How fast is fast? What unit and transeducer? What sort of depths? Do you intend to only make depthcharts, or bottom composition as well?

I am only making depth charts at this stage as not even sure what the bottom composition function is?

Basically wondering if you travel at 10knots or 40knots will there be a difference in amount of information logged? I have no idea how fast the sound echo travels etc. (sorry if this is stupid question)

We had a glass out conditions at sea last weekend and thought id log some data and in order to maximize my time could travel quickly to cover more area, which just got me thinking...

Bottom composition module:
This allows you to map bottom type in adition to depth, ie your map will show soft or hard bottom as well.

If your gear is good enough, and properly assembled and adjusted, you can log data in 40 knots without problems. (Hence my questions.)

If you want finer details to stand out on the final map 40 knots is way too fast IMO. I would never map faster than 6-7 knots, preferably slower (4-5) as I want usable data for hardness/bottom composition. But it depends on many things as Colibri says. A small boat's (such as mine) angle changes a lot depending on speed, a larger boat should be more stable and therefore allowing higher speeds.

Trial and error will give you your boats optimum mapping speed.

At the speeds you suggest you will certainly miss a lot of data. I survey at 2-3kts, which is conveniently a reasonable trolling speed too. I gather sidescan data as well as bathymetry data, and a slow speed ensures that important structure is properly scanned.

If you don't want to miss critical point depths its imperative that your speed is regulated. Your sounder may well be pinging at a very high rate - I seriously doubt your gps is providing a position for every one of those pings though.

T

Sonar pings rarely sync with the GPS but that doesn't really affect position so much...on most modern GPS devices, that is interpolated and corrected through other sensors.

One consideration is the depth of the water because at some depths, you may be forced to adjust your ping rate. The speed of sound through water is ~1484 meters/second. Usually with most tone sonars, this needs enough time to go there and back twice to account for the second echo since the transmissions are a single tone. That may sound like a lot of leeway but it really would affect your transmission speed.

Rough Max Ping Speeds
20m - 18.5 HZ
40m - 9.3 HZ
60m - 6.2 HZ
80m - 4.6 HZ
100m - 3.7 HZ
200m - 1.9 HZ
300m - 1.2 HZ
371m - 1 HZ

Even assuming that your boat is traveling level, at 40 knots, you could place, at most 1 data point every 20.6 meters in 371 meters of water and 1 data point every 1.1 meters in 20 meters of water.

With a variable frequency transducer, these problems are much less of an issue.

Here are some articles that might be of help.

vexilar.com/info/finding-hard-bottom-article/
oceanexplorer.noaa.gov/explorations/09be...ibeam/multibeam.html
www.nauticalcharts.noaa.gov/hsd/SSS.html
www.yachtingmagazine.com/electronics/chirp-technology

Greg,

I think I'd better send you some real world examples........ :)

T

AnglingCharts wrote:
I'm curious on the examples. From my experience, it's more an issue with precision than update frequency.

I have made maps with contours going 34 knots in 2-300 meters depth without problem with my small boat, but my boat is very level at this speed and my hull sends very litle disturbed water towards my transeducer. I use several Lowrance HDS (both Gen 1 and Gen 2), combined with a Point-1 GPS-antenna. My ducers are currently the OEM 83/200 and OEM 50/200

My other boat is much larger and heavier, and does not "flat out" in plane like my smal boat. Same electronics as above, but with different ducers. (Airmar P66 and LSS-1, a TM150 is on it's way). Since this boat has a very different angle on the water then my small boat, I don't use this for mapping while in plane.

Really valuable information Gregory. Thank you

For example, SLG/SL2 formats internally have a precision of 1 Mercator. If you are going slow enough, it will appear that you have had multiple sonar pings taken from exactly the same location.

Formats like XTF use a double precision X/Y in either Geographic WGS84 or UTM.

Most government transfer formats are glorified XYZ formats or CSV formats.

Kongsberg Multibeam formats use a timestamped geographical position and the geographical position at every multibeam transmission must be interpolated by the format processor to obtain an accurate position.