Cdncoldwater
Contributor
Same where I normally dive. It's easy to do object to object nav in great viz. Like land navigation in the day compared to night
Tips for navigation?
- Thread starter Janie88
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
Same where I normally dive. It's easy to do object to object nav in great viz. Like land navigation in the day compared to night
Okay, I have scanned this thread, and I see no mention of declination.
Let’s say you and your buddy are down on a shoreline dive, and find something big or heavy on the bottom that you want to return to. You surface straight up, and after your safety stop (no current) get up and look around. You see a lighthouse and a jetty, one off to your left and one off to your right. You can take bearings off those landmarks that are on a map, and pinpoint your location on that map. But you need to know declination.
The compasses you discuss, including the Teric, point to magnetic north, not true north. Usually, this is not a problem, but you cannot say you really know about compass navigation (underwater or on land) without at least mentioning declination, or how far off true north your compass is when pointing to magnetic north at your location. The Teric manual states, about declination, that you can look it up and set it into your Teric internal compass, but in most cases this is unnecessary as you are relating it only to getting back to a ship or a point on land. But if you try to relate compass readings in magnetic north (the reading on a uncompensated compass) to a map, then you need to know the declination of your are and change the reading to the actual true north reading. Navigation maps usually have this printed on them, but the declination changes over time, so don’t use it off a twenty-year-old map.
SeaRat
PS, a compass on land is useful in several ways, even without a map. If lost, it will allow you to walk in a straight line. It will also allow you to take a bearing on a landmark, so that you can transmit that to rescue forces via radio or cell phone. If you can see two landmarks, you can give a bearing to each, and they can then use this triangulation method to pinpoint your location for possible helicopter evacuation. It would be best if you could give those bearings in true north rather than magnetic north, as you would then depend upon someone else to convert your magnetic north bearing to a true north compensated bearing using the area’s declination. I always carry a compass while out hiking, and usually a very detailed contour map too if I’m not familiar with the area. In my area of northwest Oregon, that magnetic declination is 15.2 degrees east of true north, so you can see not being able to convert can put you very far off course.
Let’s say you and your buddy are down on a shoreline dive, and find something big or heavy on the bottom that you want to return to. You surface straight up, and after your safety stop (no current) get up and look around. You see a lighthouse and a jetty, one off to your left and one off to your right. You can take bearings off those landmarks that are on a map, and pinpoint your location on that map. But you need to know declination.
The compasses you discuss, including the Teric, point to magnetic north, not true north. Usually, this is not a problem, but you cannot say you really know about compass navigation (underwater or on land) without at least mentioning declination, or how far off true north your compass is when pointing to magnetic north at your location. The Teric manual states, about declination, that you can look it up and set it into your Teric internal compass, but in most cases this is unnecessary as you are relating it only to getting back to a ship or a point on land. But if you try to relate compass readings in magnetic north (the reading on a uncompensated compass) to a map, then you need to know the declination of your are and change the reading to the actual true north reading. Navigation maps usually have this printed on them, but the declination changes over time, so don’t use it off a twenty-year-old map.
SeaRat
PS, a compass on land is useful in several ways, even without a map. If lost, it will allow you to walk in a straight line. It will also allow you to take a bearing on a landmark, so that you can transmit that to rescue forces via radio or cell phone. If you can see two landmarks, you can give a bearing to each, and they can then use this triangulation method to pinpoint your location for possible helicopter evacuation. It would be best if you could give those bearings in true north rather than magnetic north, as you would then depend upon someone else to convert your magnetic north bearing to a true north compensated bearing using the area’s declination. I always carry a compass while out hiking, and usually a very detailed contour map too if I’m not familiar with the area. In my area of northwest Oregon, that magnetic declination is 15.2 degrees east of true north, so you can see not being able to convert can put you very far off course.
AfterDark
Contributor
Declination has never been an issue for me while diving. The azimuth I follow underwater will take me were I want to be declination be damned. If I was nav'ing a boat to a point many miles away declination would absolutely be an issue. Since I'm usually only making a loop or triangle zig zag pattern basically go out and back magnetic north is all that is needed.
Current drift is more of a u/w nav issue than declination IMO.
Current drift is more of a u/w nav issue than declination IMO.
Julius SCHMIDT
Contributor
Around the time of getting in the water I set a reciprocal heading for where I am, which is opposite to where I'm heading, as getting back is all you need.
AfterDark
Contributor
That's a plausible scenario where factoring declination is required. However wouldn't a SMB secured to the found large object be a better way to mark the object? Your home declination is about the same as mine here RI except in the opposite direction.
I'm about 2 1/2 hours from you just south of Pittsburgh. I teach an UW Nav course that I wrote myself after determining the majority of courses by every agency including the ones I've certified through, are not very well set up to provide a solid foundation if taught by the book and to the standards of the course. They have objectives that are unrealistic for divers not familiar with navigation and don't require enough time. They are 3 or 4 dives, one was actually only two (YMCA but it assumed they had YMCA OW which spent some time on Nav.), and the exercises are really not set up for lower vis and other challenging conditions.
The course I offer is 6 dives with realistic goals and time to achieve them through repetition and builds on each skill. It also uses lines and reels, has students creating a map, works on buoyancy and trim as well as using strict buddy skills to share the task loading.
It's not cheap. 275.00 per for a 2 person team or 375 for a private class. Not including gear, quarry fees, etc.
But you will get an education and have a solid foundation to work with.
AfterDark
Contributor
OP jump on this you'll never get a better offer than this! Heck I'd take myself if I were closer.
I think you mean to say magnetic declination or variation.
Declination without the modifier is the angular distance of a celestial object from the equator. E.g., At the winter solstice, when the sun is overhead at the Tropic of Capricorn, its declination is 23.5 degrees south.
Variation (the term usually used in piloting) and magnetic declination (the term used in scientific circles) refer to the same thing: the angular difference between true and magnetic north. That is the value you can find by looking at the inner compass rose on a navigational chart. It is usually irrelevant to underwater navigation for two reasons: first, the compact size of dive compasses makes them difficult to take precise bearings; the compass is designed to follow general headings. Second, underwater navigation for divers seldom involves moving between charted objects or over distances great enough for a few degrees to matter.
However, variation is only one form of compass error. Deviation is the sum of all other errors in your magnetic compass, usually caused by metal near the compass. It varies according to your compass heading and can cancel out, magnify, or have negligible effect on the variation for your position. Inspected passenger vessels are required to have deviation tables for their compasses. I do not know any divers who have ever measured deviation for their dive compasses, nor can I imagine ever needing to. It's important to mention it here because knowing variation without knowing deviation means having no clue as to your overall compass error.
The scenario you describe is not underwater navigation. It is coastal piloting without a chart or suitable navigation tools.
The best way to mark the spot you describe with only a dive compass would be to identify three charted objects, measure the angular difference between A and B and between B and C, then use a three-armed protractor to mark the location on the chart. This method will be still be imprecise because of the limitations of the compass, but it will be better than taking two imprecise bearings with a compass of unknown deviation, which will yield only a very rough estimated position.
As a practical matter, your best bet for finding the location again would not involve using a compass at all: simply identify two sets of natural terrestrial ranges from that spot that are at least 30 degrees apart.
Cdncoldwater
Contributor
Agreed and I adjust my land compasses for declination but I don't see that as an option on a large percentage of common dive compasses like common Sunnto or XS Scuba wrist or console mount types. You can find the declination for the area that you are diving and work that in you nav plan but you (generally) don't travel the same distances under water so there is less impact on the outcome.
Similar threads
