Grid Topology Issues List

Issue: slicing between axis coordinates

Description: What is the WCS subsetting result if a slicing coordinate is between two point coordinates on that axis? See also: petascope subsetting discussion

Alternatives:

• Alt 1: empty result
• Alt 2: take nearest value
• Alt 3: interpolate between values

Discussion:

• Pro 1:
• Con 1:

Decision: Alt x.

Issue: trimming/slicing on axes not aligned with CRS axes

Description: The result of a WCS Core subsetting may not be a grid (ie, array) any more. However, the result of a subsetting, as per Core, of a gridded coverage should result in a gridded coverage again. This situation can occur if the coverage's domain grid establish axes irregular or warped or rotated wrt. the Native CRS axes.

Definition:

• Domain Grid = the grid established by the points of the domain set as expressed in the coverage's Native CRS. For Grid Coverages, this is a grid by construction.
• Index Grid = the grid established by the (integer) position of the coordinate positions in relation to each other. Index Grid and Domain Grid are topologically isomorphic.

Observations:

• we believe that the origin of a coverage does not need to be inside array (as per GML 3.3)
• trimming (and slicing?) generate grids where some points need to be existent for reasons of toppology, but do not have any meaning (nor in value, nor do they have a a valid geo position)
• GetCoverage establishes an intersection of domain D and bounding Box B. Intersection must maintain grid property, which is not naturally fulfilled.
• offset vectors do not pose a problem for any coverage type (regular: offsets are the same everywhere, so position independent; irregular axes and GCPs: offsets are absolute, ie, depending on origin only and not on neighbor offset)
• this is funcitonality of the WCS Core, so it needs to be expressed in terms of Native CRS, without resorting to resampling etc.

Alternatives:

• Alt 1: reconstruct grid by omitting elements of the intersection
• Alt 2: reconstruct grid by introducing dummy nodes (like black pixels in reprojected images)
• Alt 3: reconstruct grid by including adjacent nodes outside B, including their range values
• Alt 4: reconstruct grid by including the minimal set of adjacent, but outside nodes necessary to copmlete the grid; set their range values to 0 or nil

Discussion:

• Con 1: loses data
• Con 2: dummy nodes also have no valid coordinates (offset vectors or GCPs!), not only invalid data
• Con 3: includes data "not wanted"

Decision: Alt x.