Changes between Version 6 and Version 7 of WCS+interoperability


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Timestamp:
Nov 11, 2017, 12:48:25 AM (7 years ago)
Author:
Ari Jolma
Comment:

fix wikiwords

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  • WCS+interoperability

    v6 v7  
    55The data is further tied to the coordinate system with starting point of data, location of the origin of the data in the coordinate system, and with offset vectors. The starting point of data is by default the first data point. The offset vectors give coefficients for converting data point locations into coordinates. The offset vectors are in the order defined by the sequence rule. The magnitude of the data is given as an envelope, which gives the number of data points along each data axis.
    66
    7 The situation is much complicated by the fact that coordinate systems defined for the surface of the earth can be set up in many ways. It is common to have the first axis to point east but it is possible to have it pointing north. Also the data can organized internally for computer in many ways. GDAL traditionally calls the organization sequences of pixels that form lines, which form sequences of lines. In GDAL the first data point is assumed to be at the location of (0,0) in the pixel,line coordinates, and that the pixel,line coordinates increase linearly. GDAL also traditionally associates pixels with X axis of the coordinate system and lines with Y axis of the coordinate system. GDAL uses the concept of a GeoTransform to tie the data points to the coordinate system. The GDAL GeoTransform defines the minimum X, maximum Y coordinates associated with the first data point (it assumes the data point represents a rectangular area with the point in the center), and the coefficients (above offset vectors) for computing X,Y coordinates for any data point (again, the min X, max Y coordinates associated with it). Finally, GDAL assumes X to increase eastward and Y to increase northward.
     7The situation is much complicated by the fact that coordinate systems defined for the surface of the earth can be set up in many ways. It is common to have the first axis to point east but it is possible to have it pointing north. Also the data can organized internally for computer in many ways. GDAL traditionally calls the organization sequences of pixels that form lines, which form sequences of lines. In GDAL the first data point is assumed to be at the location of (0,0) in the pixel,line coordinates, and that the pixel,line coordinates increase linearly. GDAL also traditionally associates pixels with X axis of the coordinate system and lines with Y axis of the coordinate system. GDAL uses the concept of a !GeoTransform to tie the data points to the coordinate system. The GDAL !GeoTransform defines the minimum X, maximum Y coordinates associated with the first data point (it assumes the data point represents a rectangular area with the point in the center), and the coefficients (above offset vectors) for computing X,Y coordinates for any data point (again, the min X, max Y coordinates associated with it). Finally, GDAL assumes X to increase eastward and Y to increase northward.
    88
    99GDAL has functions to detect the cases where coordinate systems differ from the GDAL concept of first axis east, second axis north. When such coordinate systems are detected in WCS, the order of all things related to the two axes is swapped. This means axis labels, and coordinate values among other things. The swapping happens in all communication, that is, when parsing a response from a server and when constructing a request.
    … …  
    1111to write: what happens if the internal organization of data that is served over WCS is not according to what GDAL assumes (data axis order, sequence)
    1212
    13 to write: WCS servers may allow asking data in CRS that is not the CRS the coverage is described. If GDAL sets the CRS of the dataset into that CRS, it needs to set its GeoTransform and raster size accordingly. That requires projection transformation.
     13to write: WCS servers may allow asking data in CRS that is not the CRS the coverage is described. If GDAL sets the CRS of the dataset into that CRS, it needs to set its !GeoTransform and raster size accordingly. That requires projection transformation.
    1414
    1515== WCS Best Practices for Interoperability