电子海图与数字海图的异同

海军大连舰艇学院本科生毕业论文（外文翻译）译文名称：电子海图与数字海图的异同专业：地图学与地理信息工程班次：2006级制图本科学员：王少帅指导教员：彭认灿教授评阅人：完成时间：2010年6月30日电子海图与数字海图的异同理查德.卡彭特，英国海道测量局卡里斯 2008，巴斯发展的本质电子航海图（ENC）与数字海图（DNC）都是由政府部门生产的数字导航产品；电子航海图由世界各国的海道测量机构生产，电子海图由美国国家空间信息署制作，该机构同时也为加拿大制作相关海图。

数字海图标准随着科学技术的进步而不断发展和完善。

1983年，在美国召开的以“电子海图”为主题的海道测量研讨会上，当讨论到全球海图数字化处理的数据量时，理查德.思克瑞（国家海洋与大气管理局）指出：“即使使用最大型的计算机设备，处理海量的数据依旧十分困难。

”而在今天，即使是普通手机都拥有处理大量信息的功能。

文中对此问题的简单论述只为强调一点，即早期对于ENC和DNC发展及相关产品的预测是很有意义的。

特别是IT界曾经历过对其技术发展的悲观估计和对其应用的过高评价。

概念和起源20世纪70年代中期，各国海道测量机构相继开展了海图数字化研究工作，在之后的数十年里，IHO研究制定了相应的数据交换标准；相关概念也迅速扩展到数据提供和用户系统的方方面面。

这项工作导致了S-57的出版，至今，该项规范已发展至第三版的S-100 和正在修订中的S-101。

如下观点具有一定的启迪性：在上述的1983年研讨会中，尼尔安德森（CHS）指出，IHO所做工作承自1919年在伦敦召开的旨在制订纸质海图编绘规范的国际海道测量大会，该项工作在64年之后的1983年依旧具有重要意义。

同样，在其之后的二十多年内该项工作尚无法结束。

在这段时间内（世纪末的二十多年），世界各国的海道测量机构根据最新要求，达到了能生产具有高度兼容性数据产品的水平。

其主要动因是较纸质海图具有更高安全性的ECDIS导航的实现。

20世纪80年代，美国海军发现随着舰载计算机系统的广泛应用，以单一导航图象为主的自动（导航）系统越来越凸显出其重要价值。

这与淘汰机械标图系统的需求不谋而合。

电子海图被视为相关解决方案的主要内容。

国防制图局制订了系统的发展计划，并决定在制作全球电子海图的同时应用相应的海图规范，并逐渐形成了当前的矢量产品数据格式（VPF）。

发展的动因很明确，海图产品应用需求中称：“现行的海军测绘体系急需调整。

因为这种老旧的机械标图系统无法满足海军的实际操作需求。

”定义如下定义清楚指出，电子航海图和数字海图有着相同的目的：可在非纸质的情况下为海上航行安全提供数字形式的海图数据。

数字海图是一种支持航海和地理信息系统应用的通用总体数据库。

(MIL-PRF-89023)电子海图：内容、结构和格式都经过标准化设计的一种数据库。

由政府部门授权的海道测量机构出版发行，并与ECDIS配套使用。

ENC包含保障航行安全的所有的海图信息。

(IHO S-32)通用性各自的标准一发布，人们就开始研究两者的数据格式转换。

1997年，参加IHO和北约DGIWG的部分国家通过一体化委员会完成了一个“界面控制文件”。

这种图形化的信息类型符合两种标准，同时有利于识别区域中相同的部分。

2002年，英国皇家测绘局（UKHO）和美国国防部国家测绘局（现名NGA）联合委托加拿大IDON技术公司进行了一项研究，该项研究的最终完成联合了CARIS公司的力量。

此项研究通过不同标准间的相通之处评估其通用性，其目的是针对当前的数据维护提供稳定的、操作性强的数据转换策略。

这项工作导致了2004年海道测量信息标准化工作组的成立。

该组织的目的是为当前和未来制订标准化的数据格式（及相关的扩充模式），以提高矢量化的海道测量数据的通用性。

其工作立足于用户的实际需求和建立实用的信息数据库，目前已得到DGIWG和IHO的正式授权。

该工作组在2006年发布了一项研究报告，在报告中推荐使用基于XML 的数据格式，其中包含一个针对通用数据模型的符合HIHWG要求的独立“非确定产品”信息库，以利于两种不同产品的生产输出。

遗憾的是先前对于ENC和DNC的比较研究多集中于两者的区别，特别是有一部分研究是基于对两者基本属性的错误理解上的。

例如，曾经认为DNC更适宜于表征地形信息，但实际上地形信息的多寡是与描述尺度的大小相关的，而非通过产品形式区分。

ENC通常具有更丰富的编码方法。

问题不在于差别之多而在于差别之少。

例如在数据模型中，残骸的编码与水深要素的编码是一样的，只是其属性定义为残骸。

两种产品（ENC 和DNC）对于相似的问题都是采用同一种做法，以便利生产方和使用方。

事实上，IDON的研究发现，“产品之间的通用性远不止目前所显现的程度”。

产品差异通过多年的研究，人们已经发现了两者之间的诸多差异，并广泛应用于产品数据之间的相互转换或评估产品通用性对用户所产生的影响。

下文阐述了两者之间的部分差异。

更新更新通产被视为两者之间的主要差异所在，但实际上却鲜有不同：两者都需要不断更新，并拥有一套完整的更新机制。

对于DNC，其数据结构中本身就含有关于更新的信息记录；对于ENC，则依据工业标准采用独立软件实现更新操作。

拓扑关系、图层ENC中要素对象分为两类：一类是构成地球表面的要素对象，另一类是所有位于第一类要素表面的要素对象。

两类要素都以几何图形的形式表征。

DNC有12个数据层，层与层之间通过拓扑和几何关系相联。

为了说明数据模型对最终产品的影响，以岸线的如下情形为例：（岸线）构成陆部的外边界；（岸线）构成深水区的内边界；（岸线）构成控制区的部分边界。

在ENC中，上述情况使用相同几何要素进行描述；在DNC中，上述情况下的线要素会被置于不同的数据层，但几何符号是相同的（有些DNC 在生产时允许出现数据层之间的拓扑关系；当要素是独立采集时（即不存在相关性），要素间的差异应通过相关软件进行识别）。

拓扑-节点早期DNC中的VPF数据不允许孤立点与线要素末点地理位置重合。

（当发生这样的情况时），将线要素的末点回移至其上一点，但应确保在显示时不能太过明显，但这样做可能会导致进行GIS空间查询的时候出现问题。

ENC and DNC - congruence and contrastRichard Carpenter, United Kingdom Hydrographic OfficeCARIS 2008, BathThe nature of developmentFor those unfamiliar with them, the Electronic Navigational Chart (ENC) and the Digital Nautical Chart (DNC) are both marine digital navigation products produced by government agencies; ENC by national hydrographic offices world-wide and DNC by the National Geospatial-Intelligence Agency (NGA) in the USA supported by Canada for Canadian waters.These digital chart standards have developed and matured during a period when technology has moved forward at unprecedented pace.In 1983, at the Hydrographic Society USA seminar on “The Electronic Chart”, in commenting on a then estimate of the data volume for worldwide digital charting, Richard Schiro (NOAA) noted that “dealing with a billion bytes is no simple task even for the biggest of computer facilities” – today an order of magnitude greater capacity is common even in a mobile-phone.I mention this simply to emphasise that the early estimates for the development and production of both ENC and DNC were actually quite good. Typically the IT industry has a history of underestimating technological developments and being over-optimistic about their application.Conception and BirthBy the mid-1970s various hydrographic offices were experimentingwith chart digitisation and early in the following decade the International Hydrographic Organization (IHO) was developing a standard for the exchange of digital data between agencies; a concept soon extended to encompass the supply of data to user systems. This work led to the publication of S-57, now in its third edition and currently undergoing fundamental revision to become S-100 and S-101.Again some perspective is instructive;at the 1983 seminar mentioned above, Neil Anderson (CHS) reminded those present that the IHO had been formed following the 1919 Hydrographic Conference in London to work on standards for paper charts and that this task was still going strong 64 years later. We can add that another quarter of a century has not seen the end of it. The same period has, with the application of the latest consistency recommendations, achieved a position whereby highly compatible digital products can be published by HOs around the world. A key driver has been the realisation that ECDIS-based navigation is inherently safer than a paper-based solution.During the 1980s the US Navy found that the automatic provision of a single navigational picture became increasingly important as ship-borne computer based systems proliferated. This coincided with a need to replace electro-mechanical chart plotter systems. A digital chart was seen as an essential component of this solution. Given the system development timescales, the Defense Mapping Agency (DMA – now part of NGA) made the decision to utilise the nautical prototypes being produced as part of the development for the Digital Chart of the World (DCW) which led to the currentVector Product Format (VPF).The driver for development was clear, the Statement of Requirements for the chart product notes that, “The current Navy plotting systems are in desperate need of upgrading. They are all old electro- mechanical systems which no longer meet the operational needs of the navy.”DefinitionsAs their definitions below make clear, both ENC and DNC clearly are intended to achieve the same end –the provision of the chart data needed for safe navigation in a digital form which permits the adoption of a paper-less solution.“The DNC is a general purpose global database designed to support marine navigation and Geographic Information System (GIS) applications.” (MIL-PRF-89023)“ENC: The data base, standardized as to content, structure and format, issued for use with ECDIS on the authority of government authorized hydrographic offices. The ENC contains all the chart information necessary for safe navigation…” (IHO S-32) InteroperabilityAlmost as soon as the respective standards were published, groups around the world began to look at how data in one format could be converted into the other.In 1997 an ‘Interface Control Document’ was completed by a number of nations participating in both the IHO and the Defence Geographical Information Working Group (DGIWG) of NATO through a Harmonization Committee. This mapped information types across the standards and sought to identify areas of potentialharmonisation.In 2002 the UKHO and the National Imagery and Mapping Agency (NIMA – now retitled NGA)) jointly commissioned a study from IDON Technologies Inc.of Canada which was carried out in conjunction with CARIS. Their remit was to evaluate interoperability through hydrographic standards harmonisation, aiming to provide implementable strategies for conversion that would maintain continuity for the current data holdings.This work led to the creation of the Hydrographic Information Harmonisation Working Group (HIHWG) in 2004.Its aim was to promote and develop the harmonisation of both existing and future standards leading to improved interoperability of hydrographic vector data. This undertaking was accomplished by defining a direction with a focus on Maritime user requirements and pragmatic database solutions. It was formally recognized by DGIWG and the IHO.The group produced a final report in 2006 recommending an XML based solution with producers maintaining a single ‘product neutral’repository compliant with the HIHWG’s recommendations for a Common Content Model so as to allow output of either product.It is a pity that comparisons of ENC and DNC have necessarily focussed on their differences, especially as many were based on misapprehensions as to the nature of one or other product. For example, it has been said that DNC is more suited to topographic information while in reality any differences lie mostly in the greater depiction of land detail on NGA paper charts and hence the inclusionof more topography in the digital product. ENC often has richer encoding options.The question is not why are there so many differences but why are there so few? For example if we look at data modelling basics it could be just as valid to encode a wreck as a depth object carrying an attribute that identifies it as a wreck rather than natural seabed. That both products chose to use a wreck object carrying a depth attribute – in conjunction with all the other similar choices – makes the life of the user and producer much simpler.Indeed, the IDON Study went so far as to say that, “there is more compatibility buried in these products than is apparent….” Product DifferencesMany of the differences have of course been widely publicised over the years; mainly as a by-product of efforts to achieve conversion between products or to assess interoperability rather than as a consideration of their impact on the user. The sections below highlight some that reflect the different approaches adopted by the two products.UpdatingThis is an area that is often cited as a major difference but in many respects is of little significance – both products were always intended to be updated; both have a working update mechanism. That for one product it is an inherent part of its record structure and the other uses an industry-standard, stand- alone software tool is largely irrelevant.Topology - LayersENC divides object classes into two types: those which make up the电子海图与数字海图的异同海军大连舰艇学院本科毕业翻译第10页 ‘skin of the earth’ and together form a complete and unbroken layer, and all others that conceptually sit on top of the first group. Geometry is wholly shared across both types.DNC has 12 data coverages with topology and geometry being internal to each. As a simple illustration as to how these different models will impact their products, we can consider a situation where the coastline forms:The outer edge of the land areaThe inner edge of the depth areasPart of the boundary of a controlled areaIn an ENC they will all share the same geometry; in DNC they will all be in separate coverages and the geometry may be repeated (some DNC is produced with software that permits cross coverage topology during collection; when collected independently a software tool is used to identify any variances).Topology - NodesThe early form of VPF adopted for DNC does not allow isolated nodes to share position with a connected node ending a linear feature. For example, the end of a leading line has to be offset from its back marker. Typically, though, the offset should be so small that it could not be seen at normal display scales though the lack of coincidence may impact some GIS queries.。