Thayer Watkins

The Logical Structure of a Map in ArcXML

The purpose of this page is to examine the system by which ArcXML stores the information for a map. Ultimately the ArcXML system will be compared with that of the Geography Markup Language (GML) to see if the two systems are logically equivalent or if one of the systems contains components related to map encoding not found in the other. Note that ArcXML, ArcIMS and ESRI are trademarks of the Environmental Systems Research Institute, Inc.

ArcXML was created by ESRI for transfering information within its ArcIMS mapping software package. Therefore ArcXML is a much more extensive instrument than GML.

ESRI provided for the public a programmers' manual for ArcXML. This document contains the information needed to examine the formal structure of the language. The problem is in separating elements related to the content of a map document from the code necessary for creating a presentation of the information; i.e., the graphic display of the map.

The root element of an ArcXML file is the <ARCXML> tag. There are four possible subtags to the root tag but only one, the <CONFIG> tag has to do with content and structure. The others have to with queries and updating the data for a document.

The logical structure of the <CONFIG> tag is shown below.

The <MAP> is shown with subtags but the <SCALEBAR> tag has no subtags. The information required for specifying a scalebar is code as attribute values within the <SCALEBAR> tag.

The <PROPERTIES> element of ArcXML generally contains metadata, information about the data, which in this case is contained in the <LAYER> elements. A <MAP> must contain one and only one <PROPERTIES> element but it can contain any number of <LAYER> elements.

The <LAYER> element is the heart of the map document. Its logical structure is shown below.

The above diagram indicates that most subtags to the <PROPERTIES> tag are optional as denoted by the ? following the name. Only the <ENVELOPE> element is mandatory. The subtags to the <PROPERTIES> tag so not themselves have subtags. All required data for these tags are given as values of attributes within the tags.

The essential element of a map is a layer. The <MAP> element may have any number of <LAYER> elements.

As shown above a <LAYER> element can contain any number of <OBJECT> elements which are defined by POINT, LINE, POLYGON and TEXT elements. The SCALEBAR may also be included. The <EXTENSION> tag allows provision for geocoding through a <GCSTYLE> tag and any number of <GCFIELD> subtags. A <LAYER> may also have a dataset which can involve any number of partitions.

The POINT, LINE, POLYGON and TEXT elements may have subelements but these are symbols which not part of the content structure and are merely presentation features and therefore they are not shown. The geometric content of the of <POINT>, <LINE> and <POLYGON> tags which is not shown is given by attribute values, such as a list of coordinates.

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