GeoAPI

• Type: Improvement
• Status: Open
• Priority: Trivial
• Resolution: Unresolved
• Affects Version/s: 2.2
• Fix Version/s: None
• Component/s: coverage
• Labels:

  None

The OGC 01-004 ("Grid coverage service") implementation specification defines a SampleDimension class which contains among other four attributes: scale, offset, noDataValue and units:

units, offset and scale javadoc

GeoAPI provides an extension attribute called sampleToGeophysics which is really nothing else than scale, offset and noDataValue packaged in a MathTransform1D object. It really doesn't introduce any new concept that OGC 01-004 didn't already have. This packaging was introduced for two reasons:

• Easier for users since a single call to MathTransform1D.transform(double) takes care automatically of all relevant attributes including the "no data values".
• Allows generalization to non-linear transforms, especially logarithmic transforms like the one used in Chlorophyll-a concentration from SeaWiFS data.

sampleToGeophysics javadoc

However discussion on the GeoTools mailing list suggests that this transform is not always well understood. Some users do not realize that this is nothing else than a packaged form of scale, offset and noDataValues - they rather see this transform as a new, weird and unusual concept. Other users point out that "sample to geophysics" sound like a very complex transform to them, like calculation of geophysics values from raw data including sensor calibration, etc. It has been suggested that getSampleToUnits() would be a better name for the following reason:

• Make it clear that this transform is not something complex, but rather sometime simple in the spirit of "unit conversion" (converting from the packed values commonly stored in files to unpacked, geophysics values).
• Make the relationship with the units attribute apparent. Those units are the target of the scale + offset conversion.
• Would be consistent with the GridCoverage.getGridToCRS() method where the name implies the destination (the CRS as returned by Coverage.getCoordinateReferenceSystem()). In this case, the destination of SampleDimension.getSampleToUnits() is the units as returned by SampleDimension.getUnits(). In both cases the source is some form of gridded data.

• Assignee:

  Martin Desruisseaux

  Reporter:

  Martin Desruisseaux

• Created:

  05/Mar/08 8:26 AM

  Updated:

  28/Jan/11 10:33 AM