Geographic coordinate system
The geographic coordinate system is used to indicate positions on Earth. We do this by displaying the latitude and longitude. The equator divides the earth in the northern and southern hemispheres. On the vertical side of the map there is also an N of North or S of South. You can read on which latitude you are on the vertical side of the sea map. At the equator you are at 0 degrees latitude and the north and south pole are 90 degrees north / south. Each of those degrees is divided into 60 minutes and each minute is divided into 10 parts. At the sides of the sea map, the minutes are black and white. Note that the minutes on one sea map can be divided into ten little parts, each box being one-tenth of a sea mile. On other sea maps, every minute is divided into 5 parts, each corresponding to 2 tenths of a minute. The horizontal lines on the map indicating the latitude are called parallels. They run parallel to the equator. Meridians are vertical lines from the north pole to the south pole that indicate the longitude. The 0 degrees meridian that runs through London / Greenwich divides the earth into two parts, the eastern and western hemispheres. The meridians increase to 180 degrees to the east and 180 degrees to the west. On the horizontal side of the map you can see if you are on the (E) eastern or (W) western hemisphere.
There is a standard method to write down the position, for example like this: 51° 00,6'N and 003° 25,9'E. So with leading zeros to ensure that there are always the same number of characters and note the sign for degrees and minutes.
Only the minutes on the vertical side of the map correspond to 1 Nm (= nautical mile) and are displayed in a more stretched out way on a higher latitude. Therefore in case you measure a distance on the sea chart please use 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 sea mile can be calculated by dividing the circumference of the earth in minutes on the circumference of the earth in Km (circumference = 40.000km). The circumference of the earth is 360 degrees and every degree consists of 60 minutes. The circumference is thus 360 x 60 = 21600 minutes. 40,000 km / 216,000'= 1,852 km.
The sea charts we use are made with the mercator projection method. This method is used to draw the earth on a flat piece of paper. On nautical charts made with the Mercator projection, courses and compass bearings are straight lines, as that is easier than drawing curved lines on the nautical chart. The mercator projection is a conformal map projection. But the map is, of course, a distortion of reality, because the earth is full and the map is flat. Firstly, the distance between the meridians is the same throughout the map, while the meridians in reality meet each other at the poles. Secondly, the distance between two parallels is actually the same everywhere in reality, but on the map the distance between the parallels increases with the latitude, because the minutes on the vertical edge increase as the latitude increases.
Great circle routes
Imagine that we want to sail from New York to Porto. If we then draw a straight line in the map, we find that we have to sail 90 degrees. The straight line in the map is called a rhumb line, rhumb, or loxodrome and in this case happens to be parallel to a parallel, because the two places are at the same latitude. So if we were to sail straight east, we would arrive in Porto. We then sailed over that loxodrome (and in this case by chance also over a parallel). On the map it seems like we have sailed the shortest route. In reality that is not the case. In reality, the shortest way would be a great circle route. If you were to pull a rope tight from NY to Porto on a globe, then the rope would not go over the parallel where those places are located, but the rope would go north of it (see the red line in the picture below). That red line indicates the shortest route: the great circle route.
A great circle is a line around the earth that would divide the earth into two equal parts.
In the photo below you can see the difference between the great circle route and the loxodrome. The great circle route, however, is a curve on the map, in this case with the concave side towards the equator.
Symbols and abbreviations
The title of the map shows the name of the area, Chart sounding datum (to which depths are displayed) and the projection method. The nautical chart shows depths and heights (lines), buoys, beacons, light houses, types of seabed, wrecks, obstacles, traffic separation schemes, precautionary areas, tidal information and variation. Also, there are often warnings, instructions and explanations such as: "there are more blind barrels (without lamp) than charted". You could consider purchasing Chart No. 1 Nautical Chart Symbols Abbreviations and study it carefully.
On nautical charts with a small scale, we can set a long course without having to put several cards next to each other. On nautical charts with large scale, so with many details we can navigate along the coast. The coastal charts often contain maps of the ports. In the almanacs there are charts to get an overview of ports, but these are not updated, so not suitable for navigation.
Notices to Mariners Nms
In the Notices to Mariners (NMs) you can find changes to the situation (changed depth, removed buoys, beacons, obstacles, wrecks, lights that have been removed, closed locks, etc.) and update the nautical chart. You can find these NMs on the internet and also in magazines. Download and study an example of a NMs to get a better idea of what Nms exactly are.
Books that can be found on board are the following:
- Nautical Almanac with local regulations, general information, information about ports, bridges and locks. This has to be renewed regularly. The Netherlands Coast Pilot (HP1) for example provides detailed information about the sea area of the Dutch and Belgian coast, and about the seaports and accesses to it.
- Light list (HP2) gives a description of the coastal lighting and fog signals, with separate chapters about radio beacons, RACON's, light buoys and super buoys.
- Tidal Atlas (HP33) is an annual publication in which the water levels and tidal currents along the Dutch coast and adjacent area are included.
- Chart No. 1 Nautical Chart Symbols Abbreviations is a standardized book about the symbols, abbreviations and terms used on the Dutch sea charts and other international nautical charts.