Class

• <DRM Grid Overlap>

Superclass

• <DRM SEDRIS Abstract Base>

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 four 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 an <DRM Alternate Hierarchy Related Geometry>, or disjoint <DRM Time Constraints Data>.

2. A and B have matching <DRM Classification Data>.

3. At least one <DRM Table Property Description> in A has a meaning matching a <DRM Table Property Description> in B.

4. Both A and B actually have data for that matching <DRM Table Property Description> at L.

When such an ambiguity occurs, <DRM Grid Overlap> instance(s) indicate how the data provider intended the consumer to calculate the <DRM Table Property Description> value intended at each such location.

When <DRM Grid Overlap> instances are required and are present, resolution only occurs within an overlay group. The resolution process is performed on data from <DRM Property Grid> cells that contain a given location (choose the first priority group that includes all relevant grids.) The resolution process is as follows:

STEP 1:

Start with priority 0. Each priority group shall have exactly one <DRM Grid Overlap> instance with priority 0. The <DRM Property Grid> instance for this <DRM Grid Overlap> instance shall overlap the other <DRM Property Grid> instances in the given priority group. The operation for priority 0 shall be BASE.

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 priority. Priorities within an overlay group need not be consecutive, but they shall be unique. Extract the cell data from the <DRM Property Grid> that has this <DRM Grid Overlap> as a component. Operate on this and the current data according to the <DRM Grid Overlap> operation. The result of the operation becomes the current data for the next step.

REPLACE means that this data overrides the current data from the last step.

ADD and MEAN can only be applied to numeric data.

MERGE operations are dependent on the classification of the <DRM Property Grids>, and use methods documented outside SEDRIS.

STEP 3:

Look for next priority. If found, goto 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 6.118 — DRM_Grid_Overlap

Constraints

• Publishable Object

Example(s)

• Low resolution grid A covers a large area, and contains smaller (but higher resolution) grids B, C, and D. The <DRM Grid Overlap> scheme 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 intersection A & B, B data overrides A.
  In intersection A & C, C data overrides A.
  In intersection A & D, D data overrides A.
  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 modeled as a grid M of elevation offsets above the underlying bathymetry in grids A and B. The <DRM Grid Overlap> scheme 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

  In intersection A & M and outside of B, add M offsets to A bathymetry values.

  In intersection B & M and outside of A, add M offsets to B bathymetry values.

  In intersection A & B, average A and B bathymetry values.

  In intersection A & B & M, first average A and B bathymetry values, and then add offsets from M to the average.