Query Parameters

Specific multiresolution queries are obtained by suitable instantiation of the focus condition and the LOD threshold in TIN extraction. Application programs in the outer layer (MT-clients) may either implement their specific queries directly, or make use of parameters provided in a specific module of the outer layer, which implements a set of predefined LOD thresholds and focus conditions.

In the current implementation, only a small number of LOD thresholds and focus conditions are provided, which have been used to implement applications described in Section .

Focus conditions that depend on spatial location of triangles are provided. For each such condition, only triangles intersecting a given spatial entity, either on the two-dimensional terrain domain, or in the three-dimensional space, are active. The following spatial entities can be considered as regions of interest:

• Points in 2D (the terrain domain). This implements a point location query, and can be used to evaluate local terrain characteristics (e.g., elevation and slope at a given point).
• Polylines in 2D. These are used to get the terrain configuration (profile) along a relevant feature such as a road, a river, a boundary, etc.
• Circles in 2D, which are the basis for range queries. Getting the terrain configuration within a distance d from a given location reduces to a local query with a circular region of interest.
• Axis-parallel boxes in 2D, which implement window queries; boxes both in two-dimensions (on the xy-plane) and in three dimensions (in the 3D space) can be used.
• Angular sectors, which have been used to implement the view frustum in terrain visualization (see Section ).
• Horizontal planes at a given height, which are used for contour line extraction (see Section ).

Such focus conditions can be also combined with logical operators AND and OR, which consider the intersection and the union, respectively, of the corresponding regions of interest.

The following LOD thresholds that depend on position and elevation accuracy are available:

• Constant threshold functions, expressed by means of a single real value which bounds from above the elevation error admitted for a triangle.
• Threshold functions increasing with the distance from a reference point. The distance can be evaluated either on the xy-plane, or in 3D space. In both cases, the function is distance-increasing and it is defined by specifying: the reference point R; the threshold value for elevation error in R; a reference distance d; the threshold value at distance d from R; and the dependency law, which may be linear, quadratic, or exponential.

Additional LOD thresholds and focus conditions can be defined easily by using the subclass mechanism of C++ in the context of the object oriented library that is provided to support system extensions.