Web Services Publishing¶
Best Practice Guide for Federal Geospatial Data Managers¶
I. Introduction¶
This guide introduces the importance of geospatial web services, emphasizing their role in enabling easy access, sharing, and integration of spatial data for efficient analysis and decisionmaking among GIS users. The guide aims to assist federal geospatial data managers in understanding and implementing best practices, covering topics such as the benefits of web services, geospatial metadata standards, choosing the right web service types, preparing geospatial data, testing and validation, and ongoing maintenance for accuracy and relevance. Spatial data refers to information that is associated with specific geographic locations or positions on the Earth’s surface. Geographic information expands spatial data to include attributes and additional details that are associated with the spatial features. Geospatial web services are essential for GIS users as they enable easy access, sharing, and integration of spatial data across different applications, allowing for efficient analysis and decision-making. These services enhance collaboration by providing a standardized framework for accessing geographic information and fostering interoperability among different GIS platforms and tools.
II. Understanding Web Services¶
A web service is a standardized method created to facilitate communication and data exchange between various applications or systems over the Internet. A geospatial web service is a type of web service that provides access to and facilitates the exchange of geographic information, allowing applications to retrieve and display spatial data over the internet.
Key Components of Geospatial Web Services:¶
Data Repository / Source: The geospatial data that the web service provides or manages, which can include vector data (points, lines, polygons), raster data (imagery), or other types of geospatial information.
Metadata: Information about the geospatial data, such as its source, format, projection, and other relevant details. Metadata helps users understand and effectively use the geospatial information provided by the service.
Server: The server is responsible for hosting and managing the geospatial web service. It processes client requests, retrieves data from the data repository, and sends the appropriate responses.
Service Interface: An interface outlines a series of requests to be made to an object, such as a server that hosts geospatial web services, in order to carry out a specific task, like a transformation or query.
Common Standards for Geospatial Web Services¶
Geospatial data standards are essential as they establish a uniform framework for collecting, storing, and sharing geographic information. To learn more, please see A Guide to the Role of Standards in Geospatial Information Management.
The Open Geospatial Consortium provides standards for the following types of Geospatial Web Services:
Each service serves a specific purpose and will be discussed in more detail later in the guide. There are additional standard groups such as the International Organization for Standardization (ISO). ISO has published ISO 19128:2005 Geographic information Web map server interface, which was the product of OCG and ISO collaboration and was built on the OGC WMS and WMTS standard specifications. ISO also produces many geospatial metadata standards, some of which will be discussed later in this guide.
Please see the following list of additional standards offered by government agencies and interagency committees:
III. Benefits of Using Web Services for Geospatial Data¶
Web services provide a standardized way to access geospatial data over the Internet. Users can access data from anywhere, reducing the need for physical storage and facilitating remote collaboration. Additionally, the standardized format promotes interoperability among different software applications and systems. This enables seamless integration of geospatial data into various platforms and applications.
Web services facilitate collaboration by allowing multiple users to access and work with geospatial data concurrently. This makes it easier to query and request specific data and perform analyses, promoting collaborative efforts in a common spatial framework.
IV. Geospatial Metadata Standards¶
The Geospatial Data Act of 2018 (GDA) requires covered agencies to provide metadata to GeoPlatform. The metadata must be provided through a geospatial metadata standard. A geospatial metadata standard is a set of guidelines and specifications that define how information about geospatial data should be documented and organized.
Examples of geospatial metadata standards include ISO 19115 (International Organization for Standardization) and the Federal Geographic Data Committee (FGDC) Content Standard for Digital Geospatial Metadata (CSDGM) in the United States. ISO geospatial metadata standards and CSDGM use eXtensible Markup Language (XML) to organize the metadata content into an interoperable metadata record that can be read by: geographic information systems, data catalogs, web maps, and other online services. Both standards are endorsed by FGDC.
CSDGM Overview¶
CSDGM is a widely used metadata standard developed by the U.S. Federal Geographic Data Committee (FGDC) for documenting geospatial data. Although CSDGM is widely used, the standard is somewhat outdated and does not provide elements for unique identifiers or web services.
Table 1. CSDGM Benefits and Drawbacks.
| CSDGM Benefits | CSDGM Drawbacks |
|---|---|
| ● Familiar to most metadata creators. ● Available editor and validation tools. ● User-friendly CSDGM Workbook supports implementation. ● Profiles support theme-specific vocabularies and features. ● Can be published via Data.gov and GeoPlatform. | ● Includes a large number of mandatory elements. ● Does not fully support current technologies and capabilities. ● Provides no elements for unique identifiers or web services. ● Conflicts with GDA requirements. |
ISO Suite Overview¶
The ISO 191** suite is a collection of geospatial metadata standards. ISO 19115 and the 2014 update, ISO 19115-1, serve as the core of the suite of standards. The original ISO 19115 standard was based on CSDGM. Additional standards extend the fundamental standard. The suite enables users to combine standards as needed, based on the data type, content, use, and data management strategy of the organization.
The ISO geospatial metadata standards contain both content standards and implementation standards. The two are usually paired together. The content standard specifies the information to be documented and the implementation standard specifies the format of the content in the metadata record.
Table 2. Corresponding ISO content and implementation standards.
| ISO Content and Description Standard Name | ISO Number | Corresponding XML Implementation Schema Standard | ISO Number |
|---|---|---|---|
| Geographic Information – Metadata | 19115 | Geographic information -- Metadata -- XML schema implementation | 19139 |
| Geographic Information – Metadata – Part 1: Fundamentals | 19115-1 | Geographic information -- Metadata -- Part 3: XML schema implementation of metadata fundamentals | 19115-3 |
| Geographic Information – Metadata – Extensions for Imagery and Gridded Data | 19115-2 | Geographic information — Metadata — XML schema implementation — Part 2: Extensions for imagery and gridded data | 19139-2 |
| Geographic Information – Methodology for Feature Cataloging | 19110 | XML encoding included as Annex E | XML encoding included as Annex E |
| Geographic Information – Data Quality | 19157 | Geographic information -- Data quality -- Part 2: XML Schema Implementation | 19157-2 |
ISO metadata contains code lists that standardize metadata content by providing a controlled vocabulary, ensuring consistency and interoperability across different systems and organizations. Topic categories are one of the few mandatory ISO elements and are used by catalogs and other data management systems to discover and sort data in a standardized manner.
Additionally, ISO geospatial metadata standards enable the use of identifiers to direct readers to existing content that is relevant to the resource being documented. Identifiers are universal labels that can be used by people and machines to ensure that they are referring to the same thing and can be used to locate those things on the web.
Table 3. ISO Suite Benefits and Drawbacks.
| ISO Suite Benefits | ISO Suite Drawbacks |
|---|---|
| ● Robust, flexible, and current with regard to technology (services, data models, data acquisition). ● The modular nature supports interoperability. ● Fewer mandatory elements because the applicability of the elements varies so widely. As a result, ISO geospatial metadata standards enable users to go into greater details where needed and less, or no, detail where not relevant. ● Utilizes standardized content such as code lists, topic categories, and unique identifiers. ● Can be published via Data.gov and GeoPlatform. | ● Official ISO standards must be purchased. ● ISO standards require a technical understanding of UML diagrams and XML notations. ● Limited options for non-proprietary metadata editors and validation tools. |
Selecting a Geospatial Metadata Standard¶
In addition to the GDA requiring the agencies to provide metadata to GeoPlatform, the GDA also specifies that the chosen metadata standard should be consistent with international standards. Given this specification, users should create metadata using the ISO metadata standards where appropriate.
V. Choosing the Right Web Service Type¶
Choosing the right type of geospatial web service is important because different services cater to specific data types and use cases. Please see the following service types and their associated used cases:
Web Map Service (WMS) for Map Rendering: A Web Map Service (WMS) is a standard protocol for serving georeferenced map images over the internet. It allows clients to request maps and spatial data from a server in the form of images, which can then be displayed in a web browser or other mapping applications. Since WMS provides images, it can be bandwidth-efficient compared to fetching raw vector data. The client receives only the map images needed for visualization. The WMS standard is defined by the Open Geospatial Consortium (OGC).
Web Feature Service (WFS) for Feature-Level Access: WFS allows clients to request and receive geographic features and their attributes in a standardized format. This is in contrast to Web Map Service (WMS), which serves pre-rendered map images. WFS supports spatial and attribute-based queries, allowing clients to request specific subsets of data based on criteria such as location or attribute values. If granted authorization, the WFS has the capability to update or delete geographic features.
Web Coverage Service (WCS) for Coverage Data: A Web Coverage Service (WCS) provides access to multi-dimensional coverage data via the Internet. Coverage datasets can include satellite imagery, remote sensing data, and digital elevation models.
Web Processing Service (WPS) for Geospatial Processing: Web Processing Service (WPS) is an Open Geospatial Consortium (OGC) standard protocol facilitating geospatial processes over the Internet. The standard establishes guidelines for the format of inputs and outputs (requests and responses) in geospatial processing services.
Table 4. Geospatial Web Service Summary Table.
| Service Type | Data Type | Data Format Example | Access Type |
|---|---|---|---|
| WFS | Vector | Shapefile, File Geodatabase Feature Class | Returns geospatial features. |
| WMS | Vector and Raster | Vector: Shapefile, File Geodatabase Feature Class Raster: GeoTIFF, Esri Grid | Returns static maps (rendered as pictures by the server). |
| WCS | Raster/Image | GeoTIFF, Esri Grid | Returns coverage datasets, with access to attributes. Can be used in processing tools. |
VI. Preparing Geospatial Data for Web Services¶
Data Preparation, Quality, and Accuracy¶
If you are publishing a service with multiple layers, arrange the drawing order so that the largest polygon layers are towards the bottom and the smallest point, line layers are towards the top. The layer’s symbology should be customized so that it enables the effective communication of spatial information. Attribute fields should have descriptive names assigned as their aliases to ensure clarity. Additionally, all links stored within attribute fields should be tested. The data manager should validate that the data being used is the approved source that meets the requirement requested by the data/project sponsor.
Standardized Metadata¶
Comprehensive metadata enhances the usability, interoperability, and reliability of the data product, allowing users to make informed decisions when accessing and utilizing spatial data. Metadata creation and maintenance should follow FGDC Technical Guidance. ArcGIS Online and ArcGIS Pro are each equipped with a metadata editor that can be used to populate the required information outlined in the FGDC Technical Guidance. mdEditor is a free, open-source metadata editor that can also be used to create FGDC-compliant metadata. mdEditor and ArcGIS Online/ArcGIS Pro store metadata using their internal formats. Additionally, both platforms provide translation capabilities from their internal formats to either FGDC CSDGM or one of the ISO standards (19115, 19115-1, etc.).
Coordinate Reference Systems (CRS) Considerations¶
Service should support the proper WGS 84 Web Mercator or Web Mercator Auxiliary Sphere projection. Please see the National Map Projection article from USGS to learn more.
VII. Testing, Validation, and Maintenance¶
Services should be evaluated to determine how the web service performs in diverse situations, considering different scales, simultaneous requests, and various data complexities. Confirm that the response time aligns with acceptable standards. When applicable, geospatial web services should utilize caching strategies like tile caching, where pre-rendered map tiles are stored and quickly delivered to users, resulting in faster map rendering. The published geospatial web services should be tested across diverse geographic information systems and platforms. The service should also be tested outside of the home agency network. Check the attributes associated with spatial features for accuracy, completeness, and consistency. Ensure that the geospatial data consistently uses the correct coordinate system. Confirm that the geospatial data is suitable for the intended scale of visualization and analysis. Data managers should set up a regular schedule for data updates and maintenance to keep geospatial web services accurate and current. Effective communication channels are needed to notify users about upcoming changes, modifications, or disruptions to service functionalities.
VIII. Conclusion¶
Federal geospatial data managers can leverage geospatial web services to provide easy access, sharing, and integration of spatial data. Key practices involve adhering to geospatial metadata standards, selecting appropriate web service types, preparing geospatial data, and ensuring ongoing maintenance for accuracy. By implementing these best practices, federal geospatial data managers can play a pivotal role in fostering collaboration within the geospatial community, enabling concurrent access and collaboration among multiple users, and promoting seamless integration across different GIS platforms and tools.
