Class Name: Grid Overlap
Definition
The Grid Overlap class provides
information on how to resolve
data ambiguities at a location inside a grid cell of two or more
Property Grids of the same data_table_type or
of data-compatible compatible types, which cannot be resolved by other means
such as disjoint Time Constraints Data
or aggregation under
Alternate Hierarchy
Related Geometry, etc.
In such cases, resolution only occurs within a priority group. The
resolution process is performed on data from
Property Grid cells that
contain a given location (choose the first priority group that includes
all relevant grids). The process is as follows:
STEP 1:
Start with priority 0. Each priority group must have exactly one
Grid Overlap in the priority_group with
priority 0. The Property Grid
for this Grid Overlap should overlap
the other grids in the given
priority group. The operation for priority 0 must be
SE_DT_OP_BASE.
Extract cell data from the Property Grid
which has this Grid Overlap
as a component. This becomes the current data.
STEP 2:
Find the next priority. Priorities within a priority group need
not be consecutive, but they must be unique. Extract the cell data from
the Property Grid that has this
Grid Overlap as a component. Operate
on this and the current data according to the
Grid Overlap operation.
The result of the operation becomes the current data for the next step.
SE_DT_OP_REPLACE
means that this data overrides the current data from the last step.
SE_DT_OP_ADD and
SE_DT_OP_AVERAGE
can only be applied to numeric data.
SE_DT_OP_MERGE
operations are
data_table_type-dependent and use methods documented outside SEDRIS.
STEP 3:
Look for next priority. If found, goto step 2. Otherwise use
the current data.
Primary Page in DRM Diagram:
Example
- Low resolution grid A covers a large area, and contains smaller,
but higher resolution grids B, C, and D. The
Grid Overlap scheme is:
| Prop_Grid | priority_group | priority |
operation |
| A | 10 | 0 |
SE_DT_OP_BASE |
|---|
| B | 10 | 1 |
SE_DT_OP_REPLACE |
|---|
| A | 20 | 0 |
SE_DT_OP_BASE |
|---|
| C | 20 | 1 |
SE_DT_OP_REPLACE |
|---|
| D | 20 | 2 |
SE_DT_OP_REPLACE |
|---|
In intersection A & B, B data overides A.
In intersection A & C, C data overides A.
In intersection A & D, D data overides A.
In intersection A & C & D, D data overides 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
Grid Overlap scheme is:
| Prop_Grid | priority_group | priority |
operation |
| A | 1 | 0 |
SE_DT_OP_BASE |
|---|
| B | 1 | 1 |
SE_DT_OP_AVERAGE |
|---|
| M | 1 | 999 |
SE_DT_OP_ADD |
|---|
| B | 2 | 0 |
SE_DT_OP_BASE |
|---|
| M | 2 | 999 |
SE_DT_OP_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.
FAQs
- Why is this class needed?
- It is possible and allowable in SEDRIS for more than one
Property Grid to cover the same location
and to contain the same Table_Property
Descriptions.
Grid Overlap allows a transmittal preparer to
express how a consumer is intended to resolve this ambiguity, i.e. how
to calculate the Table_Property
Description value intended at each location.
- Are there real datasets that need this capability?
- There are numerous numerical models in the atmosphere and ocean
community that start by computing a coarse grid over a large
area and then use this grid as boundary and initial conditions
for calculating a more finely-sampled grid over a smaller area.
In many cases, the process is repeated several times, producing
a "nest" of grids that all cover the same area. It is also
possible to implement variable-resolution grids in SEDRIS by
constructing a base grid covering a large region at a coarse sample
spacing suitable for describing 'ambient' conditions, and then to
inset finer grids at locations with detailed features of interest.
- When must Grid Overlaps be
present?
- A Grid Overlap is required whenever multiple
grids contain values for the same
Table Property Description at the
same Location within the simulated
environment, since otherwise the transmittal is ambiguous.
- When are Grid Overlaps not
needed?
- Whenever they do not cause ambiguity in the transmittal. If the
Property Grids are explicitly disjoint due
to some higher organizing structure such as mutually exclusive
branches of an Aggregate Geometry
there is no ambiguity and a Grid Overlap is
not required. If grids covering the same location have no common
Table Property Description
contents, they
do not create ambiguity and do not need a
Grid Overlap.
- Can a Property Grid have more than one
Grid Overlap?
- Yes. A base Property Grid could have
disjoint overlaps with several
different 'insets'. Although it is usually possible to choose priority
levels within a single group to resolve the ambiguities, use of multiple
groups may make the situation clearer and easier for the consumer. There
are also less common situations of multiple overlaps that can't be
resolved using a single group.
- What happens when cells of overlapping grids are not spatially
aligned with each other?
- The operation rules described in the definition apply at a
single point location, so alignment of cells is not strictly
required. However, it is likely that combining values from misaligned
cells will not produce a sensible value. As basic guidance, preparers
of transmittals should avoid this situation when possible, since it is
confusing to consumers. Grids should be resampled before preparing the
transmittal so as to achieve alignment whenever possible.
Constraints
None.
Component of (one-way)
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