Class

• <DRM Reference Surface>

Superclass

• <DRM SEDRIS Abstract Base>

Subclass

• None.

Definition

An instance of this DRM class specifies, for the hierarchy instance(s) of which it is a component, a surface which is to be used to resolve the elevation of <DRM Location 2D> instances in the component tree rooted at each hierarchy instance.

In addition, a <DRM Reference Surface> instance specifies how the surface is to be used in the resolution process.

A hierarchy instance requires a <DRM Reference Surface> instance if

1. there are <DRM Location 2D> instances below the hierarchy,
2. the <DRM Location 2D> instances are in the scope of a 3D spatial reference frame, and
3. the data provider wants the locations to lie on a surface other than the last default surface (The initial default is the spatial reference frame’s vertical offset model).

The field values of <DRM Geometry Hierarchy> and <DRM Reference Surface> instances define a surface for the resolution process in several cases as follows: 

1. The <DRM Geometry Hierarchy> instance is a <DRM Property Grid Hook Point> instance that aggregates at least one <DRM Property Grid> instance with these qualifications:

   1. its <DRM Classification Data> instance matches the <DRM Reference Surface> instance’s classification field,

   2. it has two spatial axes corresponding to the horizontal coordinates of the SRF, and

   3. it has a <DRM Table Property Description> for height, elevation, or bathymetry.

   If the <DRM Property Grid> instance meets the above criteria, it defines a resolution surface.

2. The <DRM Geometry Hierarchy> instance is a <DRM Union Of Primitive Geometry> instance that aggregates <DRM Surface Geometry> instances with <DRM Classification Data> instances matching the <DRM Reference Surface> instance’s classification field. In this case, all such <DRM Surface Geometry> instances combine to form the resolution surface.

   NOTE The multiplicity_rule field deals with surface complexity).

3. The <DRM Geometry Hierarchy> instance is a Distance, Index, Map Scale, or Spatial Resolution <DRM LOD Related Geometry> instance that aggregates (directly or indirectly) <DRM Geometry Hierarchy> instance cases a and/or b above under an LOD branch selected by the <DRM Reference Surface> instance lod_rule field.

4. The <DRM Geometry Hierarchy> instance aggregates some combination of cases a, b, or c.

In general, the set of all <DRM Surface Geometry> and <DRM Property Grid> instances under the <DRM Geometry Hierarchy> instance is culled by matching the <DRM Reference Surface> instance classification field (and <DRM Property Grid> instance qualifications) and matching LOD branches to the lod_rule field. The remaining geometry is the resolution surface used for ray intersections.

Within the resolution surface, only geometry matching this (possibly elaborated) classification shall be used.

The multiplicity_rule field specifies a rule to select a single point from multiple intersections of a ray with a resolution surface.

The lod_rule field value specifies a rule to select one LOD branch.

Class diagram

Figure 6.255 — DRM_Reference_Surface

Constraints

• Model spatial reference frame
• Publishable Object

Example(s)

• Consider an <DRM Environment Root> instance E, having both a <DRM Union Of Geometry Hierarchy> component G and a <DRM Union Of Features> component F.

  The <DRM Union Of Geometry Hierarchy> instance contains a <DRM Union Of Primitive Geometry> instance P1 with a <DRM Classification Data> component specifying ECC_TERRAIN_ELEVATION, and contains <DRM Polygon> instances that inherit the <DRM Classification Data>. That is, P1 is a polygonal representation of terrain, forming part of the larger environmental representation E.

  F, the feature representation of E, has a <DRM Reference Surface> component that associates to the <DRM Union of Primitive Geometry> and has these field values:

  multiplicity_rule = HIGHEST
  classification    = ECC_TERRAIN_ELEVATION
  lod_rule          = ALL

  Figure 6.256 depicts a UML diagram of this example.

  

  Figure 6.256 — <DRM Reference Surface> example

  Consequently, <DRM Location 2D> instances found in the <DRM Union Of Features> aggregation tree of F use the terrain polygons of P to resolve elevation.

• Continuing example 1, the <DRM Union Of Geometry Hierarchy> instance G under E contains another <DRM Union Of Primitive Geometry> instance P2 containing <DRM Polygon> instances classified as ECC_INLAND_WATER_ELEVATION. F, in turn, aggregates a <DRM Union Of Features> instance F2 that is classified as ECC_ENGINEERING_BRIDGE and contains <DRM Linear Feature> instances using <DRM Location 2D> instances. F2 also contains a <DRM Reference Surface> instance with <DRM Classification Data> tagged as ECC_INLAND_WATER_ELEVATION, and associated to P2.

  Consequently, the <DRM Location 2D> instances of the ECC_ENGINEERING_BRIDGE F2 will have elevation values derived for them by being evaluated against P2.

• Consider a <DRM Reference Surface> instance R for which the geometry is a <DRM Spatial Index Related Geometry> instance S. Each branch of S is a polygonal representation, part of which represents terrain surface, part roads, and part forest canopy. R associates to S, and its classification field is set to ECC_TERRAIN_ELEVATION. The resolution process then ignores the road and canopy polygons, but sees all the terrain polygons regardless of in which union they exist.

  Consider a <DRM Linear Feature> instance L representing a road, which mostly stays on the road geometry but sometimes strays off. L is placed in a <DRM Union Of Features> instance aggregating a different <DRM Reference Surface> instance R2 with associates to the same <DRM Spatial Index Related Geometry> instance but has classification = ECC_ROAD, but which like R associates to S. The resolution process for R2 sees the road <DRM Polygon> instances and ignores the others. For <DRM Feature Node> instances that stray off the road, the corresponding <DRM Location 2D> instances’ rays will fail to intersect any road polygon, so the resolution process (as per case c) applies, and the resolution process falls back on the previous override, which was the terrain surface.

• Consider a terrain representation organized in three minute regions that are grouped into one-degree cells collected under one <DRM Union Of Geometry Hierarchy> instance. In the same transmittal, <DRM Feature Representation> and non-terrain <DRM Geometry Representation> instances are organized under a corresponding spatial organization. Each three-minute hierarchy has a <DRM Reference Surface> instance associated to the corresponding three-minute terrain. Each one-degree hierarchy has a <DRM Reference Surface> instance associated to the corresponding one-degree terrain. Each of the highest level feature and non-terrain geometry hierarchies has a <DRM Reference Surface> instance associated to the terrain <DRM Geometry Hierarchy> instance.

  In this scheme, a <DRM Location 2D> instance in a three-minute region finds its resolution surface in the corresponding three-minute terrain. If a <DRM Location 2D> instance strays outside its region (that is, strict_organizing_principle = FALSE), the containing one-degree terrain resolves the <DRM Location 2D>. If the location ray fails to intersect the one-degree surface, then the full terrain <DRM Union Of Geometry Hierarchy> is used. If ray / surface intersection still fails, the elevation is resolved by the vertical offset model.