Wednesday, October 4, 2017

Magnetic Dip and Zone Balancing

Traveling from Seattle to the South Pacific?

To preserve compass accuracy when traveling large distances, the dial card in your compass should be balanced in order to compensate for the dip caused by the Earth's magnetic field. The picture below is the  back of a compass card, showing the two magnets that make it work, along with the daubs of solder on the corners that are applied to balance the card.

Here are a few related notes from the Ritchie Compass company
"Ritchie compasses come standard balanced for Zone 1, which essentially includes all of the Northern hemisphere. If you're requesting balancing for Zones 2-7, simply indicate the zone that is most central to your boating area."

"Once your compass is balanced for a specific zone, it will maintain accuracy for one Zone north or south. Ritchie recommends using a compass that is balanced for the zone where the boat will be operated most frequently."

To help you judge this more specifically, see this global plot of the dip angle (inclination).

You can also look at just the vertical component of the magnetic field as another way to judge this. A high res pdf of that, along with one of the inclination shown above, plus world variation and other useful tools is at NOAA's World Geomagnetic Data Center, which is now part of the NCEI, see especially the link to these types of images (they are high res pdfs). It could be valuable to save the pdf of world variation—they call it "declination."

Here is another company's plot of the zone balancing overlaid on one of these dip maps... at some (unknown) date in the past. The lines have slipped north somewhat over whatever time period is reflected here.

Tuesday, September 26, 2017

Minor Light Symbols on ENC in OpenCPN

OpenCPN is a powerful open-source (free to the user) navigation program that has many features of top of the line commercial programs. Downloads and detailed documentation are at There is a Mac and PC version, as well as Linux. It shows both raster navigational charts (RNC), which are graphic images of the paper charts, as well as electronic navigational charts (ENC), which are vector products that include extended navigation information and offer many user display options. We cover ENC usage in our new book Introduction to Electronic Chart Navigation.

OpenCPN is also unique in offering one of the truest presentations of vector chart (ENC) symbols as specified in the IHO standard S-52. This is a complex system, and many navigation programs have either taken shortcuts to simplify the presentation, or they willfully decided they have better ways to present certain symbols that are more useful to the mariner, and they use those, rather than the ones specified in S-52.  Some of the diversions from the S-52 standard are minor and do not matter much. Others can be distracting, and hinder the use of ENC charts.

We see numerous examples of this in several nav programs, but as noted this is extremely rare in OpenCPN, which is very good with the S-52 standard of presentation. The one exception I note now has to do with the presentation of minor lights... and I must add immediately that they are aware of this, and it will be fixed shortly, at which time this note will be deleted.  But since we use this fine program in our navigation courses, we point this out for now.

A "minor light" is one that has nominal range < 10 nmi. Major lights are shown as a ring (about 1" in diameter on the chart, regardless of selected chart scale); whereas minor lights should be a single flare symbol (teardrop) oriented toward 135T.  The existing latest version of OpenCPN (v4.80) still shows the minor lights as small rings. So when using this version or earlier, please keep that in mind and know that later versions will have this fixed. Below is a sample from OpenCPN along with a couple samples that show the minor lights right, but then show the major lights wrong.

This view of OpenCPN shows the major light (Ediz Hook Lt VALNMR = 16M) properly with two rings one for each color (white and green... white is shown as yellow on ENCs), but the minor lights (<10 M) are shown as small rings, and these should be plain flare symbols as shown on the buoys.

Below we see a presentation from another program that has this almost right. It has the minor lights correct, but shows only one ring on the major light.  It also has it too big.  Hard to discern this in these pics, but ENC symbols should not change size with view scale

Below is another example from still another program showing the minor lights correctly but a different error on major one. Shows a tear drop for the major light, and indeed only one, where it should be two.

Note that in traditional chart navigation and in the use of RNC, we tend to think of a signle light having two colors, rather than this object being two different lights.  Indeed, in many cases it is just one light with different colored lenses that rotate around the light.  But in ENCs, this configuration is called two lights, and each one is listed separately with its own specs. Often the nominal range of a color sector is less than that of the white sector—although this is not the case with the specific major light we look at here. See our ENC text for such details.

Below still another error. It shows there are two lights there, green and white, but uses the wrong symbol. This is from NOAA themselves.  They have acknowledged this error and say it will be fixed in the near future.  Again, though, we see there is no question about the minor lights.  OpenCPN simply has this wrong for now, and as noted, they will be fixing this shortly.

This minor flaw in OpenCPN does not at all distract from our high recommendation of the program. If navigation within a specific harbor in practice or in a practice exercise is being distracted by these little rings, then just turn the lights off (an ENC option) until you need them.

I look forward to deleting this blog post.