= Grid Topology Issues List =

[[TOC]]

== 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: [PetascopeSubsets 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.