Hey, the unit you are talking about is a fishfinder/sounder only, I think you can plug it into the furuno gps modules like the gp320 maybe? and it will show lat and long, but it wont have charts.  As a sounder they are probably the last monochrome (b&w) furuno sounder.  I have not personally used one, but have heard for a b&w unit they were not too bad.  The 50/200 selection is based on the depth you are fishing, but you can run both in split screen.  Generally 50kHz for shallower stuff and 200kHz for deeper, the reason being the 50kHz has a wider beam angle, so it will show a bigger 'footprint' under the boat, but lacking penetration and the 200 has a narrow beam angle for more penetration and less ground shown on the bottom.

https://www.whitworths.com.au/pdf/32450_LS-4100_Brochure.pdf On the front page they are running it in 50/200 split screen and you can see what I mean even though it was in 22mt.

Bryan

Hi Fishguts,

The Ls4100 is a great entry sounder.  When bottom fishing, use the bottom lock function. 

Chris

 Sorry forgot to add tjat in that depth water, I would personally use 200 Khz.

as ctan1968 said that depth definately 200 Khz is the go at that depth,

sarcasmO has it part right but 200k is for shallower work it has less pentration than the lower frequency and the narrow cone angle and smaller "footprint" of 200khz will give a clearer image of the ground you are looking at. 

The 50 Khz for deep water and it has a wider cone angle giving a larger "footprint"  That means you end up with a lower quality image of the bottom and you probably not going be able to pick up little ledges etc. So if you had a split screen going and you can see fish on the 50K screen but not the 200K you would know your close to them but not running over them. If you get the drift