One of the major hurdles to understanding how to use electronic charts is decoding the alphabet soup of file names and conventions associated with the process.

When you download charts from NOAA or the U.S. Army Corps of Engineers, a program called ChartServer bundles, compresses and delivers all of the charts in a single folder to your desktop. What you get, however, is different for each chart format.

For example, the name of a downloaded NOAA raster chart folder is a download ticket number that has nothing to do with the particular charts you have selected. Similarly, NOAA vector charts or those from the Corps are delivered in a folder titled ROOT with a CATALOG.031 file and nested chart folders within. Huh?

Here are the files that come with a typical raster chart download.

To better make sense of all this, let∞s look at an example using two downloads for the Port Canaveral area: one is a NOAA raster chart and one a NOAA vector chart. Once you understand raster and vector file naming conventions, which is what the alphabet soup is tell you, you'll find that file types and file names do matter-and that there is a method to the madness.

NOAA RASTER CHARTS

Raster charts share the same naming convention as their paper counterparts, and so it helps to understand the coding system. Let∞s take RNC 11478. A paper chart∞s first digit refers to a world region. Region 1 covers U.S. and Canadian waters. The second digit, coupled with the first represents a sub-region. The Gulf of Mexico to Cape Hatteras is subregion 11. The remaining three digits are assigned counterclockwise within the sub-region and are typically sub-grouped by chart scale.

Each chart download includes several files. RNC 11478 downloads seven files: two KAP files, one TXT file, three PTC files and one BSB file. Although raster charts are colloquially referred to as BSBs, the charts are actually the larger KAP files. A chart has at least one KAP file, and each KAP file represents a panel of the chart, such as a main panel, an extension or an inset. There are two chart panels for this chart: Port Canaveral (1:10,000) and Canaveral Barge Canal Extension (1:40,000).

The TXT file is a small text file of the user agreement with legal terms and conditions. PTC files provide some software applications the ability to download smaller update files to patch charts with the latest corrections. This compression technology was much more important prior to today∞s high Internet data transfer speeds. The BSB file is a companion file with parameters informing the application software about scale, metrics and geospatial references. Chart notes are also included.

Although not all files are necessary to display the chart, it∞s better not to meddle. The KAP and BSB files are fundamental. Although you don∞t technically need the TXT or PTC files, they are very small and it∞s always safer to keep chart files as a complete package.

VECTOR CHARTS

Vector charts use a different numbering convention. Continuing our example, we∞ll use the Port Canaveral chart, which is called ENC US5FL82M.

Here are the files that come with a typical vector chart download.

The first two digits indicate the country hydrographic office, which in this case is the U.S. Office of Coast Survey. The third digit communicates the scale: 1 is an overview, 2 is general, 3 is coastal, 4 is approach, 5 is harbor and 6 is berthing. The fourth and fifth characters are state postal codes. The next two digits are the ENC number with a final letter M indicating metric measurement.

Corps of Engineers charts, called IENCs, cover the inland river system and use a slightly different naming convention with several important clues to their location. Rather than a state postal code, the two--letter designator is a river code, such as UM for Upper Mississippi. The last three digits indicate that chart∞s beginning mileage. For example a chart name ending in UM155 begins at mile 155 of the Upper Mississippi.

Our example of ENC US5FL82M downloads six files: three chart files 000, 001 and 002, as well as three TXT files. Notice that while the raster chart had two panels, the vector has three cells. Vector cells do not match raster panels. Rather, vector charts use what is called the "S-57" standard, which is internationally recognized and is limited to 5 megabytes, which results in a different "quilting and stitching" paradigm.

There are also three TXT files in this case, rather than a single generic user agreement text file. Vector charts typically include TXT files for notes and cautions. Some chart cells also include JPG image files for photo or illustration references (not shown in this case). For example, the vector charts produced by the U.S. Army Corps of Engineers include many illustrations of bridges and locks with important annotations and photos of recent wrecks or other navigation hazards.

Once you understand naming conventions, you can easily search and display charts. Simply type in a chart number instead of resorting to a slow graphic process, or otherwise use the file name. For example, you can type "UM" if you want to see all charts for the Upper Mississippi.