Rational for WCS and WFS Combination for point data
Objective
While WCS was designed for gridded data, we have found it to be a good delivery standard for station data also. With a minimal convention it's possible to use the combination of WCS and WFS for the purposes that they were designed for, and create an interoperable station data convention.
Experience with Station Point Data
We have used the combination of WCS and WFS at Datafed for station data for six years. Here are a few observations:
- Typical station data looks like 3D, but is in reality 2D.
- In cube data, you have typically at least 4 dimensions, lat, lon and time.
- In point data, you have typically 2 dimensions, location and time.
The station has 2 attributes: lat and lon, but it's still one dimension. This is an important distinction which was not in the design requirements for the WCS standard. Fortunately, the combination of WCS and WFS works great.
Web Coverage and Feature Services
The Web Coverage Service was designed to deliver continuous coverages. The basic dimensions are geographic boundingbox and time. In general, each coverage point has little metadata about itself.
The Web Coverage Service was designed to deliver rich metadata about stationary objets. While the feature has a boundingbox, the lat and lon are not dimensions. They are attibutes of features.
A Coverage can be Irregular
The WCS DescribeCoverage supports both regular and irregular coverages.
[Regular Data](http://data1.datafed.net:8080/NRL?service=WCS&version=1.1.2&request=DescribeCoverage&Identifiers=NAAPS_press)
<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.
[Irregular Data](http://data1.datafed.net:8080/AQS?service=WCS&version=1.1.2&request=DescribeCoverage&Identifiers=AQS_D)
<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, bot example fine grid along the equator and sparse on the poles. On in this case, the coverage is not a grid at all, but a station network.
More Dimension Metadata in DescribeCoverage
The regular lat, lon, and time dimensions are described in the DescribeCoverage document
<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>
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,
Missing Metadata 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 geolocated objects with varying amount of metadata.
<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>
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.
