Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 52
APPENDIX D Data Formals and Chart Systems DATA FORMATS s-57 S-57 (previously called DX-90) was designed for the magnetic media inde- pendent transfer of digital hydrographic data between hydrographic offices and other interested parties. S-57 allows data to be exchanged in support of ECDIS (Electronic Chart Display and Information System). Within S-57, real-world objects of interest to hydrography are modeled by a two-dimensional Hydrographic Situation Model (HYSIM). This process allows rules related to the display of information to be independent of the HYSIM. The advantage of this approach is that it allows a number of different display methods (e.g., specialized ECDIS implementations or general geographic information sys- tem software) to refer to one HYSIM. A byproduct of this characteristic is that feature symbolization codes are carried as attributes of features and are included as a part of the S-57 model. S-57 includes four application profiles that are associated to particular sub- sets of information from all of S-57. The application profiles are: new ECDIS information (vector information allowing electronic chart base display) revised ECDIS information (vector information that allows display of electronic chart revisions) vector data for generation of paper charts · hydrographic survey vector data 52
OCR for page 53
APPENDIX D 53 S-57 data are represented in either feature records or in segment modules. Features are geometric points, lines, or areas (represented by bounding lines). Features may be simple, compound, or complex. Each feature record refers to one or more segment records that contain the spatial data for that feature. S-57 also has two special feature classes: soundings and cartographic objects such as com- pass rose symbols. The soundings feature class allows efficient coding of single sounding types associated with many sounding values, each one a discrete, unre- lated point. Spatial information is included either as lists of coordinate pairs, circular arcs, elliptical arcs, or polynomial curves. The chain-node data model is used to structure the spatial data. Through use of the chain-node model and coordinate pair lists, topological data structures can be transferred, although S-57 would carry topology implicitly, depending on the target system software used to imple- ment explicit topology. Most features are stored in only a two-dimensional geo- metric representation, although soundings are represented as x, y, and z. S-57 feature attribution is implemented through a coding catalog (S-57, part A). Interchange formats such as S-57 use the paradigm of magnetic tape transfer as an underlying model a sequential file constructed to be read once from be- ginning to end to load a database into the target system. S-57 complies with International Standards Organization (ISO) 8211, a standard developed to pro- vide a self-describing exchange. Files in compliance with ISO 8211 must be read in strictly sequential order for their content to be understood. It is the approach most commonly used for the purpose of cartographic data exchange. S-57 is specified as the internationally accepted feat for transfer of spatial data to ECDIS. There is great interest in S-57 in the international maritime com- munity; however, to date, most S-57 data sets have been developed only as proto- types. High-volume data production of S-57 data has not yet occurred, although at least one large S-57 data set of the St. Lawrence Seaway has been prepared by the Canadian Hydrographic Service. Although S-57 will no doubt undergo some future modifications (as do all international standards), S-57 is currently the most widely adopted spatial data transfer standard in the hydrographic community and the only spatial data standard adopted by the International Hydrographic Organi- zation. [This section is paraphrased from A Report on the Compatibility of the IHO and DIGEST Transfer Standards for Digital Geographic Data. British Crown copyright 1992/MOD. Published with the permission of the Control- ler of Her Britannic Majesty's Stationery Office.] Vector Product Format Vector Product Format (VPF) is another data format. It is an example of the response of the Defense Mapping Agency (DMA) to the U.S. Department of Defense (DOD) customer demands for fully digitized products. VPF data sets contain digitized features and terrain that allow the exploitation of these
OCR for page 54
54 NAUTICAL CHART PROGRAM relational data sets by geographic information systems (GISs) technology. VPF is a standard framework that allows products to be specified within it. DMA's published goal is to provide 70 percent of its products in fully digitized form over the next five years and to provide these products as digitized data freely accessible (subject to security constraints) for browsing by customers over net- works such as Internet. DMA has adopted VPF as its standard data format for digital products. DMA developed VPF to allow its data users the DOD, defense mapping organiza- tions in NATO-member nations, the U.S. intelligence community, and the inter- national hydrographic community to read its spatial product data directly, with out conversion. VPF was developed to be a direct-use standard, not a transfer standard, and in this regard it is different from S-57. The design goal for VPF was a machine- independent database format that would allow database products to be used di- rectly by GIS software. Thus VPF is designed for media that can be used directly and interactively by magnetic hard disks and CD-ROMs with ISO 9660. As a result, VPF must provide random access to each record. Performance of VPF databases would be unacceptable if a whole CD-ROM or hard disk had to be scanned to find a single record. Therefore, VPF must provide most of the services of an internal database manager in an integrated GIS software package, including descriptive schema definition tables. Similar to data sets implemented in ISO 8211, VPF is "self-describing," in that its schema definition tables are built into the data content; these schema definition tables can be accessed by spatial and thematic indices whenever they are needed, instead of in strictly sequential order. Therefore, these schema tables can be accessed to determine the design of the database. The schema tables (also known as metadata, or data about data), can also be displayed directly to the user to support on-line data dictionary functions, legend functions, data quality de- scriptions, library reference (or "inset") maps, or the presentation of other infor- mation that is normally found on the borders of printed maps. A data standard intended for direct use must also be able to represent a wide variety of databases. VPF is based on a single general model that supports a wide variety of database designs, including integrated or layered designs and simple or complex designs. An interchange format, in contrast, must construct specific in- terchanges between each data model that it attempts to exchange. Thus an inter- change format cannot be directly read by any single application. Because VPF relies on a single data model, it permits the development of generic software and allows the direct use of geographic data. VPF has a neutral, machine-independent format design. VPF does not con- tain information-like feature coding schemes or special relationships between features, which may vary from one database product to another. VPF allows such information to be encoded and included, but this information is not part of VPF itself. Neither does VPF define the geographic entities and objects to be
OCR for page 55
APPENDIX D 55 represented. Instead, these product-specific entities must be defined in a product specification for a given VPF product. A complete description of VPF is found in the Vector Product Format Military Standard (MIL-STD-2407~. [This section is paraphrased from Development of the Digital Chart of the World, DCW Final Report. 1992. Defense Mapping Agency, U.S. Department of Defense, Philadelphia, Pa.] Digital Nautical Chart The Digital Nautical Chart (DNC) is an ocean surface navigation database that will provide an alternative to the 4,500 paper nautical charts currently in use for marine navigation. In 1993 DMA designed the database for this product. The database was to be capable of supporting a wide variety of spatial analysis and geographic queries, was to include maritime features significant to marine navi- gation (including all of the world's coastal and shoreline areas), and was to be specified in VPF. The final DNC design is thematically layered and topologically structured, supporting a variety of spatial analyses and geographic queries. The digital data in each of the resulting databases may contain as many as four libraries roughly corresponding to four navigation-scale bands (harbor, approach, coastal, and gen- eral), depending on source chart type and scale. Each library may contain as many as 12 feature coverages (thematic layers), depending on the real-world fea- tures present. [This section is paraphrased from Digital Nautical Chart. U.S. Department of Defense Military Spec. Mil-D-89023. 19 October 1993. Defense Printing Office, Philadelphia, Pa.] CHART SYSTEMS Electronic Chart System Electronic chart system (ECS) is a generic term that refers to systems that display an electronic (digital) chart on a computer monitor. Such systems are not the legal equivalent of a paper chart, and the hydrographic database used need not be "official" (i.e., provided by a national hydrographic office). Electronic position- ing is required, and a radar overlay is optional. Such systems are in use on some commercial vessels as a navigation aid, with paper charts fulfilling the legal carriage requirement. Standards for ECS are under revision by the Radio Technical Commis- sion for Maritime Services SC-109 Category 3 Working Group (NRC, 1994b). Electronic Chart Display and Information System ECDIS is a specific type of electronic chart system that uses a database pro- vided by a national hydrographic office and that meets international standards
OCR for page 56
56 NAUTICAL CHART PROGRAM established by the International Maritime Organization (IMO) and the Interna- tional Hydrographic Organization (IHO). The IMO's performance standard for ECDIS has been approved by the IMO Assembly, which clears the way for ECDIS to be used as the legal equivalent of paper charts (WEND, 1996~. Certification procedures for ECDIS equipment are being prepared by the International Electro- technical Commission, and national hydrographic offices are beginning to pro- duce official ECDIS data sets. An ECDIS receives position data from radio navigation instruments and in- tegrates it with a voyage plan and an "official" hydrographic database to provide a real-time display of the ship's position with respect to the chart and voyage plan. Electronic positioning is required, and a radar overlay is optional. First- generation models are in limited use. ECDIS is expected to become a basic navi- gation technology in the future, as radar and voice communications are today (NRC, 1994b).
Representative terms from entire chapter: