The SEDRIS Data Representation Model
APPENDIX A - Classes
Reference Surface
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Class Name: Reference Surface
Subclasses
This DRM class is concrete and has no subclasses.
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 <Location 2D>
instances in the
component tree rooted at each hierarchy instance.
In addition, a <Reference Surface>
specifies how the surface is to be used in the resolution process.
A hierarchy instance requires a
<Reference Surface> if
- there are <Location 2D> instances
below the hierarchy,
- the <Location 2D> 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 <Geometry Hierarchy>'s and
<Reference Surface>'s field values
define a surface for the resolution process as follows; there
are several cases.
- The <Geometry Hierarchy> is a
<Property Grid Hook Point> that
aggregates at least one <Property Grid>
with these qualifications:
- its <Classification Data>
matches the <Reference Surface>'s
classification field,
- it has 2 spatial axes corresponding to the horizontal coordinates
of the SRF, and
- it has a <
Table Property Description> for height, elevation, or bathymetry.
If the <Property Grid> meets the above
criteria, then it defines a resolution surface.
- The <Geometry Hierarchy> is a
<Union Of Primitive Geometry> that
aggregates <Surface Geometry> with
<Classification Data> matching
the <Reference Surface>'s
classification field.
In this case, all such <Surface Geometry>
instances combine to form the resolution surface. (Note that the
multiplicity_rule
field deals with surface complexity).
- The <Geometry Hierarchy> is a
Distance, Index, Map Scale, or Spatial Resolution
<
Level of Detail Related Geometry> that aggregates (directly or
indirectly) <Geometry Hierarchy>
cases 1 and/or 2 above under an LOD branch selected by the
<Reference Surface>
lod_rule field.
- The <Geometry Hierarchy> aggregates
some combination of cases 1, 2, or 3.
In general, the set of all
<Surface Geometry> and
<Property Grids> under the
<Geometry Hierarchy> is culled by
matching the <Reference Surface>
classification field
(and <Property Grid> qualifications) and matching
LOD branches to the lod_rule
field. The remaining geometry is the
resolution surface used for ray intersections.
Primary Page in DRM Diagram:
Secondary Pages in DRM Diagram:
This class appears on only one page of the DRM class diagram.
Example
- An <Environment Root> has both a
<Union Of Geometry Hierarchy> and a
<Union Of Features> component.
The <Union Of Geometry Hierarchy>
contains a
<Union Of Primitive Geometry>
with a <Classification Data> (specifying
ECC_TERRAIN_ELEVATION). This
<Union Of Primitive Geometry>
contains <Polygons>, which inherit the
<Classification Data>. The
<Union Of Features> has a
<Reference Surface> component, which
associates to the
<Union of Primitive Geometry>
and has these field values:
<Location 2D> instances found in the
<Union of Features>
aggregation tree use the terrain polygons to resolve
elevation.
- Continuing example 1, the <
Union Of Geometry Hierarchy> under the <
Environment Root> contains another
<Union Of Primitive Geometry>
with <Polygons> classified as
ECC_INLAND_WATER_ELEVATION. The
<Union Of Features> aggregates a
<Union Of Features>, which is classified as
ECC_ENGINEERING_BRIDGE and contains
<Linear Features> using
<Location 2D> instances. The
<Union Of Features> also contains a
<Reference Surface> with
<Classification Data> tagged as
ECC_INLAND_WATER_ELEVATION, and
associated to the <
Union Of Primitive Geometry>.
- Suppose we have, as our <
Reference Surface>'s geometry, a
<Spatial Index Related Geometry>
whose components are all <
Classification Related Geometry>. Each
<
Classification Related Geometry> contains 3
<Union Of Primitive Geometry>
instances: one for terrain surface polygons, one for road
polygons (which may or may not be part of the road surface),
and one for forest canopy polygons.
<Reference Surface> would associate to
the <
Spatial Index Related Geometry>, and its
classification field
would be 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 <Linear Feature> representing a road,
which mostly stays on the road geometry but sometimes strays off. This
<Linear Feature> is placed in a
sub-<Union Of Features> aggregating a
different <Reference Surface> instance, which
associates to the same <
Spatial Index Related Geometry> but has
classification =
ECC_ROAD. The resolution process for this
<Reference Surface> sees the road
<Polygons> and ignores the others. For
<Feature Nodes> that stray off the road, the
corresponding <Location 2Ds>' rays will fail
to intersect
any road polygon, so we are in case 3: empty intersection set.
We then fall back to the previous override, which was the
terrain surface.
Using a specific plane for elevation resolution, such as
a carrier deck, or a landing plate. Represent the
plane with one or more <Polygons>. Put these
<Polygons> under a
<Union Of Primitive Geometry>
and classify them. Use that
<Union Of Primitive Geometry>
for the <Reference Surface> associated
<Geometry Hierarchy>, and use the same
classification for the
classification field.
Terrain is organized in 3 minute regions, which are grouped
into 1 degree cells, where the 1 degree cells are collected
under one <
Union Of Geometry Hierarchy>. In addition,
<Features> and non-terrain
<Geometry> are organized under a corresponding
spatial organization. Each 3 minute hierarchy has a
<Reference Surface> associated to the
corresponding 3 minute terrain. Each 1 degree hierarchy has a
<Reference Surface> associated to the
corresponding 1 degree terrain. Each of the highest level feature
and non-terrain geometry hierarchies has a
<Reference Surface> associated to the
terrain <Geometry Hierarchy>.
In this scheme, a <Location 2D> in a 3
minute region finds its resolution surface in the corresponding 3
minute terrain. If a <Location 2D>
"strays" outside its region (i.e.,
strict_organizing_principle=SE_FALSE), then the containing
1 degree terrain resolves the
<Location 2D>. If the location
ray fails to intersect the 1 degree surface, then the full
terrain <
Union Of Geometry Hierarchy> is used. If ray / surface
intersection still fails, the elevation is resolved by the
vertical datum.
FAQs
- Why does a hierarchy need a
<Reference Surface>? What do you mean
by "elevations" for <Location 2D>
instances? They don't have elevation.
- In a 3D spatial reference frame, <Location 2Ds>
are thought of as lying on a surface. Which surface is intended seems to
be subjective at best. A cartographer may prefer the vertical datum as the
<Location 2D> surface, while others prefer the
"terrain surface". Terrain surface is also a subjective term,
and terrain surfaces have been mapped to the DRM in a variety of ways.
Even if one notion of surface for <Location 2Ds>
were mandated, it would not meet everyone's requirements.
The solution is to mandate a clearly defined surface (the initial
default) and provide a flexible mechanism to override the default
for all or for selected parts of the transmittal.
- How does a <Feature Hierarchy> or
<Geometry Hierarchy> use a
<Reference Surface> to resolve elevations
for aggregated <Location 2Ds>?
- Consider a <Reference Surface> that is
associated to a <Geometry Hierarchy> that
contains a surface. A <Location 2D> corresponds to
the (unique) ray which is:
- normal to the surface of the SRF ellipsoid (*),
- intersects the ellipsoid at the same horizontal coordinates as the
<Location 2D>, and
- extends below the surface of the ellipsoid to a depth
equal to the minor radius of the ellipsoid.
(*) NOTE:
For augmented Projected SRFs, this is the projection ellipsoid. For LSR,
use the z=0 plane, where z is the coordinate axis specified by the
SRM_LSR_Parameters_3D
up_direction value.
The intersection of this ray with the resolution surface defines the
candidate set for the corresponding 3D location. One 3D location is
selected from the ray/surface intersection set according to the
following three cases:
- If the set contains 1 and only 1 element, the spatial position
of that point resolves the <Location 2D>
instance.
- If the set contains more than one element, the
<Reference Surface> field
multiplicity_rule
value is used to select exactly one element.
(For instance, if several overlapping <
Property Grids> with <Grid Overlap> components
are part of the <Reference Surface>, use
<Grid Overlap>'s rules to define the
<Reference Surface> in the overlap region
of two or more of the surface grids. If the intersection set still has
more than one point, use
multiplicity_rule.)
- If the intersection is empty, then look for other
<Reference Surface>
instances higher up the aggregation tree and repeat this resolution
process with that surface instead. If there are no other
<Reference Surface>
instances higher up the aggregation tree, then use the
vertical datum, which is guaranteed to be a case 1 surface.
(See also example 5).
- What if there is no single <
Union Of Primitive Geometry> that defines the
<Reference Surface>?
- There is no requirement that the aggregate be free of non-reference
surface geometry (See example 3). In this case, find the higher level
<Geometry Hierarchy> that aggregates
the desired <
Union Of Primitive Geometry> sub-hierarchies, and use that for the
<Reference Surface> association.
- What happens to <LSR Location 2D>
instances in an LSR <Model> when the
<Model> is instanced by a model instance object in a
non-LSR 3D spatial reference frame?
- That depends on whether or not the scoping SRF supports
<Location 2Ds>.
- If <Location 2D> instances are supported
in the scoping SRF, (e.g. the scoping SRF is 3D geodetic, for which
we have 2D geodetic), these
<LSR Location 2D> instances are converted
to <Location 2D>
instances in the instancing 3D SRF using a 3 step process:
- Otherwise, if the scoping SRF does not support
<Location 2Ds>, then
the <LSR Location 2D> is converted
to 3D by setting the tertiary axis value to zero.
(Note 1): LSR models are not permitted to contain
instances; see constraints.
(Note 2): Conformal behaviour may also be modeled with
<
LSR Location 3D Control Links>.
- Can a <Geometry Model Instance>
be used for a <Reference Surface>
association?
- Yes, if the <Model>'s spatial reference frame
matches the currently scoped 'world' spatial reference frame.
- How are <Location 2D> instances converted
consuming data in a different spatial reference frame?
- There are two cases.
Case 1 - both SRFs have the same horizontal datum.
Case 2 - The two SRFs have different horizontal datums.
In case 1 (Same horizontal datum), the ray determined by the
<Location 2D> is invariant, so the
horizontal coordinates are converted in the usual way.
In case 2 (Different horizontal datums), the ray may change, so three
steps are needed:
- If I consume a transmittal in different SRF, should I resolve
elevations in the originating or the consuming SRF?
- It should not make a difference if the two SRFs are both
"real world" or
both Augmented Projected Coordinate Systems (APCS). In the case in which
one is "real" world and the other is APCS, there are
two ways to deal with the "conversion distortion" that
tends to bend flat surfaces.
Consider a <Location 2D> whose ray intersects
the resolution surface near the centre of exactly one triangular
<Polygon>. The intersection point determines the
elevation, and therefore a corresponding <
Location 3D>. If you resolve in the first SRF, and then convert the
<Location 3D>, the location will match the
transmittal point, but it may no longer lie on the triangle's surface.
On the other hand, if you convert the
<Location 2D> and then resolve in the second SRF,
the corresponding <Location 3D> will lie on the
triangle's surface, but may differ a little
in elevation from the originating point. Therefore if absolute location
is more important than conformality, resolve in the originating SRF.
If conformality is more important, resolve in the consuming SRF.
Constraints
Associated to (one-way)
Composed of (two-way)
Component of (two-way)
Inherited Field Elements
This class has no inherited field elements.
Notes
Associated with Notes
Specifies the <Geometry Hierarchy> containing the
<Surface Geometry> and/or <Property Grids> to be used
as the resolution surface.
Composed of Notes
A <Reference Surface> instance has <Property Value> components
only when the classification of the <Reference Surface> requires
elaboration by <Property Value> instances.
Fields Notes
Within the resolution surface, use only geometry matching
this (possibly elaborated) classification.
Rule to select a single point from multiple intersections of a
ray with a resolution surface.
Rule to select one LOD branch.
Prev: Reference Origin.
Next: Reference Vector.
Up:Index.
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Last updated: October 1, 2002
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Copyright © 2002 SEDRIS™
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