NOAA Electronic Navigational Chart Data Handler Extension for ArcView 3.2x

David Stein and Scott Rutzmoser, Technology Planning and Management Corporation Contractors, NOAA Coastal Services Center

The NOAA Coastal Services Center (Center), in partnership with the NOAA Office of Coast Survey, has developed an ArcView® extension called the NOAA ENC Data Handler that allows the Office of Coast Survey’s Electronic Navigational Chart (ENC) suite to be used in the Esri® ArcView 3.2x software environment.   The ENC is a vector database of chart features built to the International Hydrographic Organization’s Transfer Standard for Digital Hydrographic Data (S-57).  The complexities of ENC files are evident by the fact that there are approximately 180 possible data layers associated with a single ENC.  As a result of this complexity, the Center developed an extension that converts, describes, and displays ENC data so that they become useful in a geographic information system (GIS).  The extension uses a software utility developed by Esri to convert the ENC objects to shapefiles.  The Center developed additional functionality that prompts users to choose the data they wish to display for a particular ENC. 

Automated functions include creating views, adding groups of shapefiles, toggling themes, applying symbols, and arranging themes in the table of contents.  All of these functions are accessible through an easy-to-use graphical user interface.  This paper will describe the development and utility of the NOAA ENC Data Handler.

A NOAA electronic navigational chart, or NOAA ENC, is a database of chart features. The structure and format of a NOAA ENC is defined by the International Hydrographic Organization S-57 standard.  The NOAA ENC is in a nonproprietary, publicly available format.   It contains information about the location of each chart feature, as well as attribution information such as color, shape, depth, and so forth.  This information is used by navigation system software to draw a display that resembles a nautical chart, but which has greatly enhanced functionality.  Key definitions describing the various components of ENCs can be found in appendix A.

 A raster chart, by contrast, is a facsimile of a paper chart, or a picture of a chart displayed on a computer screen.  It is a file of information that can be thought of as dots, or pixels, on the computer screen.  The raster file stores the color of each pixel and the computer reads these colors but cannot distinguish a black pixel that is part of a sounding from a black pixel that is part of a dangerous wreck symbol. 

Conversely, the NOAA ENC, as described above, stores the actual feature including its latitude and longitude, as well as descriptive information.  A navigation system using a NOAA ENC would "know" that the dangerous wreck in the above example is a dangerous wreck, as well as how much water is over the wreck. A navigation system using an NOAA ENC can use this information to check planned routes, sound alarms if a vessel’s projected course will move it close to dangerous features, and inform the mariner about various warnings and regulations that relate to areas that the vessel is transiting.  Further, the mariner can control the display of NOAA ENC data and remove features from the display that are not important to the safety of the vessel to unclutter the screen. This allows the important features to be more easily viewed.

Because the ENC data are stored in a vector format, they lend themselves for use in a GIS environment.  There is a wealth of valuable data in an ENC that can be used for research and resource management.  Potential applications of ENC data in a GIS include coastal mapping, environmental assessment, and fisheries management.

Development of the NOAA ENC Data Handler began in early 2001.  The first step was to obtain the S57toArc.dll, packaged with ArcView 3.2, to perform the initial conversion from S-57 objects to shapefiles.  Because of the potential for as many as 180 shapefiles per ENC, the project team made a decision to develop symbology at the feature level.  A series of ArcView legends were created using symbols from S-52, Specifications for Chart Content and Display Aspects of ECDIS, the National Imagery and Mapping Agency’s (NIMA) Geosym Library, and other intuitive symbols.  Once symbols were created, a lookup table was developed to facilitate the conversion of S-57 acronyms to descriptive naming conventions.  Listed below are descriptions of the major functions and the graphical user interface of the extension.

Six major functions were incorporated into the graphical user interface (GUI).  They include converting ENC data, creating views, adding existing ENC data to a view, toggling themes, removing themes, and the addition of a help dialog.

1. Converting ENC data.  A dialog was created that prompts users to convert ENCs to shapefiles (figure 1).  As part of the conversion process, the ENC objects are also renamed to descriptive file names.  The shapefiles are added to the folder containing the source file.  A folder is created by using the name of the source file and adding an "_shpfls".  In addition, legend files for each shapefile are copied to the same folder.

           Figure 1. Convert ENC dialog box.

2. Creating ENC views.   This function will automatically create views using the shapefiles that were previously converted (figure 2).  There are three automatic viewing options: minimum, intermediate, and maximum.  The potential shapefiles for each of the viewing options are listed in appendix A.  After a viewing option is chosen, the user is prompted to name the view.   The view is then created and the shapefiles and legends are added and sorted in the table of contents.  A sample of a maximum view for San Diego Bay is shown in figure 3.

            Figure 2.  Automated viewing options dialog box.

 

            Figure 3.  Maximum viewing option for San Diego Bay.

3. Adding existing ENC data to a view.  This function allows a user to manually add existing ENC data (shapefiles) to a view, or automatically add ENC data to a view (figure 4).  Choosing "Manually Select ENC Data" allows the user to add individual ENC shapefiles to the view. This is similar to the ArcView "Add Theme" button but it includes custom symbols and sorts the themes in the table of contents.  Choosing "Automatically Add ENC Data" opens a version of the "Automated Viewing Options" dialog.   This option will add a group of ENC data to the view and sort themes in the table of contents.

          

            Figure 4.  Adding existing data to a view.

4. Toggling and removing themes.  The toggle theme function groups ENC data by source folder and allows users to turn on and off groups of ENC themes in a view (figure 5).  This is useful when data from more than one ENC (source folder) are added to a view.  The remove theme function allows users to remove groups of ENC themes from a view.

            Figure 5.  Toggle dialog box.

5. Help dialog.  The help function launches an HTML document comprised of an overview of the extension, S-57 definitions, instructions for use, and useful Internet resources.

The ENC data available from the NOAA Office of Coast Survey are developed first and foremost for navigation.  The Office of Coast Survey is currently focusing on the major ports at the largest available scale.  This subdivision of data could provide a challenge for GIS users because many charts may have to be downloaded for a particular area.  Although the charts are seamless, the scales at which the charts were digitized may vary.  For example, the New York Harbor paper chart (chart 12327) is comprised of several different ENCs of varying scales.  As long as ENC data users understand this caveat, and that merging data across charts, in some cases, may result in a data set of varying scale and accuracy, this should not present a problem. 

Version 1.0 of the ENC Data Handler is scheduled for release in late Summer 2002.  Future versions of the extension will depend on how Environmental Systems Research Institute (Esri) plans to handle ENC data in ArcGIS 8.x.

S-57 – International Hydrographic Organization (IHO) Transfer Standard for Digital Hydrographic Data.  S-57 specifies the model, structure, rules, and format for ENC data development and exchange.

ENC – Electronic Navigational Chart.  A vector-based digital file containing hydrographic features suitable for marine navigation.  It is based on the International Hydrographic Organization (IHO) S-57 standard.

Cell – The area covered by a NOAA ENC is referred to as a "cell."  At present, each NOAA ENC corresponds to the largest scale paper chart in a given area.

Object – An identifiable set of information (e.g. channels, aids to navigation, shoreline) within an ENC as defined by S-57.  The NOAA ENC Data Handler converts each chart “object” to a shapefile.

Appendix B.  List of Shapefiles Categorized into Automated Viewing Options

References

NOAA Office of Coast Survey World Wide Web site. < http://chartmaker.ncd.noaa.gov>