Rational for WCS and WFS Combination for point data
Objective and Current Experience
While WCS was designed for gridded data, we have found it to be a good delivery standard for station data also. With a minimal metadata encoding conventions it's possible to use the combination of WCS and WFS for the purposes that they were designed for, and use existing standards for interoperable data delivery system.
The Web Coverage Service was designed to deliver continuous or discrete coverages. The basic dimensions are geographic boundingbox and time. In general, each coverage point has little metadata about itself. The Web Feature Service was designed to deliver rich metadata about stationary objects. While the feature has a boundingbox, the lat and lon are not dimensions. They are attributes of features.
By combining those two it is possible to deliver complete metadata of a station network.
Dimensions covering geographic area
- Typical station network looks like 2D, but is in reality 1D.
- In cube data, you have typically at least 2 dimensions, lat and lon.
- In point data, you have typically 1 dimension, location.
The location has 2 attributes: lat and lon, but it's still one dimension. This is an important distinction and WCS 1.1 supports such dimensions.
A Coverage can be Irregular
The WCS DescribeCoverage supports both regular and irregular coverages.
<SpatialDomain> <ows11:BoundingBox crs="urn:ogc:def:crs:OGC:2:84" dimensions="2"> <ows11:LowerCorner>-179.5000 -89.5000</ows11:LowerCorner> <ows11:UpperCorner>+179.5000 +89.5000</ows11:UpperCorner> </ows11:BoundingBox> <GridCRS> <GridBaseCRS>urn:ogc:def:crs:OGC:2:84</GridBaseCRS> <GridType>urn:ogc:def:method:WCS:1.1:2dSimpleGrid</GridType> <GridOrigin>-179.5000 -89.5000</GridOrigin> <GridOffsets>+1.0000 +1.0000</GridOffsets> <GridCS>urn:ogc:def:cs:OGC:0.0:Grid2dSquareCS</GridCS> </GridCRS> </SpatialDomain>
The GridBaseCRS element manifests the projection: "WGS 84"; type "2dSimpleGrid" origin -179.5, -89.5 and increments 1.0 and 1.0 degrees. In this case, the grid has regular lat and lon dimensions.
<SpatialDomain> <ows11:BoundingBox crs="urn:ogc:def:crs:OGC:2:84" dimensions="2"> <ows11:LowerCorner>-126.0000 +24.0000</ows11:LowerCorner> <ows11:UpperCorner>-65.0000 +52.0000</ows11:UpperCorner> </ows11:BoundingBox> </SpatialDomain>
This is a description of irregular space dimensions. The data can still be an irregular grid, for example fine grid along the equator and sparse on the poles. On point data, the coverage is not a grid at all, but a station network.
Additional Dimensions in DescribeCoverage
This is an elevation dimension. Irregular dimensions are expressed as axises, with some standard metadata like datatype and Units Of Measure. The WCS adds some CF-NetCDF metadata into the AxisMetadata node. This is still all according to the standard,
<Axis identifier="elev"> <ows11:Title>air_pressure</ows11:Title> <AvailableKeys> <Key>1000.0</Key> <Key>975.0</Key> ... <Key>10.0</Key> </AvailableKeys> <ows11:DataType>double</ows11:DataType> <ows11:UOM>mbar</ows11:UOM> <ows11:Metadata> <cf:AxisMetadata> <cf:axis positive="down">Z</cf:axis> <cf:standard_name>air_pressure</cf:standard_name> </cf:AxisMetadata> </ows11:Metadata> </Axis>
Metadata Limits in DescribeCoverage
While the extra dimensions in gridded data can be expressed with WCS 1.1, for station data the location table causes trouble. Here is a typical location table:
loc_code, lat, lon, elev 010730025, 33.55, -86.78, 54 010730028, 33.53, -86.85, 51 ....
There is no good way to express a table in the WCS, but WFS fits to the job perfectly. It's a collection of stationary located objects with varying amount of metadata.
The solution is to combine WCS and WFS. The WCS framework enumerates the location dimension keys, loc_codes, in the AvailableKeys node. Then the real rich location table is delivered via the WFS link.
<Axis identifier="location"> <ows11:Title>Location Table</ows11:Title> <AvailableKeys> <Key>010730025</Key> <Key>010730028</Key> ... </AvailableKeys> <ows11:Metadata xlink:type="simple" xlink:href="http://data1.datafed.net:8080/AQS?service=WFS&Version=1.0.0&Request=GetFeature&typename=AQS_D&filter=field:PB&outputFormat=text/csv" /> </Axis>
(In the actual implementation AvailableKeys does not enumerate loc_codes, because it's redundant. We're just showing it's possible.)
For a generic client, the necessary location metadata attributes are loc_code, lat and lon.
It is useful to add more metadata, like loc_name and elev. All these names are just a convention, and need to be documented as a best practice.
Custom Location Metadata
Many networks display a large collection of different kind of metadata that is very provider-specific, and therefore impossible to standardize. For example the AQS network:
If you press the small "m" button next to the parameter list, you'll see the Rich Metadata View
It's impossible to standardize such information, so we should just agree how to encode the URL that pops up the HTML page containing human readable information about the location.