The coordinate system on Nautical charts
The coordinate system on Nautical charts is used to indicate positions on Earth. We do this by representing latitude and longitude. The equator divides the earth into the northern and southern hemispheres. The standing side of the chart therefore shows N for North or S for South. You can read which latitude you are at on the standing (vertical) side of the chart. At the equator you are at 0 degrees and at the north and south poles at a maximum of 90 degrees north or south latitude. Each degree is divided into 60 minutes and each minute is divided into 10 tenths of a minute. The chart shows black-and-white chequered borders with minutes. Note that on one chart the minutes are divided into 10 little squares, each then being a tenth of a nautical mile. On other chart, each minute is divided into 5 squares, each corresponding to 2 tenths of a nautical minute. The horizontal lines on the chart indicating north or south latitude are called parallels. These run parallel to the equator. Meridians are vertical lines from the north pole to the south pole that indicate longitude. The 0 degree meridian that runs through London/Greenwich divides the earth into two parts, the eastern and western hemispheres. The meridians increase to 180 degrees east and 180 degrees west. On the horizontal (=horizontal) side in the chart, you can see whether you are in the (E) eastern or (W) western hemisphere.
There is a standard method of noting the position, for example: 51° 00.6′ N and 003° 25.9′ E. So with leading zeros to ensure that there are always the same number of numbers noted and note the degree and minute sign. Only the minutes shown on the vertical side of the chart correspond to 1 Nm (=nautical miles) and are therefore displayed more stretched at greater latitudes. Therefore, pick the nautical miles on the vertical side at the same latitude as the distance to be measured. 1 Nm corresponds to 1.852 KM. The length of a mile can be calculated by dividing the Earth’s circumference in minutes by the Earth’s circumference in Km (circumference = 40,000km). The earth’s circumference is 360 degrees and each degree consists of 60 minutes. So the circumference is 360 x 60 = 21600 minutes. 40,000 km / 21600′ = 1.852 km.
Mercator projection
The sea charts we use are made using the mercator projection method. This method is used to draw the convex earth on a flat chart. On the mercator chart, headings and compass bearings are straight lines, which is easier than having to draw curved lines in the chart. The mercator chart is angular. But the chart is a natural distortion of reality, because the earth is not flat. First, in the chart the distance between meridians is the same everywhere, while in reality the meridians run towards each other at the poles. Second, in reality, the distance between two parallels is the same everywhere, but on the chart, the distance between the parallels increases, because the minutes on the vertical side increase further from the equator.
Great circle routes
Imagine we want to sail from New York to Porto. If we then draw a straight line in the chart, we find that we have to sail 90 degrees. The straight line in the chart is called a rhumb line and in this case it happens to run parallel to a parallel because the two places are at equal latitude. So if we were to sail straight east, we would also end up in Porto. We would have sailed across that rhumb line (and in this case, coincidentally, a parallel). On the chart, it looks like we sailed the shortest way. In reality, we didn’t. In reality, the shortest way would be a great circle route. If you were to draw a line on a globe the line would run north of the parallel (see the red line in the picture below). That red line indicates the shortest route: that is, the great circle route.
A great circle is a line around the earth that would split the earth into two equal parts.
In the picture below, you can see the difference between the great circle route and the rhumb line. In the chart, the great circle route is a curved line, in this case with the concave side towards the equator.
Chart Symbols on Nautical charts
The chart title on Nautical charts includes the name of the area, the chart datum (to which depths are displayed) and the projection method. The chart shows depth and height (lines), buoys, beacons, fires, soil types, wrecks, obstacles, traffic separation schemes, precautionary areas, current and tide data and the variation. It also often contains warnings, indications and explanations such as: “there are more blind barrels (without light) than charted”. You might consider buying Chart 1 (INT1) and going through it thoroughly. If you want to save on costs, you can also download Chart No 1 (6 MB) for free from the US Hydrographic Service. But note that there are differences with for example the Dutch sea charts. Below some examples.
There are different types of charts: on the overview chart with a small scale we can plot a course to the other side of the North Sea without having to put several charts side by side. On the coastal chart with a large scale, i.e. with many details we can navigate along the coast. Coastal charts often have detailed plan charts of ports which a rectangle drawn. In the almanacs there are plan charts to get an overview of ports but these are not updated with the NTMs so not suitable for navigating.
Notices to Mariners (NTM)
In the Notices to Mariners, you can find changes in the situation (changed depth, removed buoys, beacons, emerged obstacles, wrecks, extinguished lights, closed locks, etc.) and update the chart. You can find BAZs on the Internet and also in water sports magazines. Just download and study a BAZ to get a better idea of what exactly it contains.
Nautical books
Books that are commonly found on board are the following:
- Almanac part 1 (regulations and general info)
- Almanac volume 2 (data on ports and bridges and locks, it must be bought again every year).
- The Netherlands Coast Pilot (HP1) provides detailed information on the sea area of the Dutch and Belgian coast, and on the seaports and accesses to them.
- HP2 provides a description of coastal lighting and fog signals, with separate chapters on radio beacons, racons and light buoys.
- High water table and Currents (HP33) is an annual publication containing water Height of tide tables and tidal currents along the Dutch coast and adjacent area.
- Chart 1 (INT1) is a standardised book on the signs, abbreviations and symbols used on Dutch nautical charts and other international nautical charts.
