opendap / fileout_gdal Goto Github PK

Updated for version 0.9.4

This handler builds a GeoTiff, JPEG2000, and GMLJP2 responses from 
a DAP data response, with some
caveats. The handler is limited to a single Grid variable and that
Grid must be either a two-dimensional grid or be constrained so that
only two dimensions remain once the constraint is applied.

The handler tests the Grid for a number of properties in addition to
the limitation that it be a 2D grid:

1. It must be a lat/lon grid, that follows CF (or be reasonably close
to following the CF specification).

2. Only two datums are supported: Spherical Earth (EPSG:4047) and
WGS84, although the default can be set from the module's conf file
using the parameter FONg.default_gcs. Any datum that GDAL supports
using a 'well known text' can be given, including EPSG:<num>. See
GDAL for more info.

3. All the GeoTiffs are returned using doubles regardless of the
datatype used for the original grid.

4. Because many datasets use 'missing' or 'no data' values that are
often very small (e.g., -1e-34) and GDAL allocates colors/gray values
so they evenly span the range of values, those missing data values are changed
so that they are closer to (but never overlap) the actual data values.

5. To find the missing data value, the 'missing_value' or '_FillValue'
attribute is used. If neither of those attributes are defined for the
DAP variable, then the scaling is not performed.

6. In the resulting GoeTiff, black is the no data value.

The handler can be extended in a number of ways.

* The handler can be extended to support more bands if the logic for
  RGB or RGBA responses can be sorted out.

* The process used to determine the Geographic coordinate system could
  be extended to more GCSs and also to include map projections.

* Support for Arrays could be added.

* The handler can be used to return other file types like JPEG2000 and
  GMLJP2. To do this, add a new FONgTransmitter class, register it with
  FONgModule and have that class call a new FONgTransformer::transform()
  method.
  
Note: The handler uses three names with BESDEBUG: fong, fong2 and fong3. 
The fong name provides the usual setup feedback for handler registration
at runtime. The names fong2 and 3 provide more detail for runtime debugging.