CATZOC and passage planning

When I had just started my sea career, I never understood how the depths of the entire ocean was measured.

I mean 70% of the earth is covered with water. How was this nearly impossible task of measuring the depths achieved?

Over the years I realized that it was not done in few days and not by few men.

This task is ever going since hundreds of years and the hydrographic offices of the countries are conducting these surveys regularly.

The only thing that has changed over the years is the method of conducting these surveys.

In early days the survey was done with hand-lead lines and single beam echo sounders.

It then changed to the wire-drag method where a wire was dragged by two or more ships with weights submerged to the fixed depth.

Wire_drag_operations

Any obstruction in the area covered would be detected by the wire getting stretched.

The modern technique uses SONAR multi-beam waves to record the depths. The collected data is processed through other data like tide to get the depth as accurate as possible.

This method, of course, would give more accurate depths.

So Let is say we both are on a vessel and we are reviewing our next passage plan.  You being master of the vessel and me as 2nd mate.

I give a brief about the passage plan and point out to you that at one section of the plan, UKC for the ship will be 1.5 meters which is more than the minimum required by the company.

You are still worried and I cannot understand why?

I ask you, hey Cap, What are you worried about? Our UKC is complying with the company’s UKC Policy.

Your reply: What if the depth mentioned on the chart is not accurate?

We both dig a bit more and find that this depth was measured in the year 1920 by hand-lead line.

Will you change your passage plan or you will trust the depth?

Of course, you cannot trust this depth to be absolutely accurate.

But then me, you and everyone at sea would be interested in knowing the answer to this one question.

How much error can we expect?

This is what CatZoc is all about.

Let us discuss Catzoc in details.

Type of expected errors in depths

When a depth is measured during a survey, there can be two errors to this depth.

  • Depth may not be accurate.
  • The position at which this depth is marked may not be accurate

The latest survey conducted with the latest technology can be considered to have better accuracy.

What is CATZOC or Zone of confidence

If the possible errors in depth and position where depth is marked were constant on the charts, it would have been easier to just apply a constant error.

But these errors are not constant.

As the technology advanced, the errors in these readings became marginalized and readings taken were more accurate.

But how do we know which readings are accurate and which are not?

Also what maximum errors can be expected in these reading?

Information about CATZOC (or zone of confidence) helps in this.

Based upon the error in position and depth, the accuracy data is divided into 6 zones of confidence (CATZOC).

CATZOC-with-ECDIS-Symbol

Each zone of confidence (CATZOC) has been assigned the maximum errors that it can have in depths and its positions marked on the charts.

So if for a chart or ENC, the CATZOC is 4 star (zone of confidence B), this would mean that

  • the position of the depths and dangers marked on this ENC may be inaccurate for about 50 meters
  • There may be an error in the depths for up to 1 meter + 2% of depth

So if at any place the charted depth is showing 20 meters, the error in this could be 1.4 Meters (1 meters + 2% of 20 meters).

CATZOC information on paper charts

It is not that CATZOC has surfaced recently. It had been there since long.

On paper charts it was in form of something called “the source diagram”.

On each paper chart, there is a source diagram which tells the source of the information on the paper chart.

For example look at this source diagram of BA chart.

Source-Diagram

It has divided the areas based on the nature of survey done for this chart.

As you can see that in Area “c and d” depths were taken by a lead line. The depths in this area can be expected to have significant error.

Navigators need to apply this error to the depths. But again, How much error?

There is a sad news.

Paper charts never gave how much error these depths can have. It was left to the mariner to make the judgment about the information provided in the source diagram.

But then UKHO realized this and updated the source diagram on the new charts with CATZOC information.

source-diagram-on-paper-chart-with-CATZOC-information
Source: Admiralty 
But till this date, that is only a handful of charts among thousands.

Catzoc on ECDIS

On ECDIS, the CATZOC information is displayed as CATZOC symbols.

These ECDIS symbols are in form of the number of stars.

Each symbol represents a particular “zone of confidence” category.

CATZOC-with-ECDIS-Symbol

An ENC with six stars would mean that the information in this ENC is highly accurate. Let us see where to find this information on JRC ECDIS.

To find this information on JRC ECDIS, go to Chart -> Chart Settings

Then go to View 1. Under this ensure that option “Accuracy” is checked.

Catzoc-in-JRC-ECDIS

This will show the CATZOC symbol on all the ENCs on the ECDIS.

CATZOC-Symbol-on-ENC

To get more information on the “zone of confidence” of a particular ENC, while the accuracy is still selected in the settings, right click on the ENC.

Then go to S-57/C-MAP/ARCS Information.

JRC-ECDIS-chart-accuracy-data

Click on the Quality of data and it will show the “Zone of Confidence” for that ENC.

chart-accuracy-JRC-ECDIS

If you now go to “Chart Legend” tab, it will give you further information for that “Zone of confidence” like position accuracy and depth accuracy values.

JRC-ECDIS-Data-quality

How to use CATZOC Information

To use the CATZOC information, all we need to do is apply the maximum possible errors wherever required.

If we are planning a planning a passage that involves passing close to a danger, we need to keep safe distance considering the CATZOC category in that area.

Let us say we have planned a passage in an area with CATZOC category of 3 stars (zone of confidence “C”).

This is one part of our passage that passes from a known danger by 0.4 NM.

passing-dangers-at-close-distance-CATZOC

As the CATZOC category is 3 stars (zone of confidence “C”), there could be position error of up to 500 meters in showing the position of this danger.

500 Meters is approximately equal to 0.3 NM.

So even though we may be thinking that we are passing clear of the danger, the danger could be right on our course if we do not consider the CATZOC in that area.

actual-position-CATZOC

Consideration for Depth accuracy

We also need to take the errors in depth in consideration because of CATZOC category of ENC.

Let us say vessel need to pass through this area which has the least depth of 8.5 meters.

Depth-accuracy-CATZOC-ENC

The minimum tide in this area is 0.9 meters. So available depth in this area is 9.3 Meters.

Vessel’s maximum draft is  7.2 meters.

Vessel expected to experience squat of 0.8 meters. The company requires the vessel to maintain UKC of 10% of the maximum draft (including squat).

A simple UKC calculation would depict that vessel will easily comply with the UKC policy of the company.

UKC-Calculation-without-CATZOC

Everything looks OK except the fact that if the data in this ENC belongs to say 4 stars CATZOC (zone of confidence “B”), there can be an error in depths up to 1 meters + 0.2% of depth.

So in this case, the error in depths can be up to 1.17 meters (1 + 0.17 meters).

In this case, not only the vessel will not comply with company’s UKC policy, there are great chances of the vessel running aground.

UKC Calculation on board needs to take CATZOC information in the account.

When we take CATZOC into account for the depth of 8.45meters and 4-star CATZOC (zone of confidence “B”), we need to assume the maximum error in this depth.

So we need to assume that the minimum depth in this area is 7.33 meters only, 1.17 meters being the error in-depth reading.

Even the company’s UKC calculation form need to have CATZOC depth error.

UKC-Calculation-including-CATZOC

Consideration for Safety Depth on ECDIS

ECDIS has safety setting feature in it. These safety settings include

  • Safety depth
  • Safety Contour
  • Shallow contour
  • Deep water contour

I had covered these setting on a different blog and I will not repeat that here except the safety depth.

Safety depth is the minimum depth at which the vessel would comply with the company’s UKC policy.

So now let us say your draft is 8 meters and expected Squat is 1 meter. That means vessel has an effective draft of 9 meters.

The company requires the vessel to maintain UKC of 10% of the draft. In this case, it would be 0.9 meters.

So the required depth to comply with company’s UKC policy would be 9.9 meters.

In this case, the safety depth would be 9.9 meters but without considering the CATZOC.

If we enter the value as it, the ECDIS will show 10 meters depth as safe depth for the vessel.

But 10 meters depth may have an error of 1.2 meters (CATZOC 4 stars) and the actual depth could be 8.8 meters only.

So when entering safety depth value in ECDIS, we need to consider the CATZOC in that area and enter the corrected value for safety depth in the ECDIS.

In the example above, if we allow the CATZOC error in depth for “zone of confidence B”, the minimum depth for complying with the company’s UKC policy will be 11.25 meters.

Since ECDIS does not take decimal values for the safety depth, in this case, we can enter 12 meters as the safety depth setting.

Conclusion

The data shown on the nautical charts and ENCs may have errors depending upon how this data was measured and when it was measured.

Older the data, the less accurate it will be because of old technology used for measuring the data.

The level of accuracy has been divided into six categories known as “zone of confidence” or CATZOC.

Each “zone of confidence” (CATZOC) has been assigned a maximum error value for the depths and its position shown on the charts.

While planning the passage, it is important that these errors are taken into account.

It becomes more important to allow for the CATZOC when calculating the UKC in shallow waters or when passing close to a danger marked on the chart.

Related Blogs

Follow us Today on
our social network

Capt Rajeev Jassal

About Capt Rajeev Jassal

Capt. Rajeev Jassal has sailed for over 19 years mainly on crude oil, product and chemical tankers. He holds MBA in shipping & Logistics degree from London. He has done extensive research on quantitatively measuring Safety culture onboard and safety climate ashore which he believes is the most important element for safer shipping.

Search Blog

69 Comments

Tuyson Gay
Tuyson Gay
Dec 22, 2017

Thank you sir, very good explanation

Rajeev Jassal
Rajeev Jassal
Dec 22, 2017

Thanks Tuyson...

Angelo Dolatre
Angelo Dolatre
Dec 23, 2017

VERY HELPFUL INFORMATIONS.THANKS.

Rajeev Jassal
Rajeev Jassal
Dec 24, 2017

Glad you found it helpful Angelo...

Sudhanshu Kishore
Sudhanshu Kishore
Dec 24, 2017

Thanks a lot sir, perfectly explained... Master's stroke.

Rajeev Jassal
Rajeev Jassal
Dec 25, 2017

Thanks Sudhansu ...

ERCAN ALTAY
ERCAN ALTAY
Dec 24, 2017

Thank you again for your clear explanation that is very useful for all officers onboard

Rajeev Jassal
Rajeev Jassal
Dec 25, 2017

Glad you found it useful Ercan...

GOPI
GOPI
Nov 14, 2021

IN DEEP SEA OR OCEAN PASSAGE DO WE CONSIDER CATZOC FOR SAFETY DEPTH CALCULATION?

Raghunathan
Raghunathan
Dec 24, 2017

The explanation is simple and clear sir.. Thx for ur effort. I just wanted to know from where or which publication can i get the position and depth accuracy values for each category of zone of confidence .

Rajeev Jassal
Rajeev Jassal
Dec 25, 2017

You can find the information in NP231 (Admiralty guide to the practical use of ENCs) and in NP100 (Mariner’s handbook).

Maxim
Maxim
Dec 26, 2017

Good explanation regarding CATZOC. But have question. For example you are approached to some port with CATZOC C , OR D AND IF YOU WILL APPLY ERRORS You Will couldn't pass without cross safetty depth in your route . What to do in such case. But normally if you use paper chart you could pass with out any restriction in same route.

Rajeev Jassal
Rajeev Jassal
Jan 17, 2018

That is the issue with transition stage where we are right now. In future, we would see more and more charts with the source data containing CATZOC information and then it will be same for ENC as well as the paper chart. But meantime we need to follow the company's navigation manual.

walter
walter
Sep 5, 2022

assess the risk...

Visveswaran
Visveswaran
Dec 27, 2017

Great work

Rajeev Jassal
Rajeev Jassal
Jan 17, 2018

Thanks Visveswaran...

Rajesh Jajaiah Soodana
Rajesh Jajaiah Soodana
Dec 27, 2017

Thank you sir for explaining CATZOC in such a simple way......

Rajeev Jassal
Rajeev Jassal
Jan 17, 2018

Glad you found it useful Rajesh...

Antonio
Antonio
Dec 28, 2017

Buona sintesi. Grazie

Rajeev Jassal
Rajeev Jassal
Jan 17, 2018

Grazie per aver letto Antonio...

Dmitrii
Dmitrii
Jan 2, 2018

Good day, Sir. How do you apply zone of confidence "U" when you are preparing you passage plan? Big amount of big scale ENCs have this category

Rajeev Jassal
Rajeev Jassal
Apr 29, 2018

Before transiting through the CATZOC U area, we need to get the information of depths from the pilots, local agents, various publications, the previous experience where the vessel passed with same or higher drafts, noted depths by other vessels in the fleet. And finally, passage through CATZOC U area need to be approved by the company.

Mohamed
Mohamed
Jan 4, 2018

Thank you capt.

Shahbaaz Ahmad
Shahbaaz Ahmad
Jan 15, 2018

hello sir, how we get 11.25 whlie applying zoc for safety depth setting?

Shahbaaz Ahmad
Shahbaaz Ahmad
Jan 15, 2018

hello sir, how we get 11.25 whlie applying zoc for safety depth setting?

rocky singh
rocky singh
Feb 2, 2018

Good Day Sir, Where do you activate the indication of chart data Accuracy (CATZOC) in the ENCs on FURUNO FMD 3200 ECDIS?

Marcin
Marcin
Feb 22, 2018

Good day. In chart display setings - general - accuracy

Aman dubey
Aman dubey
Feb 3, 2018

Good day sir, U explain all ur stuffs in such a crystal clear manner,which devops the interest of reader eliminating any ambiguity and myths on the topic. Keep sharing your valuable knowledge sir Great work# respect.

Rajeev Jassal
Rajeev Jassal
Apr 29, 2018

Glad you are finding it useful Aman...

Przemek
Przemek
Feb 24, 2018

When planning a voyage and calculating UKC you shoud cinsider ZOC (or CATZOC). As far A, B and C is clear and you include ZOC to UKC calculation then D which is worse than C so how you consider this? How much is worse than C? :) How you evaluate in this case you voyage plan? Another thing... what about if you have U? You should not proceed? :)

Rajeev Jassal
Rajeev Jassal
Apr 29, 2018

Before transiting through the CATZOC D & U area, we need to get the information of depths from the pilots, local agents, various publications, the previous experience where the vessel passed with same or higher drafts, noted depths by other vessels in the fleet. And finally, passage through CATZOC D & U area need to be approved by the company.

Przemek
Przemek
Feb 24, 2018

one more thing... you have place called Drogden Channel on Danish waters: charted depth is 8.0 meters. CATZOC is 1A, so error can be expected -0.60 m. You will assume that maximum vessel's draft should be 7.40. In Sailing directions and other publications is stated that maximum draft 7.70 m.

Rajeev Jassal
Rajeev Jassal
Apr 29, 2018
GOPI
GOPI
Nov 14, 2021

DO WE NEED TO APPLY CATZOC CORRECTION IN OCEAN WATER?

Ranjit Singh
Ranjit Singh
Mar 1, 2018

Sir you are doing a great job by sharing your knowledge and clearing doubts of all junior officers like me. thanks for your efforts. sir I have a question, if we are transiting through a dredged channel do we have to apply CATZOC? Or does the accuracy depends on when the channel was dredged?

Rajeev Jassal
Rajeev Jassal
Apr 29, 2018
Mohammed Arif
Mohammed Arif
Mar 5, 2018

Very well explained Sir, the only question that comes to my mind is , in areas like western Australian ports (Catzoz A or A1)where, the maximum sailing draft(Capesize vessels) in loaded condition is abt 18 meters with dreged channel depth being 15 to 16 meters and considering all other factors like squat,heel, swell etc the vessel's calculated UKC hardly comes close to company requirements. In that particular case considering CATZOC error further reduces the minimum calculated UKC , we know alternative route is not an option in these cases. Please comment.

Rajeev Jassal
Rajeev Jassal
Apr 29, 2018

Please see my reply to the above comment...

V.k.kaup
V.k.kaup
Apr 29, 2018

Simple and easy explanation with examples. Thank you Mr Rajeev .

Rajeev Jassal
Rajeev Jassal
Apr 29, 2018

Glad you liked it...

Sandeep Sinha
Sandeep Sinha
May 23, 2018

An important subject explaied lucidity yet effectively. Failure to cater to CATZOCs is common and ship positions are therefore misleading many a time. This article explains a Best Practice.

Maninder jit singh
Maninder jit singh
Jun 1, 2018

So helpful sir

JItender
JItender
Jun 10, 2018

Dear Rajeev, Very well explained but it is not correct to apply CATZOC correction to Safety Depth otherwise most of the ports you will not be able to comply with UKC requirements at any time. CATZOC allowances should only be allowed to Safety contour settings, so that you can monitor areas which are greater than safety depth and navigate within restricted Safety contours. Please correct this. Jitender Sanyal.

PANKAJ GAUTAM
PANKAJ GAUTAM
Jun 13, 2018

MANY MANY THANKS FOR PRECISE EXPLANATION..

Ravi Bankhele
Ravi Bankhele
Jul 4, 2018

Thanks sir for sharing information. I have query about CATZOC "JRC ECDIS shows different CATZOC rating difference at different scales for same sea area."

MANOJ KUMAR
MANOJ KUMAR
Sep 8, 2018

Thanks sir for sharing information

Nguyen hoi
Nguyen hoi
Oct 26, 2018

Good day Sir. I have a question about calculation Catzoc. Draft. 8m, squat 0.8m, UKC policy 1m, swell 0.2 m Max. Draft = 8 + 0.8 + 1 + 0.2 = 10 m Catzoc (A1) = 0.5 + 1% 10m ( Max. Draft) = 0.6 m (I use value Max. Draft to calculate Catzoc) Hight . 1m Safety contour = safety depth = 10 + 0.6 - 1 = 9.6 m Input value 9.6m on ECDIS ( Navmaster) to make a route. I am right or wrong Tks so much.

Nguuen hoi
Nguuen hoi
Oct 26, 2018

Good day Sir. I have a question about calculation Catzoc; Draft. 8m, squat 0.8m, UKC policy 1m, swell 0.2 m; Max. Draft = 8 + 0.8 + 1 + 0.2 = 10 m; Catzoc (A1) = 0.5 + 1% 10m ( Max. Draft) = 0.6 m (I use value Max. Draft to calculate Catzoc) , Hight . 1m ; Safety contour = safety depth = 10 + 0.6 - 1 = 9.6 m; Input value 9.6m on ECDIS ( Navmaster) to make a route; I am right or wrong ; Tks so much.

Besik Tsiteladze
Besik Tsiteladze
Feb 7, 2020

firstly, you did not indicate the depth from the chart. I think you should first understand what dynamic draft and static draft are.CATZOC refers to the definition of quality and confidence in relative depth.Safety depth this is a navigable place, for a ship's, you fixed of 9.6 m safety depth for 10 meters dinamic draft ?CATZOC Calculation Of confidence depth ,but not for calculating safety depth

Mac Ervs
Mac Ervs
Nov 2, 2018

This is very helpful. Mind if I grab a copy for my students? Thanks in advance.

Gerald Josephy P. Dimzon Jr.
Gerald Josephy P. Dimzon Jr.
Jan 7, 2019

Well explained even a newly promoted officer can immediately know the difference from the applied and not applied CATZOC for the UKC calculation.

Jaime Venegas
Jaime Venegas
Jan 9, 2019

Thanks for your post, very helpful, do you have any other post related about CATZOC U only, so far there are plenty of factor to considering this ZOC. Regards

Ivans
Ivans
Jan 29, 2019

Good day What about dragged channels or fairways You can see that declared depth is maintaining as the minimum. Do you need to apply catzoc for UKC calculation?

Sushil
Sushil
Apr 16, 2019

8.5+0.9=9.4

Captain Hasemur Rahman
Captain Hasemur Rahman
Apr 29, 2019

It’s really helpful post with simple but clear description. Thanks sir...

Adhiwirayuda
Adhiwirayuda
May 14, 2019

Usefull information for me Capt,as cadet. Big thanks Capt.

mannyson
mannyson
May 18, 2019

thanks for sharing very helpful.

saba karim
saba karim
Jun 14, 2019

Sir can u plzzz tell me how we can order chart in ecdis????

AMJAD ALI AWAN
AMJAD ALI AWAN
Jun 20, 2019

ITS AMAZING EXPLANATION REALLY ONE DOESNT GET BORE

Yosman
Yosman
Mar 20, 2020

Thanks it helps to understand and very easy to explain. Nice Job. I

Alerander.Lillian
Alerander.Lillian
Apr 28, 2020

It is hoped that you will have to work hard

Motiur
Motiur
Apr 2, 2021

Your writings are so easy to understan.. Thank u so much sir

CHOONG WENG CHIEW
CHOONG WENG CHIEW
May 29, 2021

very good and clear information. Thank you. Keep it up.

NANDAN
NANDAN
May 30, 2021

SIR PLZ ADVISE IF SOURCE OF INFORMATION REGARDING CHANNEL DEPTH IS PROVIDED BY PORT CONTROL IN THIS CASE ALSO WE REQUIRE TO APPLY CATZOC ERROR, AS CHANNEL CHARTED DEPTH IS 5.2 MTR AND HOT IS 6.0 MTR VESSEL DRAFT STATIC IS 10.50 MTR .PLZ ADVISE WHAT SHOULD I CONSIDER FOR SAFETY DEPTH SETTING ,AS AVOVE CHARTED DEPTH (5.2 mtr) SAFETY DEPTH SETTING WILL BE ONLY A FEED TO ECDIS WITHOUT USE. PLZ ADVISE SIR

ALGI VANO S
ALGI VANO S
Aug 25, 2021

let me say is wondering blog related knowledge for seafarers, thank you captain Rajeev ,

Anthony
Anthony
Aug 28, 2021

I have known Dr Okiti for over a year now. he is my favorite psychic, and A lot of things he revealed to me were hard to believe but as time went by, I found all of them to be true. I do not hesitate to go to him with any issue. I highly recommend His spells work correctly and faster he will even do a goodluck charm that will make things to be easier for you +2348140757352 ( https://Drokitispellhome.com )

Capt Vineet
Capt Vineet
Jun 11, 2022

Though I am well aware about CATZOC but i just started reading as your language and explanation is in very clear and simple manner, which makes me interested to read till the end. Excellent, Pls keep the same spirit as it is very useful for the new comers.

Isabella Lucas
Isabella Lucas
Nov 11, 2022

My ex-husband and I had always managed to stay friendly after our divorce in February 2017. But I always wanted to get back together with him, All it took was a visit to this spell casters website last December, because my dream was to start a new year with my husband, and live happily with him.. This spell caster requested a specific love spell for me and my husband, and I accepted it. And this powerful spell caster began to work his magic. And 48 hours after this spell caster worked for me, my husband called me back for us to be together again, and he was remorseful for all his wrong deeds. My spell is working because guess what: My “husband” is back and we are making preparations on how to go to court and withdraw our divorce papers ASAP. This is nothing short of a miracle. Thank you Dr Emu for your powerful spells. Words are not enough. Email emutemple@gmail.com Phone/WhatsApp +2347012841542.

Rit
Rit
Dec 8, 2022

Very good explanation indeed

Leave Comment