Map Context

Author:

Jeff McKenna

Contact:

jmckenna at gatewaygeomatics.com

Last Updated:

2019-11-21

Introduction

The term “map context” comes from the Open Geospatial Constortium’s (OGC) Web Map Context Specification v1.0.0, which coincides with the OGC Web Map Server Specification (WMS) v1.1.1. A map context is a XML document that describes the appearance of layers from one or more WMS servers, and can be transferred between clients while maintaining startup views, the state of the view (and its layers), and storing additional layer information.

Support for OGC Web Map Context was added to MapServer in version 3.7/4.0. This allows client applications to load and save a map configuration in a standard XML format. MapServer can read context documents of versions 0.1.2, 0.1.4, 0.1.7, 1.0.0, 1.1.0 and can export contents in versions 0.1.4, 0.1.7, 1.0.0, 1.1.0. Web Map Context 1.1.0 support was added to MapServer 4.10

This document assumes that you are already familiar with certain aspects of MapServer:

  • MapServer application development and setting up mapfiles.

  • Familiarity with the WMS spec would be an asset. Please see the following section for links to associated sources.

Implementing a Web Map Context

Special Build Considerations

Map Context support requires PROJ4, GDAL/OGR and PHP support libraries.

Build/install the above libraries on your system and then build MapServer with the “–with-wmsclient –with-proj –with-ogr –with-gdal –with-php” configure options. Also make sure that your build uses the USE_WMS_LYR and USE_OGR flags. For more details on MapServer compilation see the appropriate HowTo: Unix / Windows

Windows users can use MS4W, which is ready for Map Context use.

Map Context Mapfile

A map context document can ONLY contain WMS layers (e.g. CONNECTIONTYPE WMS). Please refer to the MapServer WMS Client HowTo for more information on declaring WMS layers.

MapFile Metadata

The following mapfile metadata are used by MapServer to handle map context information:

(Note that some parameters have width, height, format, and href, and some only have format and href. This is because width and height are only used for images and parameters that do not have them are text or html. For consistency with the spec MapServer supports height and width for all parameters, but they should only be used for images)

Web Object Metadata

  • ows_schemas_location : Location of XML schema document. Default is http://schemas.opengis.net. See http://ogc.dmsolutions.ca for an example of a valid schema tree.

  • wms_abstract : A blurb of text providing more information about the WMS server.

  • wms_address : If provided must also then provide wms_addresstype, wms_city, wms_stateorprovince, wms_postcode, and wms_country)

  • wms_addresstype : If provided must also then provide wms_address, wms_city, wms_stateorprovince, wms_postcode, and wms_country)

  • wms_city : If provided must also then provide wms_address, wms_addresstype, wms_stateorprovince, wms_postcode, and wms_country)

  • wms_contactelectronicmailaddress : contact Email address.

  • wms_contactfacsimiletelephone : contact facsimile telephone number.

  • wms_contactorganization :

  • wms_contactperson :

  • wms_contactposition :

  • wms_contactvoicetelephone : contact voice telephone number.

  • wms_context_fid : the feature id of the context. Set to 0 when saving if not specified.

  • wms_context_version : the version of the map context specification.

  • wms_country : If provided must also then provide wms_address, wms_city, wms_stateorprovince, wms_postcode, and wms_addresstype.

  • wms_descriptionurl_format : Format of the webpage which contains relevant information to the view.

  • wms_descriptionurl_href : Reference to a webpage which contains relevant information to the view.

  • wms_keywordlist : A comma-separated list of keywords or keyword phrases to help catalog searching.

  • wms_logourl_width : Width of the context logo.

  • wms_logourl_height : Height of the context logo.

  • wms_logourl_format : Format of the context logo.

  • wms_logourl_href : Location of the context logo.

  • wms_postcode : If provided must also then provide wms_address, wms_city, wms_stateorprovince, wms_addresstype, and wms_country.

  • wms_stateorprovince : If provided must also then provide wms_address, wms_city, wms_addresstype, wms_postcode, and wms_country.

  • wms_title : (Required) A human-readable name for this Layer (this metadata does not exist beyond version 0.1.4)

Layer Object Metadata

  • wms_abstract : A blurb of text providing more information about the WMS server.

  • wms_dataurl_href : Link to an online resource where data corresponding to the layer can be found.

  • wms_dataurl_format : Format of the online resource where data corresponding to the layer can be found.

  • wms_dimension :

    Added in version 4.10.

    Current dimension used.

  • wms_dimensionlist :

    Added in version 4.10.

    List of available dimensions.

  • wms_dimension_%s_default :

    Added in version 4.10.

    Default dimension value. MapServer will check for wms_time and wms_timedefault metadata when this is not specified. %s = the name of the dimension.

  • wms_dimension_%s_multiplevalues :

    Added in version 4.10.

    Multiple dimension values. %s = the name of the dimension.

  • wms_dimension_%s_nearestvalue :

    Added in version 4.10.

    Nearest dimension value. The default value is 0. %s = the name of the dimension.

  • wms_dimension_%s_units :

    Added in version 4.10.

    Units for the dimension values. The default value is ISO8601. %s = the name of the dimension.

  • wms_dimension_%s_unitsymbol :

    Added in version 4.10.

    Symbol for dimension units. The default value is t. %s = the name of the dimension.

  • wms_dimension_%s_uservalue :

    Added in version 4.10.

    User dimension value. MapServer will check for wms_time and wms_timedefault metadata when this is not specified. %s = the name of the dimension.

  • wms_format : Current format used.

  • wms_formatlist : List of available formats for this layer.

  • wms_metadataurl_href : Link to an online resource where descriptive metadata of the corresponding layer can be found.

  • wms_metadataurl_format : Format of the online resource where descriptive metadata of the corresponding layer can be found.

  • wms_name : Name of the WMS layer on the server.

  • wms_onlineresource : Required URL to access the server.

  • wms_server_version : The version of the web map server specification.

  • wms_server_title : The title of the web map server.

  • wms_stylelist : Current style used.

  • wms_style_%s_legendurl_width : Width of an image describing the style. %s = the name of the style.

  • wms_style_%s_legendurl_height : Height of an image describing the style. %s = the name of the style.

  • wms_style_%s_legendurl_format : Format of an image describing the style. %s = the name of the style.

  • wms_style_%s_legendurl_href : Location of an image describing the style. %s = the name of the style.

  • wms_style_%s_sld : URL to the SLD document of this style. %s = the name of the style.

  • wms_style_%s_sld_body : SLD_BODY document of this style. %s = the name of the style.

  • wms_style_%s_title : Title of the layer. %s = the name of the style.

  • wms_title : (Required) A human-readable name for this Layer.

Sample Map Context Mapfile

 1    MAP
 2
 3    NAME "mapcontext"
 4    STATUS ON
 5    SIZE 400 300
 6    SYMBOLSET "../etc/symbols.txt"
 7    EXTENT -180 -90 180 90
 8    UNITS DD
 9    SHAPEPATH "../data"
10    IMAGECOLOR 255 255 255
11    FONTSET "../etc/fonts.txt"
12
13
14    #
15    # Start of web interface definition
16    #
17    WEB
18      IMAGEPATH "/ms4w/tmp/ms_tmp/"
19      IMAGEURL "/ms_tmp/"
20      METADATA
21        "wms_abstract" "Demo for map context document. Blah blah..."
22        "wms_title" "Map Context demo"   #### REQUIRED
23      END
24    END
25
26    PROJECTION
27      "init=epsg:4326"
28    END
29
30    #
31    # Start of layer definitions
32    #
33
34        LAYER
35          NAME "country_bounds"
36          TYPE RASTER
37          STATUS ON
38          CONNECTION "https://demo.mapserver.org/cgi-bin/wms?"
39          CONNECTIONTYPE WMS
40          METADATA
41            "wms_title"                     "World Country Boundaries"  #### REQUIRED
42            "wms_onlineresource"            "https://demo.mapserver.org/cgi-bin/wms?"  #### REQUIRED
43            "wms_srs"                       "EPSG:4326"
44            "wms_name"                      "country_bounds"
45            "wms_server_version"            "1.1.1"
46            "wms_format"                    "image/gif"
47            "wms_dimensionlist"             "time,width"
48            "wms_dimension"                 "time"
49            "wms_dimension_time_unitsymbol" "t"
50            "wms_dimension_time_units"      "ISO8601"
51            "wms_dimension_time_uservalue"  "1310"
52            "wms_dimension_time_default"    "1310"
53            "wms_dimension_time_multiplevalues"  "1310,1410"
54            "wms_dimension_time_nearestvalue"  "0"
55          END
56        END
57
58    END # Map File

Testing Map Context Support

  1. The first thing to do is to save your mapfile using the saveMapContext function available from the PHP/MapScript library. An example script is shown below:

    <?php
  if (!extension_loaded("MapScript")) dl(MODULE);
  $oMap = ms_newMapObj("mapcontext.map");
  $oMap->saveMapContext("mapcontext_output.xml");
?>

  2. Scan the XML output to look for <!– WARNING: … –> comments. Then make the necessary changes to fix every warning that you encounter. At the end of this you should have a mapfile compatible with the Map Context specification.

  3. Now you can load your new Map Context document into an application using the loadMapContext function from the PHP/MapScript library.

Sample Map Context Document

The following is a sample Map Context document:

 1    <?xml version='1.0' encoding="ISO-8859-1" standalone="no" ?>
 2    <ViewContext version="1.1.0" id="mapcontext" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ogc="http://www.opengis.net/ogc" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.opengis.net/context" xmlns:sld="http://www.opengis.net/sld" xsi:schemaLocation="http://www.opengis.net/context http://schemas.opengis.net/context/1.1.0/context.xsd">
 3      <General>
 4        <Window width="400" height="300"/>
 5        <!-- Bounding box corners and spatial reference system -->
 6        <BoundingBox SRS="EPSG:4326" minx="-180.000000" miny="-90.000000" maxx="180.000000" maxy="90.000000"/>
 7        <!-- Title of Context -->
 8        <Title>Map Context demo</Title>
 9        <Abstract>Demo for map context document. Blah blah...</Abstract>
10        <ContactInformation>
11        </ContactInformation>
12      </General>
13      <LayerList>
14        <Layer queryable="0" hidden="0">
15          <Server service="OGC:WMS" version="1.1.1" title="World Country Boundaries">
16            <OnlineResource xlink:type="simple" xlink:href="https://demo.mapserver.org/cgi-bin/wms?"/>
17          </Server>
18          <Name>country_bounds</Name>
19          <Title>World Country Boundaries</Title>
20          <SRS>EPSG:4326</SRS>
21          <FormatList>
22            <Format current="1">image/gif</Format>
23          </FormatList>
24          <DimensionList>
25            <Dimension name="time" units="ISO8601" unitSymbol="t" userValue="1310" default="1310" multipleValues="1310,1410" nearestValue="0" current="1"/>
26          </DimensionList>
27        </Layer>
28      </LayerList>
29    </ViewContext>

Map Context Support Through CGI

MapServer CGI allows you to load a map context through the use of a CONTEXT parameter, and you can point this parameter to a locally stored context file or a context file accessible through a URL. For more information on MapServer CGI see the CGI Reference.

Support for Local Map Context Files

There is a new cgi parameter called CONTEXT that is used to specify a local context file. The user can then use MapServer to request a map using the following syntax:

http://localhost/mapserver.cgi?MODE=map&MAP=/path/to/mapfile.map&CONTEXT=
                     /path/to/contextfile.xml&LAYERS=layer_name1 layers_name2

Nota

All layers created from a context file have their status set to ON. To be able to display layers, the user needs to add the LAYERS argument in the URL.

Support for Context Files Accessed Through a URL

The syntax of using a web accessible context file would be similar to accessing a local context file:

http://localhost/mapserver.cgi?MODE=map&MAP=/path/to/mapfile.map&CONTEXT=
            http://URL/path/to/contextfile.xml&LAYERS=layers_name1 layer_name2

Due to security concerns loading a file from a URL is disabled by default. To enable this functionality, the user needs to set a CONFIG parameter called CGI_CONTEXT_URL in the default mapfile that will allow this functionality. Here is an example of a map file with the CONFIG parameter:

# Start of map file
NAME "map-context"
STATUS ON
SIZE 400 300
EXTENT -2200000 -712631 3072800 3840000
UNITS METERS
IMAGECOLOR 255 255 255
IMAGETYPE png
CONFIG "CGI_CONTEXT_URL" "1"
...
WEB
  ...
END
LAYER
  ...
END

END

Default Mapfile

To smoothly run a MapServer CGI application with a Map Context, the application administrator needs to provide a default mapfile with at least the basic required parameters that will be used with the Context file. This default mapfile can contain as little information as the imagepath and imageurl or contain a list of layers. Information coming from the context (e.g.: layers, width, height, …) would either be appended or will replace values found in the mapfile.

Here is an example of a default map file containing the minimum required parameters:

 1NAME "CGI-CONTEXT-DEMO"
 2STATUS ON
 3SIZE 400 300
 4EXTENT -2200000 -712631 3072800 3840000
 5UNITS METERS
 6IMAGECOLOR 255 255 255
 7IMAGETYPE png
 8#
 9# Start of web interface definition
10#
11WEB
12  MINSCALE 2000000
13  MAXSCALE 50000000
14#
15# On Windows systems, /tmp and /tmp/ms_tmp/ should be created at the root
16# of the drive where the .MAP file resides.
17#
18  IMAGEPATH "/ms4w/tmp/ms_tmp/"
19  IMAGEURL "/ms_tmp/"
20END
21END # Map File

Map Context Support Through WMS

MapServer can also output your WMS layers as a Context document. MapServer extends the WMS standard by adding a request=GetContext operation that allows you to retrieve a context for a WMS-based mapfile with a call like:

http://localhost/mapserver.cgi?map=/path/to/mapfile.map&service=WMS&
request=GetContext&version=1.1.0

The VERSION parameter controls the version of context document to return.

GetContext is disabled by default because it could be considered a security issue: it could publicly expose the URLs of WMS layers used (cascaded) by a mapfile.

To enable it, set the «wms_getcontext_enabled» web metadata to «1» in your WMS server’s mapfile.