| Author: | Jeff McKenna |
|---|---|
| Contact: | jmckenna at gatewaygeomatics.com |
| Revision: | $Revision: 8451 $ |
| Date: | $Date: 2009-01-23 10:42:36 -0800 (Fri, 23 Jan 2009) $ |
| Last Updated: | 2008/04/18 |
Table of Contents
A WCS (or Web Coverage Service) allows for the publication of “coverages”- digital geospatial information representing space-varying phenomena. In the MapServer world it allows for unfiltered access to raster data. Conceptually it is easy think of WCS as a raster equivalent of WFS. The following documentation is based on the Open GIS Consortium’s (OGC) Web Coverage Server Interfaces Implementation Specification version 1.0.0.
In order to enable MapServer to serve WCS data, it MUST be compiled against certain libraries:
For WCS 1.1.x support (MapServer 5.2) there is an additional requirement:
Please see the MapServer UNIX Compilation and Installation HowTo for detailed instructions on compiling MapServer with support for these libraries and features. For Windows users, MapServer for Windows (MS4W) comes WCS Server support.
Much as in the WMS and WFS support, WCS publishing is enabled by adding certain magic METADATA keyword/value pairs to a .map file.
MapServer will serve and include in its WCS capabilities only the layers that meet the following conditions:
The following is an example of a simple WCS Server mapfile. Note the comments for the required parameters.
NAME WCS_server
STATUS ON
SIZE 400 300
SYMBOLSET ../etc/symbols.sym
EXTENT -2200000 -712631 3072800 3840000
UNITS METERS
SHAPEPATH "../data"
IMAGECOLOR 255 255 255
FONTSET "../etc/fonts.txt"
#
# Start of web interface definition
#
WEB
IMAGEPATH "/ms4w/tmp/ms_tmp/"
IMAGEURL "/ms_tmp/"
METADATA
"wcs_label" "GMap WCS Demo Server" ### required
"wcs_description" "Some text description of the service"
"wcs_onlineresource" "http://127.0.0.1/cgi-bin/mapserv.exe?" ### recommended
"wcs_fees" "none"
"wcs_accessconstraints" "none"
"wcs_keywordlist" "wcs,test"
"wcs_metadatalink_type" "TC211"
"wcs_metadatalink_format" "text/plain"
"wcs_metadatalink_href" "http://someurl.com"
"wcs_address" "124 Gilmour Street"
"wcs_city" "Ottawa"
"wcs_stateorprovince" "ON"
"wcs_postcode" "90210"
"wcs_country" "Canada"
"wcs_contactelectronicmailaddress" "blah@blah"
"wcs_contactperson" "me"
"wcs_contactorganization" "unemployed"
"wcs_contactposition" "manager"
"wcs_contactvoicetelephone" "613-555-1234"
"wcs_contactfacimiletelephone" "613-555-1235"
"wcs_service_onlineresource" "http://127.0.0.1/cgi-bin/mapserv.exe?"
END
END
PROJECTION
"init=epsg:42304"
END
LAYER
NAME bathymetry
METADATA
"wcs_label" "Elevation/Bathymetry" ### required
"wcs_rangeset_name" "Range 1" ### required to support DescribeCoverage request
"wcs_rangeset_label" "My Label" ### required to support DescribeCoverage request
END
TYPE RASTER ### required
STATUS ON
DATA bath_mapserver.tif
PROJECTION
"init=epsg:42304"
END
DUMP TRUE ### required
END
END # Map File
OK, now that we’ve got a mapfile, we have to check the XML capabilities returned by our server to make sure nothing is missing.
Using a web browser, access your server’s online resource URL to which you add the parameters “SERVICE=WCS&VERSION=1.0.0&REQUEST=GetCapabilities” to the end, e.g.
http://my.host.com/cgi-bin/mapserv?map=mywcs.map&SERVICE=WCS&VERSION=1.0.0&REQUEST=GetCapabilities
If you get an error message in the XML output then take necessary actions. Common problems and solutions are listed in the FAQ at the end of this document.
If everything went well, you should have a complete XML capabilities document. Search it for the word “WARNING”... MapServer inserts XML comments starting with “<!–WARNING: ” in the XML output if it detects missing mapfile parameters or metadata items.
Note that when a request happens, it is passed through WMS, WFS, and WCS in MapServer (in that order) until one of the services respond to it.
Here is a working example of a GetCapabilities request:
OK, now that we know that our server can produce a valid XML GetCapabilities response we should test the DescribeCoverage request. The DescribeCoverage request lists more information about specific coverage offerings.
Using a web browser, access your server’s online resource URL to which you add the parameters “SERVICE=WCS&VERSION=1.0.0&REQUEST=DescribeCoverage&COVERAGE=layername” to the end, e.g.
http://my.host.com/cgi-bin/mapserv?map=mywcs.map&SERVICE=WCS&VERSION=1.0.0&REQUEST=DescribeCoverage&COVERAGE=bathymetry
Here is a working example of a DescribeCoverage request:
The GetCoverage request allows for the retrieval of coverages in a specified output format to the client.
The following is a list of the required GetCoverage parameters according to the WCS spec:
VERSION=version: Request version
REQUEST=GetCoverage: Request name
COVERAGE=coverage_name: Name of an available coverage, as stated in the GetCapabilities
CRS=epsg_code: Coordinate Reference System in which the request is expressed.
BBOX=minx,miny,maxx,maxy: Bounding box corners (lower left, upper right) in CRS units. One of BBOX or TIME is required.
TIME=time1,time2: Request a subset corresponding to a time. One of BBOX or TIME is required..
WIDTH=output_width: Width in pixels of map picture. One of WIDTH/HEIGHT or RESX/Y is required.
HEIGHT=output_height: Height in pixels of map picture. One of WIDTH/HEIGHT or RESX/Y is required.
RESX=x: When requesting a georectified grid coverage, this requests a subset with a specific spatial resolution. One of WIDTH/HEIGHT or RESX/Y is required.
RESY=y: When requesting a georectified grid coverage, this requests a subset with a specific spatial resolution. One of WIDTH/HEIGHT or RESX/Y is required.
FORMAT=output_format: Output format of map, as stated in the GetCapabilities.
The following are optional GetCoverage parameters according to the WCS spec:
RESPONSE_CRS=epsg_code: Coordinate Reference System in which to express coverage responses.
So to follow our above examples, a valid DescribeCoverage request would look like:
http://my.host.com/cgi-bin/mapserv?map=mywcs.map&SERVICE=wcs&VERSION=1.0.0&REQUEST=GetCoverage
&coverage=bathymetry&CRS=EPSG:42304&BBOX=-2200000,-712631,3072800,3840000
&WIDTH=3199&HEIGHT=2833&FORMAT=GTiff
Here is a working example of a GetCoverage request (note that a 3MB tif is being requested, so this may take a few seconds):
WCS 1.1.0 and later versions of the WCS protocol are supported by MapServer 5.2. For the most part the map file setup for WCS 1.1.0 is similar to WCS 1.0.0, but the actual protocol is substantially changed.
The GetCapabilities request is the same as WCS 1.0 but with a different VERSION value:
SERVICE=WCS&VERSION=1.1.0&REQUEST=GetCapabilities
The format of the returned capabilities document is substantially altered from WCS 1.0, and makes use of OWS Common for service descriptions.
The DescribeCoverage request is similar to WCS 1.0, but the IDENTIFIER keyword is used instead of COVERAGE to name the coverage being requested:
SERVICE=WCS&VERSION=1.1.0&REQUEST=DescribeCoverage&IDENTIFIER=spaceimaging
The format for GetCoverage is substantially changed from 1.0. The following is a list of GetCoverage required parameters:
VERSION=version: Request version
REQUEST=GetCoverage: Request name
IDENTIFIER=coverage_name: Name of an available coverage, as stated in the GetCapabilities
BOUNDINGBOX=minx,miny,maxx,maxy,crs: Bounding box corners (lower left, upper right), and the CRS they are in. The CRS is described using a URN.
FORMAT=output_format: Output format (mime type) of grid product, as stated in the GetCapabilities.
If an alternate spatial resolution is desired, then the following set of keywords must be used to specify the sample origin and step size of the output grid to be produced. The produced grid will be of a number of pixels and lines as can be fit in the BOUNDINGBOX starting at GridOrigin, at GridOffsets resolution.
GRIDBASECRS=crs: The grid base CRS (URN).
GRIDCS=crs: The grid CRS (URN).
GridType=urn:ogc:def:method:WCS:1.1:2dGridIn2dCrs: This is the only supported value for MapServer.
GridOrigin=x_origin,y_origin: The sample point for the top left pixel.
GridOffsets=xstep,ystep: The x and y step size for grid sampling (resolution). Both are positive.
As well, the following optional parameters are available.
RangeSubset=selection: Selects a range subset, and interpolation method. Currently only subsetting on bands are allowed. Depending on rangeset names, this might take the form “BandsName[bands[1]]” to select band 1, or “BandsName:bilinear[bands[1]]” to select band 1 with bilinear interpolation.
So a simple GetCoverage might look like:
SERVICE=WCS&VERSION=1.1.0&REQUEST=GetCoverage&IDENTIFIER=dem&FORMAT=image/tiff
&BOUNDINGBOX=43,33,44,34,urn:ogc:def:crs:EPSG::4326
A more complex request might look like:
SERVICE=WCS&VERSION=1.1.0&REQUEST=GetCoverage&IDENTIFIER=dem&FORMAT=image/tiff
&BOUNDINGBOX=33,43,34,44,urn:ogc:def:crs:EPSG::4326
&GridBaseCRS=urn:ogc:def:crs:EPSG::4326&GridCS=urn:ogc:def:crs:EPSG::4326
&GridType=urn:ogc:def:method:WCS:1.1:2dGridIn2dCrs
&GridOrigin=33,44&GridOffsets=0.01,0.01
&RangeSubset=BandsName:bilinear[bands[1]]
It should also be noted that return results from WCS 1.1 GetCoverage requests are in multi-part mime format. Typically this consists of a first part with an xml document referencing the other parts of the message, and an image file part. However, for output formats that return multiple files, each will be a separate part. For instance, this means it is possible to return a jpeg file with a world file, the OUTPUTFORMAT is appropriately configured.
In WCS 1.1 protocol coordinate systems are referenced by URN. Some typical URNs are:
urn:ogc:def:crs:EPSG::4326
urn:ogc:def:crs:EPSG:27700
urn:ogc:def:crs:OGC::CRS84
The first two are roughly equivalent to EPSG:4326, and EPSG:27700 while the third is a CRS defined by OGC (essentially WGS84). One critical thing to note is that WCS 1.1 follows EPSG defined axis/tuple ordering for geographic coordinate systems. This means that coordinates reported, or provided in urn:ogc:def:EPSG::4326 (WGS84) are actually handled as lat, long, not long,lat. So, for instance the BOUNDINGBOX for an area in California might look like:
BOUNDINGBOX=34,-117,35,-116,urn:ogc:def:crs:EPSG::4326
And, likewise the bounds reported by GetCapabilities, and DescribeCoverage will be in this ordering as appropriate.
WCS support tries to leverage existing WMS or OWS metadata definitions. The rationale being that many values will be the same regardless of the type of service. The module will look for the following metadata prefixes (in order): wcs, ows and finally wms unless otherwise noted.
The following metadata are available in the setup of the mapfile:
wcs_abstract
wcs_accessconstraints
wcs_address, wcs_city, wcs_contactelectronicmailaddress, wcs_contactfacimiletelephone, wcs_contactorganization, wcs_contactperson, wcs_contactposition, wcs_contactvoicetelephone, wcs_country, wcs_postcode, wcs_service_onlineresource, wcs_stateorprovince
wcs_description
wcs_fees
wcs_keywords
wcs_label
wcs_metadatalink_format
wcs_metadatalink_href
wcs_metadatalink_type
wcs_name
wcs_responsibleparty_address_administrativearea, wcs_responsibleparty_address_city, wcs_responsibleparty_address_country, wcs_responsibleparty_address_deliverypoint, wcs_responsibleparty_address_electronicmailaddress, wcs_responsibleparty_address_postalcode, wcs_responsibleparty_individualname, wcs_responsibleparty_onlineresource, wcs_responsibleparty_organizationname, wcs_responsibleparty_phone_facsimile, wcs_responsibleparty_phone_voice, wcs_responsibleparty_postionname
wcs_abstract
wcs_description
wcs_extent
wcs_formats
wcs_keywords
wcs_label
wcs_metadatalink_format
wcs_metadatalink_href
wcs_metadatalink_type
wcs_name
wcs_nativeformat
Axes Descriptions
MapServer allows you define a number of these for a layer. Individual axis are identified by name when defining specific metadata (e.g. description). All defined axes must be listed in the rangeset_axes metadata tag so MapServer knows in advance what to expect. A special rangeset for multiband date is automatically generated by adding the name “bands” to the rangeset_axes list. If found MapServer will automatically generate metadata for the image bands. You may of course extend that basic support using the naming conventions below.
wcs_rangeset_axes
Description: (Optional) Delimited list of defined range sets. If defined, you can also use the following nine metadata items, where rangeset axis matches the axis name provided in this wcs_rangeset_axes metadata:
{rangeset axis}_semantic
{rangeset axis}_refsys
{rangeset axis}_refsyslabel
{rangeset axis}_description
{rangeset axis}_label
{rangeset axis}_values
{rangeset axis}_values_semantic
{rangeset axis}_values_type
{rangeset axis}_interval
wcs_rangeset_label
wcs_rangeset_name
wcs_srs
wcs_timeitem
wcs_timeposition
MapServer has long supported a method of breaking a dataset into smaller, more manageable pieces or tiles. In this case a shapefile is used to store the boundary of each tile, and an attribute holds the location of the actual data. Within a MapServer mapfile the layer keywords TILEINDEX and TILEITEM are used to activate tiling.
Consider the example where an organization wants to serve hundreds or even thousands of MODIS scenes. Five images cover the spatial extent and each group of five varies by date of acquisition. This turns out to be a fairly common scenario for organizations interested in WCS, one that the existing tiling support does not adequately address. In previous versions of MapServer a developer would have to create one tile index and one layer definition for each group of five images. This could result in configuration files that are prohibitively long and difficult to manage.
In order to more efficiently support the WCS specification a new tiling scheme has been implemented within MapServer. One that supports spatial sub-setting, but also ad hoc sub-setting based on any attributes found within tile index. In many cases a temporal attribute could be used, but sub-setting is not limited to that case. The new scheme introduces the concept of tile index layers, that is, a separate layer definition is used to describe the tile index dataset. With this we get all the benefits of any MapServer layer, most importantly we can apply MapServer filters to the data. Filters can be defined at runtime using MapServer CGI, MapScript or via the WCS server interface. The syntax for the layer using the index remains unchanged except that the value for Tile Indexes refers to the index layer instead of an external shapefile.
So, looking at the example above again we can reduce our MapServer configuration to two layer definitions, one for the tile index and one for the imagery itself. Extracting a single dates worth of imagery is now a matter of setting the appropriate filter within the tile index layer.
Developing these tile indexes is more difficult than basic indexes simply because there are no ready-made tools to do so. Fortunately we can leverage existing tool available within MapServer or supporting libraries such as GDAL by post processing their output.
Taking the above example, building an index is relatively simple task if you are willing to roll up your sleeves and write a bit of code. First, the basic spatial index needs to be built. The GDAL utility gdaltindex already does this. Simply point gdaltindex at the directory containing the collection of MODIS images and it will build a shapefile index suitable for use with MapServer. The next step would be to add the temporal information. The pseudo code would look something like:
This general approach could be used for many cases. A scripting language such as Perl, PHP or Python works well since they all have readily available modules for manipulating .dbf files. A worst case would involve hand editing the resulting .dbf file using a desktop tool such as Mircosoft Access or ESRI Arcview.