Technical Tips
Implementing Open GIS Web Mapping into ArcGIS
by Ben Johnson, Senior GIS Consultant |
What is WFS?
What is WMS?
ArcGIS Support for
WMS and WFS
Adding WMS data to
ArcGIS
Map Service properties
Results of an Identify
Request
Finding Data
URLs to Australian
Examples
Sample Layer Files
Legends
Further Information
What is WFS?
The Web Feature Service Interface Specification (WFS) provides
a common XML interface to data stored in different spatial
formats. It provides a standard for communicating geospatial
data at the feature level through Geographic Markup Language
(GML). Through feature streaming, WFS clients can support
inserts, deletes and updates of spatial data from multiple
data sources.
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What is WMS?
Web Map Server (WMS) is an OpenGIS Consortium specification
for an online service that provides maps in one of a number
of standard image formats, including .jpg, .png, .gif and
.tiff. WMS provides a standard for the provision of transparent
map image data, allowing data from many different sources
to be overlayed and delivered to client programs as an integrated
map image. WMS specifies two primary capabilities: GetCapabilities
which allows for the exposure of metadata about a specific
map and GetMap which allows for the delivery of a map image.
In addition, an optional capability, GetFeatureInfo, can
be made available by the WMS service provider to allow for
the delivery of feature information via the web. The provision
of coordinate information as GML is not a part of WMS. Exposing
the geometry of the underlying data through GML can be an
issue when data is subject to stringent license constraints.
For this reason, most Australian government agencies are more
likely to provide WMS services only for the provision of GIS
data.
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ArcGIS Support for WMS and WFS
At ArcGIS 8.3, Map Server provided support for WMS and WFS
through the ArcGIS OGC Interoperability Add-On 1.0. This allowed
users to effectively save GML data exposed through WFS as
a shape file or geodatabase feature class.
At ArcGIS 9.0 and above, this add-on is no longer available,
however native support for WMS map services is now provided
for ArcView users out-of-the box. (SP2 is required)
For access to WFS services, users are now required to purchase
the ArcGIS Data Interoperability extension.
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Adding WMS data to ArcGIS
In ArcCatalog you can now load three different types of GIS
Server; ArcGIS Server, ArcIMS Server and WMS Server.
Figure
1
The following example accesses the URL for the GA WMS service
which contains approximately 170 datasets. This WMS map service
provides access to a range of datasets that are freely downloadable
from GA’s
website.
- To add a WMS Server, expand the GIS Servers dialog in
the left hand side of the Catalog View and select ‘Add
WMS Server’. This will activate the following dialog
box. You will need to know the URL of the WMS Service that
you are trying to access.
- Enter the published Geoscience Australia URL into the
URL: dialog box. http://www.ga.gov.au/bin/getmap.pl?dataset=national&
- Click on the GetLayers button. This will perform a ‘getCapabilties’
query on the WMS service and return metadata about the layers
available in the WMS service, a title and an abstract, if
available. The level of metadata detail will be dependent
on the attributes that have been documented by the service
provider.
- Click on OK to save your WMS service connection.
- As with any layer in ArcCatalog, once it is loaded you
can minimize your map window and drag the WMS service over
into your map window in ArcMap.
Figure
2
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Map Service properties
Projections
WMS services can be published in a range of projections as
specified by the WMS service provider. In the XML file returned
by a GetCapabilities statement, a range of EPSG projections
will be listed within an SRS tag.
Figure
3
The European Petroleum Survey Group (EPSG), now the OGP,
publishes and maintains a common set of known projections
that are known as EPSG codes. ArcMap recognise these codes
and can construct a request to the WMS service in the SRS
of the data frame, provided it is supported by the WMS service.
When you load up a map service into an ArcMap document that
is not in the projection of your data frame, you will receive
the standard ‘ArcMap does not support this projection’
message. To resolve this problem:
- Right-click on the provider properties button.
- Select the Change Coordinate System Button.
- Choose from one of the available projections in the drop-down
list box. ArcMap has interpreted these based on the EPSG
projections made available by GA in the SRS tag above.
- Your data will be projected into the correct projection.
Figure
4
Performance
The GA website currently loads 173 datasets into your view,
although not all of these datasets are turned on by default.
This provides a significant overhead at the server level,
just in terms of looping through each of the datasets and
determining whether they fall within current display scale
ranges. This makes this WMS service virtually unusable in
ArcMap with all the layers turned on.
If you are interested in seeing only a subset of datasets,
you can control the layers that are made available through
the WMS layers properties window and increase your performance.
- Right-click on the WMS Service and select properties:
Figure
5
- Select the Layers Tab.
- Click on the ‘Just Layers listed below” radio
button.
- Select individual layers that you want to display and
click on the greater than symbol to make them active.
- Select OK. Only these layers will now be visible within
your map frame.
Figure
6
The other tab available via the Properties window is the
‘Save As Layer File’ button. If you are happy
with the grouping of layers, you can save them as a layer
file and provide this other staff members within your organisation
or email it to a client.
The advantage of the layer file is that only stores the path
to the data as a URL so it can be used by anyone within your
organisation and beyond, provided they have ArcGIS version
9.0 SP2 and a connection to the internet, preferably a broadband
connection.
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Results of an Identify Request
ArcMap users can query information provided through a WMS
service, provided its attributes have been exposed by the
WMS map service provider. A WMS query on a feature returns
its results as HTML. This means that ArcMap cannot present
this information in the standard way that it does for other
attribute queries. Context menu options such as zoom-to are
also not available. Of the Australian services I have checked,
the only service that allows identifies is the GA site.
Figure
7
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Finding Data
The WMS standard does not support search capabilities. Therefore
these options will not be available for ArcMap users. Users
will need to have searchable layers such as towns loaded as
feature layers (shapefiles or Personal geodatabase etc. )
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URLs to Australian Examples
Provider |
Geoscience Australia |
Description |
Provides access to the full 173 datasets
currently made available freely for download from the
GA website. |
URL |
|
Notes |
Can identify features. Supports a
full range of Australian projections. |
Provider |
Bureau
of Meteorology - BOM |
Description |
Spatially enabled live weather information
service under development by the Australian Bureau
of Meterorology. |
URL |
|
Notes |
Not a guaranteed service. |
Provider |
Department of Environment
and Heritage |
Description |
Australian Natural Resource Atlas
(ANRA). Provides a very good range of environmental
datasets. |
URL |
|
Notes |
Can not query. Available in a range
of projections with some good environmental data. |
Provider |
WALIS |
Description |
Western Australian Atlas – Provides geology polygons. |
URL |
|
Notes |
Can
not query and only available in GCS_WGS84. |
Provider |
MapShed |
Description |
Provides good satellite imagery visible
to 1:60,000 for promotional purposes and good topographic
data including street directory style roads and public
land. |
URL |
|
Notes |
Available in all MGA projections,
GDA94 and WGS84 so it should work for most Australian
users. Not Queryable. There is a background image included
with this by default meaning that it can’t be
used transparently with other WMS based layers or image
datasets. |
Provider |
ER Mapper |
Description |
Provides a great WMS service showcasing
the power of Image Web Server. A number of detailed
images are available. A sample layer file for a 1:1,000,000
map image and a photo mosaic of the Otways region are
provided in MGA54. |
URL |
|
Notes |
There are many images published here
in different projections, based on the projection they
were geo-referenced to. You will have to select an image
and change to its published projection in order to view
it. |
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Sample Layer Files
Attached are some sample layer files based on the WMS services
outlined above. You should be able to download these and load
them into ArcMap directly provided you are running ArcGIS
9.0 (SP2) or later. Note that if the projection of your ArcMap
data frame is not in one supported by the WMS service you
will have to change the coordinate system of the data frame
by right-clicking on the WMS layer and selecting ‘Change
Coordinate System’. No message comes up automatically
by default.
Layer File |
Description |
|
A subset of petroleum layers from the GA website. |
|
This
1:1000,000,000 topographic image is delivered using
Image Web Server from ER MapperIWS. |
|
If
you were ever skeptical about the potential of the
web to deliver detailed imagery, try this service
from ER Mapper. You will need to change to MGA54 to
view this photo mosaic of the Otways region. |
**Please note: To download layer
files, right-hand mouse click on icon and click on 'Save
Target As...'
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Legends
For some WMS layers you will notice that you can get legend
information by clicking on the left hand side of the image.
WMS supports a request called GetLegend which requires a height
and width parameter if the service provider has made this
option available. The height and width will be a property
of the number of legend classes within each WMS layer. ArcMap
has no way of knowing the number of classes before it makes
this request and therefore the legend is often returned in
an illegible squashed box. This problems may be resolvable
at later versions of the WMS standard or ArcMap.
Squashed legends will also be a problem in layout view. When
adding these legends to a layout using the legend wizard,
you will have to be wary of this problem. You can adjust the
patch size for a particular legend item to accommodate a large
legend but invariably you will get poor results. One alternative
may be to directly query the WMS service and iteratively adjust
the height and width parameters you pass. These results can
then be saved as an image that can be included on your map.
The following request will return the legend image for roads
from the GA site in your browser window:
http://www.ga.gov.au/
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Further Information
Issues
with working with WMS layers with ArcGIS 9.0
For further information about how Spatial Vision may assist
you, please contact:
Ben Johnson on (03) 9691 3000
email: ben.johnson@spatialvision.com.au
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Copyright © Spatial Vision, Monday, 23-January-2006 |
