Table 6.98 — DRM_Grid_Overlap

Property

Description

Class

  • <DRM Grid Overlap>

Superclass

Subclass

  • None.

Definition

An instance of this DRM class specifies how the data provider intended the consumer to resolve data ambiguity at a location falling within a grid cell for two or more <DRM Property Grid> instances, such that the ambiguity cannot be resolved by other means.

An ambiguity occurs at a location L lying within two <DRM Property Grid> instances A and B if all five of the following conditions hold:

  1. A and B are not disjoint due to any higher-level organizing structure in the transmittal, such as membership under different branches of a <DRM Alternate Hierarchy Related Geometry> instance, or disjoint <DRM Time Constraints Data> instances.
  2. Cells of A and B overlap the same spatial location.
  3. A and B have matching <DRM Classification Data> components.
  4. At least one <DRM Table Property Description> component of A has a meaning matching a <DRM Table Property Description> component of B.
  5. Both A and B actually have data for those matching <DRM Table Property Description> components at L.

When such an ambiguity occurs, the <DRM Grid Overlap> instance for A and the <DRM Grid Overlap> instance for B shall indicate how the data provider intended the consumer to calculate the cell values at each overlapping cell corresponding to the applicable <DRM Table Property Description> instances.

Resolution only occurs within an overlay group. An overlay group is a set of <DRM Property Grid> instances that are identified by the overlay_group fields of their respective <DRM Grid Overlap> components as requiring common processing of ambiguity. Within an overlay group, the priority fields of their respective <DRM Grid Overlap> components determine the sequence in which the <DRM Property Grid> instance cell data is processed. Priorities within an overlay group need not be consecutive, but they shall be unique. The operation field specifies how the cell data interact with each other.

Each overlay group shall have exactly one <DRM Grid Overlap> instance with priority = 0. The operation field value for priority = 0 shall be BASE.

To resolve an ambiguity, identify the corresponding overlay group and process as follows:

STEP 1:

Start with priority = 0. Extract cell data from the <DRM Property Grid> instance that has this <DRM Grid Overlap> instance as a component; this becomes the current data.

STEP 2:

Find the next higher priority field value. Extract the cell data from the <DRM Property Grid> instance that has this <DRM Grid Overlap> instance as a component. Operate on this and the current data according to the operation field of the <DRM Grid Overlap> instance. The result of applying the operations specified by the operation field becomes the current data for the next step.

STEP 3:

Look for next higher priority. If found, go to step 2. Otherwise use the current data.

The overlay_group field specifies the overlay group within which the resolution process is to occur.

The priority field specifies a priority that is meaningful within the specified overlay_group.

The operation field specifies the operation to be performed on the data during the resolution process.

Class diagram

Figure 6.117 — DRM_Grid_Overlap

Inherited field elements

Field name

Range

Field data type

None

   

Field elements

Field name

Range

Field data type

overlay_group

 

Short_Integer_Positive

priority

 

Short_Integer_Unsigned

operation

 

Grid_Overlap_Operator

Default field values

Default field values for DRM_Grid_Overlap

Associated to (one-way) (inherited)

  • None.

Associated to (one-way)

  • None.

Associated by (one-way) (inherited)

  • None.

Associated by (one-way)

  • None.

Associated with (two-way) (inherited)

  • None.

Associated with (two-way)

  • None.

Composed of (two-way) (inherited)

  • None.

Composed of (two-way)

  • None.

Composed of (two-way metadata) (inherited)

  • None.

Composed of (two-way metadata)

  • None.

Component of (two-way) (inherited)

  • None.

Component of (two-way)

Constraints

Clarifications

None.

Example(s)

  • A low resolution <DRM Property Grid> instance A covers a large area, and contains smaller (but higher resolution) grids B, C, and D. The scheme specified for the <DRM Grid Overlap> components of A, B, C, and D is:

    <DRM Property Grid> overlay_group priority operation
    A 10 0 BASE
    B 10 1 REPLACE
    A 20 0 BASE
    C 20 1 REPLACE
    D 20 2 REPLACE

    In the intersection of A & B where the overlay_group is 10, B data overrides A.

    For overlay_group 20, the following is specified:

    1. In intersection A & C, C data overrides A.
    2. In intersection A & D, D data overrides A.
    3. In intersection A & C & D, D data overrides others.

    B should not intersect either C or D as this scheme will not provide ambiguity resolution.

  • A seamount is modelled as a <DRM Property Grid> instance M of elevation offsets above the underlying bathymetry in <DRM Property Grid> instances A and B, where A and B overlap. The scheme specified by the <DRM Grid Overlap> components of A, B, M is:

    <DRM Property Grid> overlay_group priority operation
    A 1 0 BASE
    B 1 1 MEAN
    M 1 999 ADD
    B 2 0 BASE
    M 2 999 ADD

    For overlay_group 1, the following is specified:

    1. In intersection A & M and outside of B, add M offsets to A bathymetry values.
    2. In intersection A & B, average A and B bathymetry values.
    3. In intersection A & B & M, first average A and B bathymetry values, and then add offsets from M to the average.

    For overlay_group 2, in intersection B & M and outside of A, add M offsets to B bathymetry values.