Raster charts are essentially digital copies of traditional paper nautical charts. They're created by scanning paper charts and then used in computer-based navigation systems.
Understanding Raster Charts
Here's a detailed breakdown of raster charts:
Definition
- A raster chart is a scanned reproduction of a paper chart. This means it's a digital image, like a photograph, of a traditional nautical chart. The chart data, including depths, shorelines, navigation aids, and other symbols, are represented in pixel format.
How They Work
- Raster charts are displayed on computer systems, often in conjunction with GPS or other positioning devices.
- The system integrates the chart with the user's current position, providing a real-time view of their location on the chart.
- This allows mariners to navigate using familiar chart layouts while benefiting from electronic display capabilities.
Key Features and Characteristics
- Based on official paper charts: Raster charts are derived from charts issued by national hydrographic offices or under their authority, ensuring accuracy and reliability.
- Pixel-based: Unlike vector charts, raster charts store information as pixels. This means that zooming in too far will result in a loss of detail and the image will appear pixelated.
- Integrated with positioning systems: They utilize continuous electronic positioning systems to track the vessel's location and display it on the chart in real-time. This integration enhances situational awareness and navigational safety.
- Familiar interface: They provide an interface that resembles a traditional paper chart, making them easier for mariners familiar with those charts to adopt.
- Cost-effective: Raster charts can be less expensive to produce and distribute than vector charts.
Advantages of Raster Charts
- Familiarity: They provide a traditional chart look that many mariners are already comfortable with.
- Cost: They tend to be more affordable than vector charts.
- Accuracy: Based on official, reliable hydrographic data.
Disadvantages of Raster Charts
- Pixel-based: Zooming in too much can cause the image to become pixelated and details can be lost, unlike vector charts.
- Limited functionality: They do not offer the same level of interactivity as vector charts. For instance, you can't select individual chart features or query them for information.
- Not easily updated: Updates require new scans of the paper charts to be released.
- Limited layers: You can't turn layers on or off as easily as you can with a vector chart.
Practical Applications
- Navigation: Raster charts are used extensively for navigation at sea, rivers, and other waterways.
- Training: They are used as a tool for training navigators on electronic navigation and familiarizing them with traditional chart reading in an electronic format.
- Backup: They serve as a reliable backup to other types of electronic navigation tools.
Example
Imagine a mariner using a computer-based navigation system. Instead of spreading out a large paper chart, they can view a high-resolution scanned image of that chart on their screen. The system displays their vessel's real-time position on the raster chart, showing them their current course and distance to landmarks.
