VTST Format

VTST is an evolving specification for an XML-based file format to describe built structures.

An earlier version is described in VTST version 1.  That version is completely replaced by the current definition.

Terminology

'Built structure' is used here to mean any permanent or semi-permanent artificial structure with significant vertical extent above the ground.  Examples are buildings, walls, and fences.  Built surfaces such as roads are not be included since they are conceptually built into the ground, not above it, although roadside railings and road signs would qualify.

Goals

The format should be:

• sufficiently flexible to allow for highly similar visible recreation of actual structures
• human-readable, by using entity names which correspond as well as possible to high-level, real-world attributes
• efficient, through good choice of default values so that many values may be implicit
• open-ended, to allow arbitrary extensions for unforeseen needs without breaking compatibility with existing software

The choice of XML as the basis for the format goes a long way toward satisfying the goals of human-readability and extensibility.  However, it involves a significant tradeoff of efficiency - file sizes are quite large.  If this becomes a significant issue in the future, it might become important to define an alternate, dense format which would most likely consist of a binary, tokenized form of VTST.

Current format definition

The overall structure of the XML format for VTST is described by the following XML Schema Document (XSD): buildings5.xsd

It is built on the definitions for GML, which is the standard for encoding of Geographic data in XML, specifically OpenGIS® Geography Markup Language (GML) Implementation Specification, version 2.1.2

Regarding components of the VTST file:

Building footprints

• the points should be specified in counter-clockwise (CCW) order
• when the building's position is to be considered on a heightfield, the footprint vertex with the lowest elevation on the heightfield is the base of the building

Edge features

• valid values for edge feature Type attribute are "Wall", "Gap", "Post", "Window", "Door"

Revisions

• rev 4, first implemented version
• rev 5, added optional "RelatedHeightFieldName" and "RelatedHeightFieldChecksum"

Example Files

• sample.vtst contains 3 structures, one of each type.