you calibration checks for the HDI GPS unit. A setup is only as accurate as the least accurate component. In your case each HDI sounding is measuring a shallowest depth over a relatively large area (compared to the accuracy of the R8).If you acheive centimeter accuracy positional data in 3 dimensions then surely you would want the depth to relate to that exact position - or why bother to go to all the effort ?! You have now started running into the problems of determining the aim of the transducer etc as well.
I fully understand your desire for perfection though, and you will be stepping the overall accuracy up a notch - though perhaps not as much as you hoped !

T

Thanks for the great insight. I am familiar with traditional land based surveying, but hydrographic surveying is new to me. I have an older Airmar DT800 (235khz) that I may use instead due to the narrower beam width for my bottom depths and also run the HDI unit to capture the raw logs (bottom composition).

One thing you might consider is running your high-precision GPS through your HDI. I'm not 100% sure you can do it with that unit, but most of the units will take (and use) a GPS antenna that outputs NMEA 0183. This would give you the best of both worlds - full sonar logging with higher accuracy. It's certainly worth running with the narrower transducer as well.