How to prepare a nautical chart

[Dan Foster]

Here's one approach to marking up your nautical chart to make it more usable for pre-trip planning and for on-the-water navigation.

 

Step 1: Identify the scale bar and figure out how far one nautical mile is on the chart. Find the lines of latitude (they are the horizontal lines going across the chart), and the degrees and minutes of latitude going up the left and right sides of the chart. If your chart has grid lines for every ONE minute of latitude, the vertical distance between two adjacent lines of latitude should be exactly the same distance as one nautical mile on the scale bar. Confirm this. Also confirm to yourself that the horizontal distance between the vertical lines of longitude IS NOT one nautical mile (unless you're paddling at the Equator).

If your scale bar is only a few nautical miles long, and you're planning to measure longer distances, now is a good time to extend that scale bar line out and add additional hash marks so you can measure longer distances accurately.

 

Step 2: Find the compass rose on the chart. The outer ring on this chart is aligned to True North. The inner ring is aligned to Magnetic North. Your kayak's deck compass (or your handheld compass) gives you bearings relative to Magnetic North, so it makes sense to always work with Magnetic bearings when sea kayaking. We will scribe Magnetic North lines across the chart so we always have a reference direction for Magnetic North handy. Begin by extending the Magnetic North line through the center of the compass rose, in both directions.

 

Step 3: We will draw parallel lines across the entire chart to create our north reference. To further simplify things on the water, we'll space these lines one nautical mile apart. Take a piece of paper, place it along the scale bar, and make marks every nautical mile. Below you'll see that it's exactly one nautical mile between the latitudes of 43° 37' and 43° 38', confirming that one minute of latitude is exactly one nautical mile.

 

Step 4: Align the edge of the paper with the magnetic north line you drew through the center of the compass rose, and make small marks to the left and right of that line, spaced every nautical mile. Slide the paper further up the line and make a second set of marks.

 

Step 5: Using a long straightedge or ruler, connect the marks to create parallel lines on your chart, one nautical mile apart.

 

Step 6: Once you've scribed Magnetic North lines across your chart, you can further enhance your chart by highlighting things like launch sites, destinations, possible bail-outs or stopping points for lunch, so that you can find them easily. You can also highlight shipping lanes, channel crossings between buoys, or other hazards that you might overlook while bouncing around on the water.

 

Step 7: Tidal predictions for smaller harbors and points along rivers (secondary tidal stations) are calculated based on a fixed time offset from the nearest large port (BOSTON, PORTLAND). Because these offsets are fixed, you can write them right on your chart. If you're paddling on the open ocean, you might not care too much about the slight differences in high tide times for locations on your chart. If you are in an area with complex tides and currents (like the Piscataqua River near Portsmouth, NH), having your chart marked up to show that high tide at Dover Point is always 1hr 33min later than high tide at Portsmouth Harbor makes it easy to plan a trip's timing, just by referring to the tide cycle for the day at the reference tidal station.

Use the attached PDF to determine the tidal time differences for the locations shown on your chart, and make note of the time offsets at those locations on your chart. Write the reference station (Boston, Portland, Newport RI) somewhere on the chart as well!

NSPN Tidal Differences.pdf

 

Going further: If there's a particular crossing you plan to use on this and future trips, draw in a straight line representing your intended crossing, and annotate it with the distance (measure against your scale bar) and the magnetic bearing and back bearing (use the compass rose, a protractor, or your hiking compass, making sure to use Magnetic North, not True North, as your reference)