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 2D Location> instances in the component tree rooted at each hierarchy instance.

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

A hierarchy instance requires a <DRM Reference Surface> if

• there are <DRM 2D Location> instances below the hierarchy,
• the <DRM 2D Location> instances are in the scope of a 3D spatial reference frame, and
• 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 datum).

A <DRM Geometry Hierarchy>'s and <DRM Reference Surface>'s field values define a surface for the resolution process as follows; there are several cases.

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

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

  2. it has 2 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> meets the above criteria, then it defines a resolution surface.

• The <DRM Geometry Hierarchy> is a <DRM Union Of Primitive Geometry> that aggregates <DRM Surface Geometry> with <DRM Classification Data> matching the <DRM Reference Surface>'s classification field. In this case, all such <DRM Surface Geometry> instances combine to form the resolution surface. (Note that the multiplicity_rule field deals with surface complexity).

• The <DRM Geometry Hierarchy> is a Distance, Index, Map Scale, or Spatial Resolution <DRM Level Of Detail Related Geometry> that aggregates (directly or indirectly) <DRM Geometry Hierarchy> cases 1 and/or 2 above under an LOD branch selected by the <DRM Reference Surface> lod_rule field.

• The <DRM Geometry Hierarchy> aggregates some combination of cases 1, 2, or 3.

In general, the set of all <DRM Surface Geometry> and <DRM Property Grid> instances under the <DRM Geometry Hierarchy> is culled by matching the <DRM Reference Surface> classification field (and <DRM Property Grid> qualifications) and matching LOD branches to the lod_rule field. The remaining geometry is the resolution surface used for ray intersections, as described in clause 5 for Reference_Surface_Elevation_Select and Reference_Surface_LOD_Select.

As described in clause 5 for Reference_Surface_Elevation_Select, a given or corresponds to the unique ray that is normal to the SRF ORM and intersects the ORM at the same coordinates as the location instance.

Class diagram

Figure 6.251 — DRM_Reference_Surface

Constraints

• Elaboration of Classification Data
• Model spatial reference frame
• Publishable Object

Example(s)

• Consider a <DRM Environment Root> instance ER, having both a <DRM Union Of Geometry Hierarchy> component UGH and a <DRM Union Of Features> component UF.

  The <DRM Union Of Geometry Hierarchy> instance contains a <DRM Union Of Primitive Geometry> instance UPG_1 with a <DRM Classification Data> component specifying ECC_TERRAIN_ELEVATION, and contains <DRM Polygon> instances, which inherit the <DRM Classification Data>. That is, UPG_1 is a polygonal representation of terrain, forming part of the larger environmental representation ER.

  UF, the feature representation of ER, has a <DRM Reference Surface> component, which associates to the <DRM Union of Primitive Geometry> and has these field values:

  multiplicity_rule	HIGHEST
  classification	ECC_TERRAIN_ELEVATION
  lod_rule	ALL

  

  Figure 6.252 — <DRM Reference Surface> example

  Consequently, <DRM 2D Location> instances found in the <DRM Union of Features> aggregation tree of UF use the terrain polygons of UPG_1 to resolve elevation.

• Continuing example 1, the <DRM Union Of Geometry Hierarchy> UGH under ER contains another <DRM Union Of Primitive Geometry> instance UPG_2 containing <DRM Polygon> instances classified as ECC_INLAND_WATER_ELEVATION. UF, in turn, aggregates a <DRM Union Of Features> instance UF_2 that is classified as ECC_ENGINEERING_BRIDGE and contains <DRM Linear Feature> instances using <DRM 2D Location> instances. UF_2 also contains a <DRM Reference Surface> instance with <DRM Classification Data> tagged as ECC_INLAND_WATER_ELEVATION, and associated to UPG_2.

  Consequently, the <DRM 2D Location> instances of the ECC_ENGINEERING_BRIDGE UF_2 will have elevation values derived for them by being evaluated against UPG_2.

• 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.

  The <DRM Reference Surface> 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 which union they're in.

  Consider a <DRM Linear Feature> L representing a road, which mostly stays on the road geometry but sometimes strays off. L is placed in a <DRM Union Of Features> aggregating a different <DRM Reference Surface> instance R2 with 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 2D Location> instances' rays will fail to intersect any road polygon, so the resolution process (as per case 3) falls back on the last override encountered, which was the terrain surface.

• Consider a terrain representation organized in 3 minute regions, which are grouped into 1 degree cells that are collected under one <DRM Union Of Geometry Hierarchy> instance. In the same transmittal, <DRM Feature> and non-terrain <DRM Geometry> instances are organized under a corresponding spatial organization. Each 3-minute hierarchy has a <DRM Reference Surface> associated to the corresponding 3 minute terrain. Each 1 degree hierarchy has a <DRM Reference Surface> associated to the corresponding 1 degree terrain. Each of the highest level feature and non-terrain geometry hierarchies has a <DRM Reference Surface> associated to the terrain <DRM Geometry Hierarchy>.

  In this scheme, a <DRM 2D Location> in a 3 minute region finds its resolution surface in the corresponding 3 minute terrain. If a <DRM 2D Location> “strays” outside its region (i.e., strict_organizing_principle=FALSE), then the containing 1 degree terrain resolves the <DRM 2D Location>. If the location ray fails to intersect the 1 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.