Nautical Charts & Understanding Their Uses for the Mariner


Understanding Nautical Charts :   You landlubbers may call them maps, but to the mariner the actual name is "nautical charts".  These charts can be quite large depending on the area encompassed.   You can purchase paper charts from many marine supply stores.  And if you need one for an area that they do not stock, most will be happy to order the ones you need.  You need to aware that the depth readings shown on these charts may be in either fathoms, (6 feet), meters (about 3 feet) or feet if for shallower bays.  On each chart there will be a title bar somewhere depicting this unit of measurement.   All charts (at least the northern hemisphere) are laid out so North points UP.

For the mariner, you will usually have to roll them tightly, then store them with a rubber band around them, as there is not much room to store 3 foot square chart in the small boat.  Some small boat owners build wooden racks attached to the cabin ceiling with round holes drilled to accommodate many of their rolled up charts.

Charts are laid out with horizontal and vertical lines overlaid on the chart.  The vertical numbers are latitude (north/south), the horizontal are longitude (east/west).  These numbers will always be shown on the edges of the chart.  The latitude number lines will not be equal as they go north/south because of the earth's being a sphere.  The reference numbers start at the equator.   The latitude reference is taken at the earth's surface at the equator.  And reference is then made to Latitude being north or south of the equator.

All readings for location of a reference point will be written as Latitude/ Longitude as this example Lat 48-26-506 N, &  Lon 124-36-125 W.   This is referring degrees, minutes and seconds.  These numbers are all in increments of 60.  However since the advent of modern technology GPS has made a change in the old seconds from 60 to now in 100ths.  So if your GPS readings were taken off a chart or are years old GPS numbers with only 2 digits, your exact location may be a few feet off using current GPS units.

You can take a reading off a chart, say for a sunken ship by using a 36" ruler, laying it horizontally thru your position, then reading the corresponding number on the vertical edge of the chart for your Latitude.  Then do the same for the Longitude by laying the ruler vertically thru your position and reading the numbers off the bottom or top of the chart.

As an example by using the information below, you can calculate your approximate position in relationship to another position

Longitude     ---   using Westport North Jetty     West 124-11-078 N as a reference

1 degree = 60.7 nautical miles at the equator                 
124    7525.8 NM west of datum line of Grenich Village (England)
1 minute = 1 nautical mile (6071')                                   124-
11   11 NM west of 124-00
1 second = 60.7'                                                             124-11-
0    0 feet west of 124-11
.010 of a second = 6.07'                                                 124-11-0
7   42.49 feet west of 124-11-0
.001 of a second = 7.28"                                                124-11-07
8    58.24 inches west of  124-11-07
.0001 of a second =.728"                                               124-11-078
 1.456 inches west of 124-11-078

Latitude   ---  using Westport North Jetty  -                   46-55-540  N as a reference, uses the same basic principle as Longitude,
                                                                                         but the first digit 00 is the Equator

Longitude  ---  Westport North jetty                             124-11-078 W
Westport Buoy #5    (west of North Jetty)                    124-12-700  (1 mile plus 4127.6'   =  approx.  1  5/8 miles west of north jetty

For instance if we use Westport Washington's buoy #3  as your position (46-55-005, 124-14-820), we see the Latitude as the first set of numbers, with the Longitude as the second set.   For Latitude, the numbers get larger as they go farther north, and the Longitude get larger the farther west you go.   So from this you can quickly calculate if your friend calls you on the radio, says he is at 47-10-501, 124-24-802, if you compare his numbers to yours, this means he fairly close by but is slightly north and somewhat west of you.

From the above you can see how precise a good GPS reading can get you.  Engineering/surveying goes one more digit (.0001) on the seconds numbers as you can see above is rather precise.

Electronic Charts ;  I envy the owners of larger boats that have computer satellite capabilities who can at the click of a mouse call up charts of about anywhere.

With today's modern technology you can tap into free internet NOAA charting at    However these are really only good for fishing if you are looking for rock or gravel structure.  I have not been able to find a way to use the mouse to provide a GPS Lat/Lon location on the screen chart however.  But you could get close by doing a split screen with Google Earth which does give you GPS numbers & do a comparison to the NOAA chart.

Chart Usage For Navigation ;   Most of us will not be normally traveling up or down the coast or real far offshore, but will be using charts/GPS to navigate under limited sight conditions.  I suggest you set down with charts for your intended area, locate by hand some locations that you foresee that could be beneficial to you at a later date and enter them in your GPS unit.  This can be buoys, piling markers, river entrance channels, boat basin entrances, jetty ends or anything that may be of assistance to get you out & then back in safely if the weather visibility turns bad.

If you run offshore 20 miles or so, always have a go-to buoy or landmark to get back to your entrance location.

Chart Usage For Fishing ;   Now the use of GPS for fishing pretty well go hand in hand with the navigation requirements, with the addition of those know spots that hold concentrations of fish.  This could be underwater rock structure if you are after bottom-fish or known current/eddy locations where bait is usually found on a particular tide.  Many times you may be able to share some of this info with other fishermen.   Or gather information yourself when you repeatedly catch a nice fish in the same locations. 

For the newcomer do not be bashful, ask and many times you will receive more than you ever dreamed was available.  Most fishermen will share GPS locations, (maybe not their secret ones, but enough to get you into fish).

All charts will give you a bottom condition, as rocky, mud, gravel etc.  This information may be critical to finding fish, depending on the specie you are targeting.

What Does This Mean to You & I ?   I have a friend that is retired, recently purchased a fishing boat, read my articles then contacted me for some assistance in expanding his knowledge.  I gave him copies of my Bathymetric charts.  In his quest for information, he purchased a NOAA CD of Bathymetric charts, got into trying to collating info into GPS coordinates.   In his previous employment he worked with more precise numbers and was in hopes this could be applied here.  Not possible.   He talked to software business, software sales representatives, navigation software companies, government chart sales offices, and the list goes on and but he wound up back where he started 3 weeks before.  His conclusion was that the charts are a good reference, but if you try to get coordinates it, is by dual observing one chart then the other and guessing at a GPS location.   These coordinates may not be all that accurate.  And as small as some of the bottom descriptions are, a minute or even several seconds one way or the other may put you off the intended location.   So his conclusion was to just use the map as a reference and hope for the best.

With the advent of GPS chart plotters, many think that they do not need to purchase the old paper charts anymore.  Well, anytime you rely on only one source of information, you are asking for trouble if for instance your power dies, your computer crashes, or you may want to set down at a table and do some planning for a upcoming trip.

If you are like me, I have sat down, used the navigation charts to locate buoys, marker piling, jetties, etc. that I then entered into my GPS unit.  I have many hours entering this data.  I have checked them for accuracy whenever I can, and most of them are surprisingly accurate. 

The first time I ever used a GPS on the water was in Grays Harbor bay probably around 1990.  We intended to go for fall Chinook off the mouth of Johns River.  When I left the Westport boat basin early that morning with weather being overcast, I made the corner, headed upriver, got to buoy #14 and the fog set in.  I had previously plotted into my handheld GPS the buoys in this area.  OK, this would be a test.  That next stretch was a straight almost easterly run and as I would approach a buoy, #15A, on the plotter screen, then #17, I made certain to pull close enough to read the number as I had never been in this location before and was not overly confident of this new GPS thing.  On to #21, then make a course change slightly north easterly.  My partner asked if I knew where I was going and I replied we were headed for buoy #25.  We kept motoring at a reduced speed, not seeing a thing, but when he asked where this buoy was, I told him about 300 feet dead ahead.  Bingo, about 20 seconds later it popped out of the fog right in front of us.  We trolled around this buoy for 3 hours and only saw one other boat.  When the fog raised, there were about a dozen other boats there, but we were on the north side of the channel and they were on the south side maybe 400' away.  This experience made a believer out of me.

I constantly check my numbers with actual encounters, then keep a note-pad on my dash to record these numbers by having a passenger write them down when I call them out as I come close to the buoy, instead of punching them in the GPS as a location, which adds to clutter.  I can then look back, then make the minor corrections if need be.   Over the years I have acquired a lot of accurate coordinates this way.

Most of us fishermen over the years have put together a loose-leaf notebook that we photo-copy sections of the actual nautical chart that pertains more to the area we may be fishing.  It is best to purchase clear plastic pouches with the ring binder holes punched so you can keep your using charts somewhat protected.

Years ago in the early 1950s when I fished with my uncle at Westport, our boats were rentals either 16' or 18' open boats that we used 7.5hp or 10hp outboard motors on.  We had no charts, did not know what a depth-finder was (other than stripping off line 2' at a time while dropping a baited mooching rig and sinker over the side).  We never used a compass as we were always within sight of land and did not have to run that far to catch a limit of salmon.  And if it was foggy, we never went beyond the end of the south jetty, and could simply follow the rocks of the jetty and then shoreline back to the boat basin.

Times have changed.  Now we would not think of leaving the boat basin without the sonar and GPS functioning.   Although while off topic here, be sure to turn on your VHF radio also.

Take Time to Learn Your GPS Unit ;  Most of us do not use our electronics enough to become fully informed as to what is available.  I have found many things that I wanted on my units, or what was there that added to cluttered screen that I wanted taken off.   Part of it was that I do not understand their terminology.   I have made it a point to write down my questions, then go to a sportsmans show where I ask the factory representatives.  Usually once we can communicate as to what my situation is, (they inevitably call it different than I do) it is easier then to at least know where to go in the menu and then stumble around until I get it thru my feeble mind to get added what I want.  I have not had much luck talking to local salespersons at even supposedly good marine stores.   They seem to have a basic knowledge, but need to be at a unit, punch the menu, try to figure out what I am trying to inquire about (but most of their quest is by trial and error).

I have a tutorial GPS/Sonar program downloaded on my computer for my Lowrance unit.  It helps, however I do spend a lot of time on my new boat with the GPS /Sonar unit learning and changing settings.  The hardest is when I make a change and did not really understand what it meant or the consequences until after I turned off something I really did not in tend to do.   Out comes the manual along with some head scratching time to figure out how to restore what I just deleted.   Keep your manual handy.  But keep in mind that these technical writers are usually writing over our heads most of the times.   My belief is that when writing these manuals, they need to pull a person off the street that has no understanding or interest in what they are writing about and before the final print, have them be able to understand what the techies have just put on paper, how to make the unit function by reading the instructions.  Also keep in mind that men and women's minds think differently (not sure if this is a condition here or not, BUT) .

But the more you understand your unit the better equipped you are and the more you can gain in the long run.  If you do not use it to near the maximum, why did you just spend big bucks when you only needed a unit that just shows you water depth and a plotter.  If you were trying to impress your fishing friends with an expensive unit, they will soon figure you out if they ever fish on your boat and you only know how to turn your unit on or off.

Personal Experience ;  One day a few of years ago, I went Columbia River sturgeon fishing with a friend in his boat.  Since we had fished this area many times, he did not turn on his sonar/GPS unit.  He seems to have an aversion to running his battery down, so he turns the sonar on only to find his fishing hole or to locate fish.   He really does not understand the GPS, only to the point of seeing the boat on the plotter screen and being able to following his trail back.  Anyway this particular morning we got outside the Cathlamet boat basin where it was slightly foggy, no problem he had done this route more times than I had, (yes I knew the area, but he was the skipper that day).  Well the fog got thicker, then when he started churning up mud off a sans spit off the lower end of Puget Island with his prop did he listen to me, finally turning on the sonar/GPS plotter. 

OK he turned to the right and we got into deeper water, but things got worse as he did not know where he was when he turned the GPS unit on, he had no reference point as to where he had been.  We now knew the water depth, and we were near the island we had just left, but that was about it.  And to add to his excitement, a freighter we could not see in the fog blew it's horn.  This freighter could probably see us on his radar.  Mind you these freighter's horns sound formidable in the fog especially when you have no idea how close she is or where she is at.   With us being unsure where we were in the fog we surely did not know whether the freighter was in bound or outbound.  

I suggested we head west knowing we would have to encounter the river side of Telleashee island.  before long, we picked up some piling on the edge of that island we knew then where we were and that we were then out of the shipping channel enough to let the freighter pass by.  The problem that we could not tell where we were because his GPS unit was set for the chart to always be forward of the boat's bow.  So every time he steered the boat in a different direction the chart changed it's appearance and nothing looked familiar.   It took a bit of looking in his manual for me change his unit to now read chart NORTH or top of the screen.  Even though he knew the river, with the GPS chart constantly changing he was being confused, not really watching his compass as he should.   He was out of his elements and did not really know exactly where he was.   This is just one example of what can happen and with the potential of even being disastrous.

Another time I accepted a invite aboard a friends boat to fish the lower Columbia River Buoy 10.  I had always fished upriver around the Megler Bridge so knew that upper section of Buoy 10, but had never fished the combat zone downriver but wanted to learn that area.    He owned a older 20' Glasply that had a Buick V6 for inboard/outboard power.

We left the Illwaco boat basin heading to the main river and it was slightly foggy.  He asked me to take the helm so he could get his rod holders in position.  I had not been out of this channel on my own for a good number of years.   Another boat passed us so I upped the throttle to keep him in sight, but was told, no don't go over his set RPM.  I found out later that his water pump belt was loose and if he upped the RPM, it started squealing.  He did not know what it was, but as long as he did not try to go faster, no noise, hence no problem. 

I soon lost sight of the other boat, so now I was on my own.  His depth-finder was reading 40', so I figured I was still in the newly dredged channel.  All of a sudden the prop is chewing up MUD.  I do a portside turn and get back into THE CHANNEL.  I ask if there is something wrong with his depth-finder? 

His answer is, well it is on demo mode and he could not figure out how to get it off.  We made it out the channel, into the river and the fog did lift.  But we saw so many of the same fish under us that day that I began naming them.  Why did he even turn it on, except he was trying to impress us, (or calm our fears).

Another time in the early 70s I was operating a 22' inboard "Kelper" (inshore commercial salmon troller) out of Westport.  This boat was not fast but very seaworthy.   All we had for navigation was a compass and a rotating flasher type depth-finder.  If it was foggy, I always made a effort to never go straight west, but to run either north or south after leaving the mouth of the river.  As with the fog it would be hard to evaluate the drift.  I did not want to be in an area where I thought I was north but was really south of the entrance, so I decided to come in. 

This particular day I ran north.  Fished until about 2PM, decided that we had better head back so I could cross the bar at slack tide.  I had no idea of how far north we were.  I ran in toward the shore until I got in about 40' of water.   The plan was to troll south following a string of commercial crab pots until I got into 50' or 60' of water, which meant I was in the river channel outside the bar, turn, and head east into the mouth of the river.

Not knowing just how far north I was, we just kept moving south.  Finally I heard a buoy slightly southwest of me.  I headed for it, and it turned out to be #3.  I was there, all I had to do now was head east, to go inside the river.  To my east about 150 yards just in sight was about a 18' outboard boat.  Out of the fog popped a commercial troller that pulled along side other the small boat, the skipper leaned out of the wheelhouse and hollered a question.  I saw one person onboard the outboard shrug his shoulders while open both palms extended outward.

This troller kept on coming toward me and asked "Which way is Westport"?  I pointed straight ahead of me to the east as and off his stern, as I was trolling in then.  I told him that I was going in and was about to pull my gear so for him to follow me.  The outboarder was watching out conversation and when I started in, he fired up his motor running ahead of me at my speed, but on my heading.  The troller was following.   Using the compass, I headed a bit to the south so if the fog was laying in thick all the way in I did not want to miss the point and wind up on the mudflats upriver.

When we got in to about 300 yards inside of #9 buoy, the fog had cleared up.  That little boat soon found high gear and headed for the boat basin.  Of the three of us I was the only one who actually knew where I was and that was because I had investigated the buoy's bell.


Copyright 2010 - 2015 LeeRoy Wisner  All Rights Reserved 

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Originated 04-04-2010, Last Updated 12-24-2014
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