The IDLnetOGCWCS::GetCoverage function method retrieves a data file from a remote OGC WCS server, and writes the file to disk. The file location is specified by the COVERAGE_FILENAME property, and will be overwritten if this property value remains unchanged between GetCoverage requests.

The URL_PATH and URL_HOSTNAME properties must be set before requesting information from a remote WCS server. You can either set these properties manually or pass a URL to the IDLnetOGCWCS::ParseUrl method prior to making a request. If you are working with a proxy server, you must also set the PROXY_HOSTNAME and PROXY_PORT properties to the correct values.

This method returns only when one of the following occur:

• Completes retrieval of information from server
• Encounters an HTTP error
• Encounters an OGC exception
• Responds to a cancel request as specified in a callback return value

You can implement a callback to return status information during the request by setting the CALLBACK_FUNCTION property. You can also use a callback to cancel a request. See Using Callbacks with the IDLnetOGCWCS Object for details. This method will throw an error if the GetCoverage request fails.

Syntax

Result = Obj->[IDLnetOGCWCS::]GetCoverage(Coverage_Request)

Return Value

The return value is string containing the path to the file returned by this request or an empty string if no file was returned.

Arguments

Coverage_Request

The Coverage_Request argument must be a string that contains the required key-value pairs separated by the & character. These elements can be extracted from the structures returned by the IDLnetOGCWCS::GetCoverageOffering method. The required elements are shown in the following table, and match those described in OGC Web Coverage Service (WCS) specification. See the OGC web site (https://www.ogc.org/) for specification details.

band=1,5,3 (e.g., radiance values in bands 1, 5, 3) 

age=0/18 (e.g., counts of people with ages under 18 yrs.) 

Keywords

None

Examples

The following example connects to an OGC server and calls GetCapabilities, GetCoverageOfferingBriefs, DescribeCoverage, GetCoverageOffering and GetCoverage to return data from the server. Elements of the structure returned by GetCoverageOffering provide the required key-value pair elements of the query defining the coverage data to return.

Note: You may need to replace the URL in the following example as there is no guarantee that the given OCG server will be available when you attempt to establish the connection.

FUNCTION ogcwcs_callback,  StatusInfo, CallbackData

PRINT, StatusInfo

; Indicate all is well.

vCancelFlag = 1

RETURN, vCancelFlag

END

PRO ogc_wcs_getcoverage_doc

; Use the GetCoverage method to return data from an OGC server. 

; The GetCoverageOffering structure provides parameters required 

; for the GetCoverage request.

; This example queries an OGC server at the following URL. Change 

; the value of this variable to access a server of your choice.

url="http://mydataserver.com:80/cgi-bin" + $ 

   "/mapserv.exe?MAP=/OGC_Data/WCS/wcs_demo.map" + $

   "&SERVICE=WCS&VERSION=1.0.0&REQUEST=GetCapabilities"

; Catch error.

CATCH, errorStatus

IF (errorStatus NE 0) THEN BEGIN

   CATCH,/CANCEL

   r = DIALOG_MESSAGE(!ERROR_STATE.MSG, $

      TITLE='OGC WCS Error', /ERROR)

   PRINT, !ERROR_STATE.MSG

   IF OBJ_VALID(oWcs) THEN OBJ_DESTROY, oWcs

   RETURN

ENDIF

; Create an IDLnetOGCWCS object and parse a known, working URL to

; set mandatory properties. 

oWcs = OBJ_NEW("IDLnetOGCWCS", $

   CALLBACK_FUNCTION="ogcwcs_callback")

oWcs->ParseUrl, url

; Return and parse a brief description of all data holdings 

; on the WCS server.

count = oWCS->GetCapabilities()

arrName=' '

; Make sure the server returned some coverage offering briefs.

IF (count NE 0) THEN BEGIN

   ; Return information about the holding using the 

   ; GetCoverageOfferingBriefs method.

   briefStruct = oWCS->GetCoverageOfferingBriefs $

      (COUNT=briefcount, NUMBER=1)

   PRINT, "Returning structures for ", + briefcount, + " briefs"

   ; Access the name element of the GetCoverageOfferingBriefs

   ; structure.

   FOR x=0, briefCount-1 DO BEGIN

      PRINT, 'CoverageOfferingBrief = ', strtrim(x,2)

      PRINT, '  name             = ', briefStruct[x].name

      ; Add the name to the string array.

      arrName = [arrName, briefStruct[x].name]

   ENDFOR

   ; Strip out empty string element.

   arrName=arrName[1:*]

   PRINT, "Number of arrName elements = " $

      + STRING(N_ELEMENTS(arrName))

   ; Call DescribeCoverage to acccess coverage details for the 

   ; first coverage in the array. 

   covName = oWCS->DescribeCoverage(NAMES=arrName[0], $

      COUNT=numCov)

ENDIF ELSE BEGIN

   void = DIALOG_MESSAGE("No information available " $

      + "from WCS Server", /ERROR)

   RETURN   

ENDELSE

; Get coverage offering details using GetCoverageOffering method. 

mainStruct=oWCS->GetCoverageOffering()

; Access elements of the structure that are required for the 

; GetCoverage request. This consists of coverage name, CRS,

; bounding box, time or parameter, width/height or resoution x/y,

; and format.

coverageName= mainStruct.co_name

crs = mainStruct.crs.request_response[0]

; Find the number of samples and lines in the full image

; from the spatial domain grid.

extent = $

   LONG(STRSPLIT(mainStruct.spat_dom_grid.limits_env_high, $

      /EXTRACT))- $

   LONG(STRSPLIT(mainStruct.spat_dom_grid.limits_env_low, $

      /EXTRACT))

nSamples=extent[0]

nLines=extent[1]

PRINT, 'Number of samples: ', nSamples

PRINT, 'Number of lines: ', nLines

pos1=STRSPLIT(mainStruct.spat_dom_lonlatenv[0].pos1, /extract)

posDims=SIZE(pos1, /n_dimensions) ? N_ELEMENTS(pos1) : 0

pos2=STRSPLIT(mainStruct.spat_dom_lonlatenv[0].pos2, /extract)

bbox=''

IF (posDims EQ 2) THEN BEGIN

bbox = pos1[0] + ',' + $

       pos1[1] + ',' + $

       pos2[0] + ',' + $

       pos2[1]

ENDIF

IF (posDims EQ 3) THEN BEGIN

bbox = pos1[0] + ',' + $

       pos1[1] + ',' + $

       pos1[2] + ',' + $

       pos2[0] + ',' + $

       pos2[1] + ',' + $

       pos2[2]

ENDIF

format=mainStruct.formats[0]

PRINT, 'format = ' + string(format)

; Hard-code width and height.

width = '1000'

height = '1000'

; Create coverage query string of required key-value pairs.

queryString = 'Coverage=' + coverageName

queryString = queryString + '&CRS=' + crs

queryString = queryString + '&BBOX=' + bbox

queryString = queryString + '&FORMAT=' + format

queryString = queryString + '&WIDTH=' + width

queryString = queryString + '&HEIGHT=' + height

dataFile = oWCS->GetCoverage(queryString[0])

PRINT, "file written to " + dataFile

data = READ_IMAGE(dataFile) 

IIMAGE, data, /ORDER

; Cleanup.

OBJ_DESTROY, oWcs

END

Version History

See Also

IDLnetOGCWCS::GetCoverageOffering