... Here, underpinned by land surveying, levelling in a tide gauge (6m LAT to HAT in Plymouth, UK), and measuring a 2.5km line both by Total Station and by two Post-processed Static GPS points (with a local CORS Station) to ensure practice with Line Scale Factor
cool
... and two from our vessel, Falcon Spirit; inside the cabin with a single beam echo sounder at the far window with the near student concentrating on the software, QINSy; and the back deck with a deployment frame and a Van Veen grab. The samples from this Second Year field week are due to be used to annotate a seabed texture chart gathered with a mini-side-scan by First Year students (and plotted in ArcGIS, in May).
Thanks for posting. What you are doing (and teaching) is so far different from what I did in my career that it is almost another profession. That's one of the things I like about this site. I may have no experience in what is being discussed but I am being educated almost daily.
Andy
Victor
Hi, would like to know what your average differences in your depth readings for cross line intersections?
You are using RTK for point positioning?
Not many practice bathymetry survey here in this forum so always good to find someone with similar work.
FrancisH, post: 364367, member: 10211 wrote: Victor
Hi, would like to know what your average differences in your depth readings for cross line intersections?
You are using RTK for point positioning?
Not many practice bathymetry survey here in this forum so always good to find someone with similar work.
Hi, Francis
A good question, ie I have been pondering how best to answer! This is my first attempt but IÛªll have to have a second go. My principal difficulty is that I donÛªt have much data to answer your question directly. Nevertheless, here are some elements:
I teach single beam echo sounding (SBES) and my colleague multi-beam (MBES). I use a 200kHz sounder with a 9 degree beamwidth, calibrated for draught by bar check and for speed of sound by a velocimeter cast (+-2cm at the instrument).
We have a predetermined Chart Datum (from the UKÛªs Hydrographic section of the Royal Navy; set close to Lowest Astronomical Tide), and it applies to the area (5km*5km in which I work (Plymouth Sound)). I have local, precise tide gauges which give relevant corrections (<+-3cm at the gauges; <6km from the survey area) related to Chart Datum.
I use Satellite Based Augmentation System (SBAS (like WAAS, but in Europe, EGNOS)) corrections, to about +-5m horizontal (I do have access to RTK GPS (+-3cm hz & +-5cm v) yet I use this for other activities not the SBES work). I work at the small scale of 1:10,000 and thus commence with a taped sensor offset survey. I do have a full Dimensional Control survey of our vessel but I use neither this nor our Motion Reference Unit.
All this because my SBES field work is the initial skills week; my colleagueÛªs MBES is the professional-level week.
And Û? all of the foregoing to say that I donÛªt have direct comparisons on my cross-lines.
Yet, according to the IHO S44 specifications ( http://iho.int/iho_pubs/IHO_Download.htm ), I ought to be obtaining depth accuracies of about 0.25m in the 12m water depths of Plymouth Sound and, with a wide set of uncertainties, +-0.5m +-0.09% measured depth (Û? +-0.6m) within 80km of a tide gauge. See Figure 3.3, p165 in Chapter 3 of C13 (available as above).
So, IÛªm going to ask the current students to extract particular values at cross points (this wonÛªt happen until after the Easter vacation, in May). IÛªm going to ask my colleague for values derived from MBES. Sorry I cannot answer you immediately.
Vic
That's ok Vic,
That's the main difficulty that I have with regards to bathymetry work. I get vertical positions for each 1 second sounding point via RTK or most of the time PPK if distances are more than 2km from nearest base.
Base is positioned on nearest MSL benchmark with an automatic tide logger recording at 5 minute intervals. The tide gauge data is used only to get MLLW value for the duration of the work.
For large area, I usually have an overlapping run/swath. The crossing lines usually done on different days would sometimes have differences less than 20cm-50cm. These are from fixed post processed/RTK positions.
I have always wondered if these are inherent errors or there is something wrong in the GPS/sonar readings.
Hi, Francis,
Interesting that you use PPK when you're greater than 2km from a base; for our RTK, we have a low power transmission (0.5 Watt at the antenna, to stay within a licence-free band) but can get about 10km.
I didn't get a flood of responses from my students. They're happy getting answers by email on a weekend but not to give me feedback to my own enquiries!
Anyway, one quotes me a single main survey line/ cross survey line check with an offset in depth of 0.25m. The actual sounding may have been coincident in northing & offset by 4cm in easting (but actually with a far greater uncertainty from SBAS and, if they were this close, near complete overlap in the insonified footprint).
Not exactly a statisically sound comparison, but something to go on and fortunately in line with the IHO expectations.
No reply from my colleague either ... ploughing a lonely furrow!
Vic
