Involving

Professional Interest Groups

 

Hydrography

Hydrographic Survey

Multibeam

 

Introduction

There is a tendency with the increasing coverage, resolution and accuracies of multibeam echosounders to dispense with the old hydrographic rule of always conducting crosslines as a quality check on the survey data. Indeed, both clients and survey contractors are questioning the need for crosslines. The first from the point of view of time and cost savings, and the second from…., well…, I am not quite sure. Contractors would surely like crosslines, more time on the job, more chargeable hours or days, but perhaps it’s for competitive advantage, deleting crosslines makes the bid look better, or just maybe it’s because of what crosslines may or may not discover in the quality of their work.

Survey contractors have used the argument that crosslines are unnecessary because the overlap of adjacent swathes detects all the errors crosslines are intended to find, or they claim their calibration and processing software and procedures have removed the need for crosslines.

Clients question whether they are really necessary and put the pressure on to reduce costs.

But where does the truth lie?

In order to gauge an industry opinion I placed the following topic on three LinkedIn discussion groups, Hydrography, Hydrographic Survey and Multibeam.

Cross Lines or Not:

I am increasing seeing a difference on the question of doing cross lines during a multibeam bathymetric Survey.
One camp says - Not necessary: The overlapping swath is sufficient to detect VoS errors, tidal error, alignment
or pitch and roll errors etc. etc.
The other camp says - Must be done: All the possible errors can not be bowled out by simple overlaps, certainly
not by just allowing the outer beams to overlap.
So, what is your learned opinion?
To do cross lines or not?

Richard Cullen said it succinctly as opening batsman for the Hydrographic Survey group.

“With the repeatability of multi-beam and better accuracy, some may move away from cross lines on large survey areas as they can uncover quite dramatic busts. Depending on how you run the survey, the correlation between swath overlaps on consecutive lines could look absolutely fine but when compared to a cross line, you can evaluate more hydrographic factors such as tide, topographic and tidal stream induced errors.”

Throughout the discussion it was obvious that the greater majority supported the use of crosslines and Stuart Raisey-Skeats clearly gave the bottom line.

“Belt & braces. If you do them, you have additional data to help resolve errors/problems. As Alexander the Meerkat says, ‘Simples’.”

Yes it is simple, but yet necessary. So necessary in fact that all the regulatory authorities still have them in their survey standards (see the references in the Appendix). But is it really necessary, or are the standards simply a hangover from the old days of single beam sounding?

The opinion from the discussion was overwhelmingly in favour of crosslines for many technical and quality reasons. In fact a persuasive argument against them, or that a mere overlap satisfies all necessary quality checks, was not presented, and nor do I believe it can be.

The few comments that did not see the necessities were overwhelmed by the aye’s and the major argument against seemed to be time and cost, not for a technically qualified reason.

 

So, What Do Crosslines Achieve?

Matt Geen seemed to have most of the answers.

“Some overlap is always a good idea, of course, to avoid getting gaps between lines. But the overlap only tells you what the far range or outer beams are doing, so it won't tell you much about speed of sound errors, for example.”

Though, the VoS element can be well covered by frequent SVPs, actually doing them seems to be the least popular choice (commercially that is), particularly in deeper waters.

Matt continues:

“In contrast, cross-lines give you:
the ability to compare along- and across- track, so you can see across-track distortions
the ability to compare data from different times in the survey, so you can see time-dependant errors, such as tide.

And all of this is on top of your lines run for patch-testing, which is another, but related, topic.”

“The crosslines are mostly useful in post processing, but I would advise doing at least a first processing pass while you are still at sea, in any case.
Here's how you check a crossline:

1. Choose the area where a crossline happens.
2. Create separate grids (DTMs) for the each of the two crossing lines
3. Subtract one grid from the other
4. You can immediately see from that:

a. Across-track bias errors (roll offset, speed of sound errors, etc.)
b. Gross error (tide, GPS height): find the signed average difference
c. Total vertical uncertainty (TVU): calculate standard deviation of the differences: you can then check your IHO S44 compliance.”

A good synopsis from Matt Geen and a clear direction for the need for crosslines. Their importance was seen by many others as well, Andrew Ternes commented that;

“I believe crosslines are an important tool for picking up errors in known problematic areas. I have seen outer beams curling up due to a fresh water layer in a river environment. The maximum we have detected is 1.2m (20-25 m/s difference in the water column in the first few layers.)  They are also good for detecting height shifts with a RTK correction (I think these are called cycle slips, the system says its in good quality but the height could be up to 20-30cm different.)”

Matthew Simpson liked the idea that crosslines provided redundancy, a concept which every good Droggy had beaten into him when he was a newt, .

“If there is one thing I have learnt as a professional Surveyor, it's "redundancy", in order to facilitate "checks". In my opinion, crosslines are fundamental and imperative.”
“Overlapping adjacent lines may show an issue (e.g. a tide issue) if you happen to conduct adjacent lines at different times in the tide cycle (e.g. one line at high tide and adjacent line at low tide) but what if you run adjacent lines from one side of the project to the other throughout the tide cycle. You will not see a significant error. Run a crossline over lines run throughout a tide cycle and you will see an error if there is one.

Crosslines are surveying 101 as far as I am concerned.”

Absolutely agree Matthew, being a well beaten Droggy I relate to the fundamental imperativeness of the need for redundancy, something the mere overlapping of lines does little to achieve in any realistic QC on the data. Rob Milne received a similar beating.

“I had x-lines beaten into me as a young droggy. They are an invaluable tool for ensuring the viability of your tidal model and gross error checking.”

Rob followed this up with some straight forward and sound advice.

“Too often a Client's lack of understanding of what can be done and, more importantly, what can't be done during the processing phase leads to catastrophes back in the office. My rule of thumb, for what it is worth, is "if in doubt, reshoot!"”

And doubt is what minimal overlaps as a QC tool leaves behind. Arguably a minimal overlap won’t even detect smaller smiles and frowns because you will be comparing the smiles/frowns on adjacent swaths, so no difference, though the resultant DTM looks like a ploughed field which should ring alarms anyway. Overlaps will detect such errors if the outer is overlapped with the nadir, which is 110% overlap of the swathes, but such surveys are rare in the commercial world (leave that to the Admiralty and Special Order Surveys).

However outers and nadirs are useful as Richard Brown explains:

“Technically crosslines provide the redundancy and independent check on your main lines. Swathe overlap will be subject to ray bending, so cross lines provide a check on the outer beams in comparison with the nadir and also enable tide, offset and position checks together with statistical assessment of the accuracy.”

But what about systematic errors? Guy Noll likes crosslines to detect those:

“I would also recommend crosslines, in addition to reasons stated above, to be acquired nearly as possible to vertical datum time (low water or mean low) and with more than one platform. More than once I've seen that there was a fairly obvious systemic error in offsets, draft, dynamic draft/squat, or other simple error found during a crossline check. Full multibeam coverage and a small reference area set aside can also find these things, but a complete survey should stand alone.”

Doing the lines at dead low water (or high) is great if you can get that in to the planning and since tidal errors can create some of the most challenging problems we face as Hydrographers, doing a cross line or two is, I think, the simplest way of checking this factor. A position totally supported by Chris Wright.

“Seems to me the utility of cross ties is mostly for assessing tide model accuracy in the absence of RTK and a defensible geoid correction. Without RTK (or for precision engineering surveys) verification of tide models/zoning absolutely requires these lines, even in areas with low magnitude tidal regimes. I think cross lines should be run first, facilitating OTF recognition of errors during acquisition of primary line data.”

And John Austin, highlighted a very important point about the subtlety of errors, how they deviously creep up on you.

“A crossline will reveal any subtle changes or degradation in the model across all lines...e.g. a slight tide error will show clearly against some mainlines; also degradation in equipment, or incorrect changes in constants, or undetected changes in SV, will also be detectable.”

Some proposed that frequent patch tests and VoS data along with carefully planned equipment mobilisations would dispense with the need for crosslines, and that would be valid if we are only talking about some systematic or random errors, even some gross errors. It was further argued that the analysis and processing software would find all the errors, but there are other sources of errors that are outside the capability of the integrated system and its associated software and procedures to detect, tidal errors being the most obvious, but also RTK errors, offset errors and blunders.

So, I think there are more uses for crosslines than we often credit them for. Matt Geen (again) gave another use for crosslines which I am sure not too many surveyors know about.

“For example, you can recover the whole tide correction table with a single cross-line.”

“A story to illustrate that: I was working with someone who had had an inexperienced team do a swath bathy survey. When they got home, they found that they couldn't post process the RTK to get accurate heights on some of the lines, because there had been a fault in the RTK base station. Luckily, they had done a few cross-lines. Using the height differences at the cross-lines, we were able to calculate a "tide table", tied into the few lines that did work, and thereby rescue two weeks' survey data.”

So how does that work? Your survey lines over the main area were done at different times and therefore different states of tide, if you run a single crossline over the whole area and then take the uncorrected (tidally) nadir beam intersection height difference, plotted against the time difference, you get a delta height which is the tidal curve.

A simplistic explanation and the theoretical example in the Appendix shows how, but perhaps someone with the time and data could produce a nice technical paper on this.

Finally, on the issues of what crosslines are good for Neil Hewitt has the last say.

“I would always recommend crosslines in all cases, it is true that multibeam overlaps will show up some issues, however the crosslines will definitely show up tidal problems, SV errors and motion artefacts. If crossline comparisons show good statistical results you will have greater confidence in the quality and accuracy of your survey, a must for all surveyors particularly if you are responsible for the results and have to sign off on the survey.
Crossline comparisons will yield better results in flat areas than uneven seabed but comparisons in uneven areas can also be useful.”

 

Quality and Cups of Tea

Neil, with his comments about responsibility, brings us nicely on to the more commercial / contractual aspects of crosslines, and that is the assurance of a job well done. Someone, at some stage, has to sign off on the survey and put his professional reputation on the line. In doing this it is sometimes not easy to stay firm in the face of other pressures. Like those we get from the Clients. Daniel Kruimel has faced those pressures and I am sure does not waver.

“It is not up to the client to understand what needs to be done for the hydrographic survey, that is why we as hydrographic professionals are engaged. It is part of our role to educate clients in what needs to be done to carry out an efficient, accurate and reliable hydrographic survey. An extra day in the field conducting checks can save days/weeks in processing time trying to rectify problems.”

And Richard Cullen has seen the effects of succumbing, and probably tried to tidy up someone else’s mess:

“While we as surveyors want to provide best practice and provide the most efficient survey practice, the client does not always understand that a little more effort / cost in conducting the survey correctly, reduces nearly two times the effort in investigating and rectifying a poor survey. I too have seen this many times in industry.”

Everyone’s interest is well served by good Q.C. The surveyor in ensuring that the job is well done and the client in obtaining a result he can trust. Remember that is at the after match function when the tea is served that the results are either justified or you are wearing the tea.

As Robert Eadie says:

“It is in all parties interest that the data is sound and rigorous. One good way of reassuring oneself all is well, is to run cross lines first. In my opinion the team that gathers the data should process it and answer any queries. The are a number of horror stories which basically arise from data going into the office for processing and reporting with no continuity of personnel.
The client is paying for useable data and should be seeking added value in the documentary verification of the works. The field at work is a verification too! The client is certainly not paying for ambiguity and confusion. If the client does not know what they are getting or how to get it; hydrographic surveyors must provide suitable advice.”

Bearing in mind the post processing and quality checks that we go through there is no simple answer if we pursue the solution by technology alone, as Matt Templeton points out.

“Each post processing software suite deals with QC in its own way, and the limitations and efficiencies of particular sensors (and contributing sensors such GPS and motion) can also play a major role in the way the QC is conducted and then interpreted. Ultimately, it would be very difficult to cater to all the options and variations in a simple form as it currently is.”

No, it is not easy to write a specification and agree a procedure with all the possible variations in systems, software, and purposes of surveys, and if we find this a challenge what about the client?

Working on the basic premise that the client is not well versed in the intricacies and mysteries of Hydrographic surveying (time to put the apron on), then as Matt Templeton suggests:

“….we (the surveyors) need to remain conscious that our clients will often want a simple answer - traditional cross lines provide an answer that nearly anyone can understand, and they very rarely lie.”

That’s pretty sound advice. Even the uninitiated can understand ‘measure twice cut once’ and that is exactly what we are doing with a cross line, the veracity of which can be explained quite easily to our client in a analogous way that he understands.

 

Frequency of Crosslines.

How many, how often, when and where? Robert Eadie first raised this point

“….another point is the frequency of cross lines. In my nautical charting experience an interval of 10cm on the scale of the survey was a standard interval. Even with MBES, RTK, HPR etc the scale of data presentation still has to be taken into consideration. X line interval may need to be reviewed for different purposes. But I would not dispense with them for area surveys.”

Yes, that is right, I remember that as a newt at HMAS Penguin where I did my Droggying and was awarded the apron. The Admiralty standard was very clear, but in these days it is not so clear.

The NOS and USACE standards make interesting reading with the number of crosslines being required stated as a percentage of the total line Km. An interesting way to put it and presumably leaving the placement totally up to the surveyor. Richard Brown had the following to say on that subject.

“But in addition to the original question we could also ask another. What density of cross line is appropriate to multibeam surveys? The traditional single beam survey with main line interval at 50m or 100m nominally had a cross line interval of about 5 times the spacing (250m or 500 metres). With MBES we can relax this requirement but by how much?”

Good question Richard. There needs to be a guideline because if there is no guidance, even though a guideline cannot cover all the possibilities, then the second law of thermodynamics takes effect and surveys tends towards chaos, or at least a don’t bother attitude. However, I do agree, to a certain extent, with Dave Richards when he says:

“So.....crosslines would be acquired in an ideal World for all the reasons outlined above in most of the posts. However, in the real World sometimes they are overlooked and, like Ian, I have seen no Real World impact on data quality. So...my advice? Acquire crosslines if you can but don't sweat if that turns out not to be “possible.”

As long as we have that in writing somewhere Dave.

We must all agree that often the purist is on a road to a breakdown if he tries to enforce too rigorous stands on the commercial world. But then even the commercial world has its dichotomies, don’t they Dave?

“Oil/Gas - yes often more stringent especially post Macondo Renewables, often less stringent - I suspect because of financial constraints.”

The frequency and layout of crosslines in the commercial world (as opposed to the standards for nautical charting) does tend to be somewhat flexible. Once again from Dave Richards:

“Some narrow corridor surveys such as pipeline or cable routes often go without crosslines more for economic reasons rather than anything based on sound surveying procedures. often, the MBES is left recording while the vessel moves about, so often "crosslines" are acquired without there being a formal crossline in the line plan.”

And from Lucas Lowe-Houghton:

“I suppose it boils down to what type of survey is being conducted, on what project and how the data will be applied.”

You have a point Lucas, and Ian Wright has a similar pot on the boil relating the need to horses for courses.

“From my personal experience the requirement to complete crosslines seems to have become more relaxed with the advent of MBES. From my perspective as a geo, the requirement to do crosslines may well depend upon the ground conditions and the end product. For example in submarine cable route surveying, the requirement has been just about done away with and there seems to be no adverse impact on end results. However, for oil/gas there seems to be a much more stringent requirement as Qc plays a far more important role. So I guess it boils down to 'horses for courses'.”

And it does. And since courses vary markedly there should be a guideline to cover all the courses without it becoming a rigid specification.

 

Specifications and Guidelines.

When the pots boiled dry should the surveyor be held responsible? Leaving it down to the man on the job to decide is fine if the only reputation at stake is his own. If you are in the wonderful position of making your own charts for your own use as Christopher Brown is, then yes, it is entirely up to you, but most of us don’t have that luxury (I picture his charts emblazoned with the words “Here be Shrimps”). In the absolute world of one ping one sounding, or even one toss of the lead, the writing of a specification was relatively simple.

Thou shalt bar check the echo sounder ……
Thou shalt not have a survey line spacing of more than 0.5cm on paper …..
Thou shalt run crossline at intervals of not more than 10cm on paper …..

But in today’s world where even ‘mowing the grass’ is a valid form of survey it is very difficult to specify, so we needs must guide, and having a guideline does protect both the client and surveyor.

With a guideline, we can let the client decide for more, or less (with the necessary warnings from the professional Hydrographer) and be it on the client’s head. That is far better than a surveyor making an on the spot decision and being keel hauled for it later, as Robert Eadie explains.

“The client's spec may be ambiguous and without a specific statement of what is required there could be a disagreement. I have no problem with disagreements, where they cannot be resolved offshore, they should be referred to project staff. What really gets my goat is when the legal profession profit from slack specs and careless review by the project staff and contractor. Be specific and keep lawyers out.”

“Do it properly, record it and everyone is protected. There is evidence that things were done properly and left/found in a certain condition. So if you have a large area to survey, cross line it and if it is not in the spec then formally raise the point and get a written answer.”

Check out our website for our take on this guideline.

 

Legal and Bid.

Speaking of lawyers….. A problem a lot of us have faced at one time or another (and a pet hate of mine) is where a whole survey proposal or ITT has been raised and there is not an Hydrographer in sight! Don’t you just hate it when you are finally brought in on a project to find that the survey spec was written by some engineer or other and they missed out key points in the scope, or failed to set any standards, or worse, set the wrong ones. Proposals and bids must be well written, especially if you are sending them to Philip Stock.

“I request a proposal to a simple spec. i.e. 5000sq.ft. full spectrum hazard survey centered on this point/between these two points. I receive several proposals and prices back from my contractors, that I review.
Why do you think all RFP state we do not necessarily accept the lowest bid?
If the proposal lacks definition (i.e. line spacing/cross line interval etc. ) I will either clarify or reject it.
This discussion asked a simple question cross lines or not. Bid a survey to me without them & you will not get the job, & if a field surveyor/party chief does not do them, the vessel will be going back out at the survey companies cost.”

Oooo Aghhh: You tell them Phillip! You and I will get on well. There has been a few survey contractors that have gone away from my office with a severe velocity of sound error showing on their faces, and I don’t mean a big smile. Robert Eadie has also met this subject head on I think.

“If the client has not specified cross lines then his project surveyor should raise the issue and if not the contractor should definitely mention it. As many jobs are fixed price it does put pressure on the contractor especially if there has been a lot of downtime for whatever reason.”

 

Cost.

Downtime and budgets, two major determinants. When the budgets tight, the crosslines go! It seems the main non-technical reasoning for dispensing with crosslines was cost, that the main driver for not doing crosslines was the budget, and yes some surveys run under very tight constraints, however, if just one crossline could uncover a multitude of sins then surely it is best to have confidence in the data for the small additional cost.

If one views crosslines as a separate function to be done after the main bathymetry has been completed, then it is a defined scope with an attached cost including the between line transits. If, however, one views them as being opportunistic, then they can be accomplished before, during and after the main bathymetry acquisition or incorporated with other parts of the scope. There is no rule as to when and how they should be completed and many surveys will include other elements, such as bottom sampling, or geotechnical coring, ROV investigations, etc. Crosslines can be ‘thrown in’ in conjunction with these activities, which often involve transits anyway, so the extra time and cost argument becomes one of optimising survey planning. But even then, as Matt Green commented.

“Even if the budget is really tight, I would advise doing at least one cross-line. It can get you out of all sorts of trouble later.”

Indeed it can, and it is far too late to try and resolve errors after the vessel has demobilised. Richard Brown said.

“Then there is the commercial issue of the costs for the additional transit of the route for the cross lines - considerable for a trans-oceanic cable.

Of course it is, but this is where guidelines come in, the survey swathe for a trans-oceanic cable or even some deep water pipeline routes could be a single swathe. In those circumstances crosslines become less relevant but even so, they should be included along the line somewhere. Certainly adhering to some rigid rule for survey conduct and planning hundreds of crosslines with hours of transits would never fly, commercially that is. That’s where a guideline on this comes in handy. We can’t live in the ideal world of the theoretical as Michael Vernik reflects.

“From the purely theoretical and idealistic point of view - I agree with you 100%.
From the practical - 180°, as there is no philanthropy in this industry, where every instant of an additional operation means more $$$.
Would you really be paying from your own pocket and losing valuable time for someone (the client) who did NOT want to do so in their own favour?!”

And later Michael commented.

“Indeed, agree with every word. Specifications govern projects, however budgets govern everything. If client can't afford cross-lines, then it should definitely be stated, no doubt at all.”

Too right Michael, give the client your professional advice and get his directions in response on paper. (leave nothing to the lawyers).

 

The Final Say.

I am leaving the last words to Doug Lockhart, who has gathered up everything and hit a six to win the game with this one.

“I have both operational and philosophical issues in surveying without cross lines. Operationally, the concerns noted above, vertical datum & SV, are of primary interest. I've also seen remote heave error mask itself as a datum error. And, I've used cross lines to sort out offset blunders. We should also keep in mind that the outer beams of most multibeams are using phase detection where the inner beams are using amplitude detection. These methods will give different results in some bottom types, particularly fluid mud found in many ports and channels.

This gets me to the philosophical concern. If we don't run cross lines we are only explicitly QC'ing our outer beams. The rest, indeed the majority, of the beams are QC'd implicitly. We are asking the client to take the leap of faith and accept QC by inference. I think that the burden should be on the surveyor to explicitly demonstrate the veracity of his data.
I've never found myself wishing that I hadn't run cross lines. But I've certainly been in the position, more times than I would care to recount, where more or even any cross lines would have been very helpful.

Leaving them out will eventually bite you.”

 

Dennis Knox
Managing Director,
OSSeas Consulting
Ocean Survey Sciences and Engineering Associates Limited
www.osseas.com

Appendix A – Some Specifications:

 

IHO STANDARDS FOR HYDROGRAPHIC SURVEYS (S-44) 5th Edition February 2008

Section A.3.2 Swath Echo Sounders (page 22): “A number of check lines should be run.”

Section A.3.3 Sweep Systems (multi-transducer arrays) (page 22): “A number of check lines should be run.”

Land Information New Zealand goes further in their specification which has become an internationally referred to standard.

 

LINZ STANDARDS FOR HYDROGRAPHIC SURVEYS (HYSEPC) V3 24^th April 2001

Section 7.5 Multibeam Survey Cross Lines (page 56)

“1. Cross lines are to be run at angles of 60º to 90º degrees to the main track-lines at intervals not exceeding 10cm on paper at the scale of the rendered sounding sheets.”

“2. A statistical comparison of raw data between the main survey swath and cross line swathes is to be undertaken to ensure that the accuracy requirements of the order of the survey are met.”

 

MARITIME SAFETY AUTHORITY OF NEW ZEALAND
Good Practise for Hydrographic Surveys in New Zealand Ports and Harbours 2004

Section 6.2 Data Collection (page 14)
"Regardless of the type of equipment in use, the running of additional lines (check or cross-lines) for the sole purpose of checking data quality at the data analysis stage is considered essential."

Section 6.4 Data Analysis (page 15)
"Cross-line comparisons and various other consistency checks are undertaken at this time."

 

FEDERAL GEOGRAPHIC DATA COMMITTEE FGDC (UNITED STATES)
Geospatial Positioning Accuracy Standards Document Number FGDC-STD-007.5-2005

Part 5: Standards for Nautical Charting Hydrographic Surveys

Section 5.12 Quality Control (page 5-10)
"Crosslines intersecting the principal sounding lines should always be run to confirm the accuracy of positioning, sounding, and tidal reductions. Crosslines should be spaced so that an efficient and comprehensive control of the principal sounding lines can be effected. As a guide it may be assumed that the interval between crosslines should normally be no more than 15 times that of the selected sounding lines."

"All swaths should be intersected, at least once, by a crossline to confirm the accuracy of positioning, depth measurements and depth reductions."

Section 5.6 Depths (page 5-7)
"As an additional check on data quality, an analysis of redundant depths observed at crossline intersections should be made."

 

NOS HYDROGRAPHIC SURVEYS SPECIFICATIONS AND DELIVERABLES
April 2011 U.S. Department of Commerce - National Oceanic and Atmospheric Administration

5.2.3.1 Instrument Error Corrections.
“In addition, frequent checks should be made between the overlap of mainscheme and crosslines collected on different days. These comparisons should be made frequently during data collection to find errors promptly, and not saved until final data processing after the field party has left the working grounds.”

5.2.4.3 Crosslines General:
“The regular system of sounding lines shall be supplemented by a series of crosslines for verifying and evaluating the accuracy and reliability of surveyed soundings and positions. Crosslines shall be run across all planned sounding lines at angles of 45 to 90 degrees.”

“…..crosslines in an object detection or complete bathymetric coverage do provide an additional semi-independent check for spatial and temporal correlation of the data set across the range of area, time, seabed relief and bottom types, survey vessels, and sonar systems represented. For this analysis to be valid, crosslines must be acquired with the same attention to accuracy and data quality as mainscheme data. Whenever possible, crosslines should be acquired under different conditions (vessel, sonar system, tide state, etc.) than main scheme data.”

(DKx Note: NOS also specify crosslines with the same criteria for Lidar).

 

HYDROGRAPHIC MANUAL FOURTH EDITION - JULY 4, 1976
U.S. Department Of Commerce
National Oceanic and Atmospheric Administration - National Ocean Survey (NOS)

Part A. —General Standards

2. Spacing of cross-check lines:  7.5 cm (3.0 in) or less.

1.4.2. Crosslines
"The regular system of sounding lines shall be supplemented by a series of crosslines (4.3.6) for verifying and evaluating the accuracy and reliability of surveyed depths and plotted locations."

"The hydrographer should exercise judgment and seek areas of regular relief where meaningful Crosslines can be run."

4.3.6. Crosslines
"Regular systems of sounding lines shall be supplemented by crosslines to provide a check and to disclose discrepancies in the main system."

4.6. Discrepancies In Hydrography

4.6.1. Discrepancies at Crossings
"Crosslines are run to disclose systematic and accidental discrepancies in soundings."

 

U.S. ARMY CORPS OF ENGINEERS ENGINEERING AND DESIGN MANUALS
EM 1110-2-1003 - 01 Jan 02 - Engineering and Design - Hydrographic Surveying

5.6 Depths
As an additional check on data quality, an analysis of redundant depths observed at crossline intersections should be made.

 

Appendix B – Contributors:

 

Appendix C – Using Crosslines to Recover Tidal Data