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.

Thanks guys, will trial out different speeds and see what happens.

Im using a thru hull b148 600W airmar transducer (new HDI quad frequency transducer)

AnglingCharts wrote: +1 I do the slow speed to get all can interesting, I use a AIRMAR TM258, and get side too, results are very good and realitics from diving! Never test at great speed, can be less good, at turning tranducer won't be straigth down, can be good for new place to get a first map.

GPS Update rate:

Can anybody outline the GPS data update process and estimate the max frequency/period updates.
At some speed it could happen that two points separated by some distance would be logged at the same Lat/Long because the GPS update process had not completed. It does seem apparent from the discussion so far that it is NOT a problem for most speeds us mere mortals are likely to travel in a boat e.g. 40kts or less so this is a curious question.

Our tests indicate that GPS positions (certainly in Lowrance units) update at a maximum of 1Hz, regardless of how often the GPS antenna is updating (we have tried with 10Hz antenna). So to answer your question, there are definitely multiple pings with the same lat/lon. We handle this by waiting for a new position before creating a new data point. Whether there is intra-file latency between position and depth is another question.

Matt

So why do they take 10Hz Rates Receivers ?
Or do you mean the stored Position is only at 1Hz ?
But on Screen you will see better results by driving at fast Speeds ?

I can't comment on what they show on screen (I don't know), but everything we have tested with 10Hz receivers still only shows updates in the logfiles at 1Hz.

This is in swedish, but the film shows Point-1 (10 hz)connected to one HDS Gen 2 Touch screen and the other screen running the internal antennae (5 hz). He runs through how to update the Point-1 before the test, so be a bit patient. :)



Keep in mind that when using a 10 hZ antennae on older units (like HDS Gen 1), the updates will still be the same as without the faster antennae. It is only HDS Gen 2 and newer that can use the 10 Hz speed of the Point-1. If you want Matt, I can send you logfiles from Simrad NSS Evo2 wich has the same internal antennae as HDS Gen 3, ie 10 Hz.

That would be very useful - thanks!

Sure thing, I'm going out with the boat on Sunday and will send you the result.

Lowrance HD Touch gen2 updates the heading and position on the nav screen with high speed if you supply it with hi speed data, in my case from a GPS compass over NMEA 0183 at 10Hz. The logging of the nav data for each ping in the SL2 file seems to be done at 1Hz and heading isn't logged at all even though it tags it as valid?!

For normal side scan speeds at 4 knots (2 m/s) a 10Hz data rate means that the position changes 0.2 m for each position update but with a resolution of only 1 m it will only detect a new position after 0.5 sec.

Below is a comparision with position from the SL2 file and the raw position from the GPS compass. The GPS uses SBAS and should be within ±1 m 95% of the time. A value of ±1 m doesn't mean that is jumping around ± 1 m with 10Hz when you are at standstill, the tests I've done at standstill shows that it stays around ± 0.1 m in short term (60 sec) and in long term it could of course move away some more.

Another thing that you should be aware of is that the heading data loads the Lowrance HD Touch so much that it lags with the ping positions in the SL2 file in sharp turns! That means that the ping positions is wrong! About 5 seconds after the turn is has recovered and gives the pings correct positions again.

Raw data from the GPS



Data from the SL2 file



/hakan

Great Infos, Thanks.