• Nautical Charts - Rastor, Vector, RNC, ENC, What does it all mean?

    Marine charting has come a long way since the days of those infamous pirate maps where X marks the spot. I am still searching for Blackbeard's stash of gold, and maybe that explains why I am such a beach comber. The history of maritime charts goes back to the days of the earliest explorers, and a detailed account would fill volumes of books. For that reason consider this article as a primer as we explore some of the more important aspects related to charting.

    Map vs Chart
    If you are reading this article, it is likely that you have entered the world of water, that is you have decided to become a mariner. If that is the case, you can put that map down and pick up a chart. There is a difference!

    A map is designed for land based travel emphasizing land forms such as roads, cities, streets, and a overview of the topography. Although maps may show areas along the coast, the depictions are very general in nature and usually don't provide enough detail for safe navigation. Maps are specifically designed to show the possible surface paths to be taken and make no allowances should you leave the depicted path. Maps do not consider your method of travel and are considered static documents meaning that you consult them for information but do not interact with them. In general, they provide you with a path from point A to point B.

    Nautical charts are designed for waterborne navigation and provide an increased level of information. Charts display detailed information regarding coastlines allowing for precise navigation. Charts also provide depth information along with other sub surface hazards, such as rocks, reefs, wrecks and other obstructions. This allows a user fix their position precisely and present many paths that can be traveled. This means it is a working document allowing the user to interact by plotting courses, mark hazards, etc. In general a chart does not provide a specific path, but instead provides you with areas to avoid.

    Originally charts were nothing more than crude drawings as mariners noted information pertinent to navigation on whatever medium was available. As time passed mariners began sharing information with each other and charts began to become more accurate and informative. However, the way various features were depicted or "symbology" was not standard throughout most of the world. Additionally the level of accuracy varied among different charts makers. It quickly became apparent that some form of standardization should be developedso mariners could confidently rely on the charts for safe navigation.

    Today the International Hydrographic Office (IHO) and the International Maritime Organization (IMO) through international agreements jointly oversee the standards and accuracy of nautical charts. These organizations established the S-57 standards that control how information is presented to the mariner and are in use today.

    Navigation Charts

    Paper Charts
    The first charts were depicted on paper or some other medium, and it was not uncommon for large ships to have hundreds of different charts. You would likely find a navigator pouring over the charts with dividers and ruler in hand plotting away. Even with the proliferation of computerized navigation equipment using electronic displays, many mariners still prefer to use the old style paper charts. Although not as compact as electronic charts appearing on a display, when you roll out a traditional paper chart, you will get a broader situational awareness of your local environment, no zooming or scrolling required to find important data. For this reason, many mariners more comfortable using paper charts.

    Raster Navigation Charts (RNC's)
    In the early 80's the first ENCs began appearing as paper charts were scanned and digitized. These files were then known as raster files and contained a grid of different colored pixels (dots). When presented on an electronic display these dots are either turned on (displaying a color) or turned off (displaying white) and as a whole depicts an image. Think back to the old dot-matrix printers and you will get the idea. Raster images are rendered pixel by pixel.

    Attachment 2196

    Example of Raster Navigation Chart, Main Channel Miami

    Raster images depict the older style navigation charts very well and when coupled with a GPS provide for an accurate source of navigation. This allows fans of these charts to be able to use them with electronic navigation systems.

    However, there are some problems with RNCs. The RNC is essentially just a picture, a snapshot that has been digitized to show on the screen. For this reason, you cannot interact with the raster chart like in the same way you interact with a vector chart as you will see later. Raster files tend to be significantly larger than vector files since every image has to be drawn pixel by pixel, this can slow the processor down in some computerized navigation system and is usually most evident when scrolling. Finally, when you zoom in on the chart, the image will become pixilated (fuzzy and illegible). Note the difference shown in the image below.

    Attachment 2194

    Both text images are the same font size and have been zoomed in over 200%. Notice the fuzziness of the raster image on top when compared with the vector image at the bottom. When zoomed the raster image begins to show the individual dots (pixels).

    Electronic Navigation Charts (ENC's)
    Okay now this can be confusing at first but hopefully this explanation will shed light on the issue. Raster charts are images of paper charts that can be displayed electronically; however, vector charts only exist as an electronic file and are not produced in paper form, therefore, they are referred to as electronic navigation charts.

    ENCs are vector files, and they draw images in a different manner. Instead of drawing an image pixel by pixel, vector files depict images using lines, circles, arcs and points. For example, to draw a square the vector file does not need to store every single pixel, instead it will store 4 points and tell the computer to draw a line between them. It will then tell the color to apply the various colors to the objects. For this reason vector files tend to be smaller and render much faster. Additionally, vector files store all the navigational information as individual objects within the file. This means you can interact with them. On most navigational systems displaying vector data you can just place your cursor over an object, a buoy, for example, and specific information about that buoy can be displayed onscreen.

    Attachment 2197
    Example of Vector Chart, Main Channel Miami

    Compare the view of the vector chart sample above with the raster chart sample of the same area. One of the best advantages of using vector charts not only is the ability to interact with the objects, but information can be displayed in layers. This allows the user to decide what information and level of detail displayed. For example, in the chart shown above the information for each buoy is being displayed, this information can be turned off to declutter the display should you desire.

    Navigating With Charts

    To ensure you navigate safely with nautical charts, there are some key points that are often overlooked or misunderstand that you need to be familiar with. Much of this information is provided immediately below the title block of the chart as shown below.

    Attachment 2200


    Most coastal and offshore charts are developed using the Mercator projection. This type of chart can best be imagined by wrapping a piece of paper around the globe and turning on a light inside the globe so shadows show through the paper. This results in the parallels of latitude and meridians of longitude maintain a vertical and horizontal relationship. This method helps to maintain directional accuracy over longer distances making a navigator's job much easier. However, these charts actually distort the land features especially near the poles. Remember those paper charts on the wall that show Greenland as a huge body of land, well it isn't really that big. This is a result of the distortion.

    The other most commonly used projection used in nautical charts is Polyconic. The best way to grasp the concept behind this type of projection is to imagine slicing the meridians of longitude and peeling them off the globe to form a flat chart. These charts do not maintain the horizontal/vertical relationship between parallels and meridians like a Mercator projection does and therefore, are not useful for long distance navigation. However, they minimize distortion and maintain accuracy of land features much better. Polyconic projections are commonly used when charting the Great Lakes and major rivers.

    Attachment 2199

    Chart scale values can be confusing to new users, but it is a ratio such as 1:40,000 which means 1 inch on the chart is equal to 40,000 inches in the real world. So if you are looking for a chart that shows less area (zoomed in) but more detail you actually go down in scale numbers, say 1:10,000 for instance. This is known as a large-scale chart, things appear larger!

    Horizontal Datum
    The horizontal datum is the reference datum upon which all the chart's measurements are made. This is to ensure consistency among the different maps. The most current datum widely in use in the US is the North American Datum (NAD-83) although you may find a few older charts that reference the older North American Datum 1927 (NAD-27). If you are using these charts with a GPS you must ensure that the GPS is set to the correct datum to match the chart, more on this later.

    Soundings are the depths of the water and various features. Just like the horizontal datum, sounding are based upon a vertical datum to ensure accuracy. Most newer charts are referenced to the more conservative mean lower low water (MLLW) datum. This datum is the long-term average of the lower of the two daily low tides. On older charts, you mind find they are referenced to mean low water (MLW). Mean low water is not as conservative as MLLW since it is a measure of the average local heights of all low tides. Since this value includes the higher of the low tides it can tend to skew the numbers to show to indicate more depth than actually may exist.

    The Dangers of GPS and Electronic Charts
    Coupling a chart to a GPS (chartplotters) in most cases can produce amazing accuracy, down to 10 feet in some cases. However, there are times when your actual positioning can be off as much as several hundred feet or even miles when compared with a chart. With a GPS achieving 10 feet of accuracy, how is this possible? Allow me to explain.

    In early chart development, the surveyors would usually precisely locate a point on the earth's surface and astronomically determine the latitude and longitude. They were very good at this and could fix their position within a few inches, amazing huh? As they began their mapping, they would start from that point and drag out their chains and rods to survey a line. From this point, they could find a prominent feature and determine the angle from each end of the line to that feature. They now had the length of the line and two of the angles. With that information, they used that stuff we hated in high school, trigonometry to compute the length of the two remaining sides of the now formed triangle.

    Eventually, surveyors became able to fix their positions so accurately using astronomical bodies that they realized one degree of latitude in one location of the world did not cover the same distance as one degree of latitude in another location. If the world were a true sphere that would be impossible. So they realized that the earth was not actually a true sphere. Oh dear now we are back to the earth is flat, well not exactly!

    Attachment 2206
    Sphere vs. Ellipsoid

    What they discovered was instead the earth was an ellipsoid. An ellipsoid is nothing more than a sphere that is flattened (wider than tall) as you can see above. This can also be referred to as a spheroid. If you look closer this will all begin to make sense.

    Attachment 2203

    Note in the image above we have zoomed in on the image and have three points placed where the sphere coincides with the ellipsoid. Imagine if you will that we take a length of string and measure from point A to point B along the perimeter of the sphere, now measure from point A to point B along the perimeter of the ellipsoid. When you compare the lengths, you would notice that the distance along the sphere is greater than the distance along the ellipsoid, yet we are technically going to the same place (the same latitude). Now if you do the same exercise going from point B to point C, you will notice the reverse is true, the distance along the sphere is less than the distance along the ellipsoid. So as you moved from one celestial derived point to another the distances were not consistent and lead to errors in charting.

    So the surveyors and geodesist realized that to accurately chart an area, they needed to base the datum for the charts on an ellipsoid and went to work. Since the distances varied depending on what part of the globe you were located, ellipsoids were developed and tailored for specific areas. This is the model that is still in use today even with satellite-based navigation, you might want to read that again and remember it as we progress further. This ellipsoid model for the US is known as the North American Datum of 1927 (NAD-27)

    As time passed, the geodesists began to notice that the ellipsoids used as a datum for the various charts were extremely accurate. However, if this same datum was applied to a different area the accuracy was severely degraded and thusly unsuitable for navigation on a worldwide scale. They realized that the earth was not even a perfect ellipsoid. Instead the earth contains rises and falls in its general shape and these are not consistent at the same latitudes in other parts around the world. Although the shape is exaggerated, think about looking at a potato and the way it has dips and rises to its skin. Additionally, the surface of the earth undulates on a local scale with valleys and mountain ranges further complicating the problem. This geodesists arrived at a shape that discounts all elevations above the mean sea-level and is known as a geoid. The geoid is the closest approximation for the shape of the earth.

    In an effort to standardize a world navigation system the geodesists continued to work toward a global solution and with the help of space based navigation technology were able to agree upon a solution. Since a geoid has no consistent shape it cannot be predicted mathematically, the geodesists agreed to create a new ellipsoid based on the shape of the geoid. This ellipsoid is known as the "reference ellipsoid" and is the basis for the datum known as the World Geodetic System (WGS-84). The WGS-84 ellipsoid is the closest mathematically defined representation of the actual shape of the sea level world and has proven to work fairly well for global navigation within certain limitations.

    Attachment 2204

    Okay to refresh our memory the geoid is a rough approximation of the earth discounting anything above mean sea-level. The reference ellipsoid is what is used by GPS and is the WGS-84 datum. With that in mind you may notice that vertically we may be below the WGS-84 ellipsoid (position A), and at other locations (position B) may rise above it especially when considering tides. This is why on occasion, mariners may find the GPS showing them at a positive or negative altitude above sea-level. The GPS is referencing the mean sea-level of the ellipsoid. Although this is not a major concern for mariners, it does point out how errors are still encountered.

    WGS-84 is a worldwide space based (GPS) navigation datum. Currently, there are no charts that have been developed solely from the WGS-84 derived ellipsoid. Chart conversions are slowly taking place but since the old charts datum is based on astronomical positioning there is no method to mathmatically convert this information to WGS-84. This requires the areas to be re-surveyed at an enormous cost.

    The bitter truth is that the GPS is extremely accurate and can tell you exactly where you are within a few feet. However, since it is referring to a totally different datum, this may not coincide with what your chart depicts. The GPS is not wrong and neither is the chart, since they are measuring from two different datums you are asking them to compare apples to oranges.

    If your GPS allows you to change from the WGS-84 datum to the chart's datum, it may be wise to do so to achieve the best accuracy. For instance, if you are using a NAD-27 chart, set your GPS datum to NAD-27. However, you must realize when you do this the GPS does a mathematical conversion, and even this is prone to error. In North America, these errors can be several hundred feet while in other parts of the world these errors can be several miles or more. Just ask anyone who has sailed extensively in the Caribbean or South Pacific about those reefs that seem to magically change locations.

    In 1983, North American charts began the slow process of coverting to a standard that is more compatible with WGS-84 known as North American Datum (NAD-83). The compatibility is normally annotated in the title block. Note how the chart below shows the 1983 datum (NAD-83) and immediately below that shows in parentheses WGS-84.

    Attachment 2200

    Some older charts may have a statement inside the title block referring to GPS or Satellite Navigation and provide a correction factor while others do not. Even worse some charts show no reference datum at all.

    Attachment 2208

    Hopefully, the diagram above will provide a clearer image of what can occur. The GPS is using WGS-84 datum to determine the position of the vessel and the reef. However, the reef was plotted using its original charted datum. The old datum latitude of the reef was charted as 31o04.0'N so I determine that I will track along the 31o00.0'N latitude and should clear the reef by roughly 3 nm. However, the error between the two different datums is 3 miles in this area. The chart plotter shows that the boat will pass well clear of the reef. However, in reality since the boat is navigating on WGS-84 data its true position is much closer to the reef, and disaster will result.

    In the days before GPS navigation mariners routinely gave obstructions a wider berth since they were unable to fix their positions accurately. Since GPS has proven to be an extremely accurate form of navigation, mariners are routinely cutting the distance by which they maneuver to avoid obstacles. This fact along with the errors mentioned above have contributed to an ever increased frequency of groundings.

    To avoid a possible calamity mariners should consult the paper charts once along coastal areas or in areas of obstructions. Since the aspect and distance ratios on paper charts are extremely accurate, confirm your position by taking bearings on charted objects and compare these with what you see on the chart plotter. Most of all be extra vigilant when navigating by GPS alone.
    Capt Tony and Mr Kins like this.


    Comments 3 Comments
    1. Chris Hayes's Avatar
      Chris Hayes -
      Well done!
    1. TimG's Avatar
      TimG -
      Thanks Chris!
    1. greenghost39's Avatar
      greenghost39 -
      ENC Downloads
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