Dr. Vladimir E. Gershenzon
This article tells about cartographic and geo-information web-sites offering access to spatial data. It describes the way seamless mosaics of satellite image are used to create online-maps, the role they play in geo-information systems of open access on web resources and during the generation of digital terrain models. It will also inform the reader about the capabilities of creating such models that contain space imagery archives and operational data reception tools.
Geo-information services and interactive online-maps
In the modern world, the ways of geographic information presentation, browsing, processing and visualization have changed along with the development of information systems and database operation methods. Such final maps have higher value if generated based on updated and appropriate geo-information data in response to user requests. Therefore cartographic and geo-information online-services start to serve an increasingly important role in meeting the demands on geographic information. In addition to interface advantages of an interactive map, the possibilities of such web-services are in dynamic data collection based on the databases and on geo-located information search results.
To understand what geo-information web-service is, it’s worth describing the functional requirements that such a web-service should meet, otherwise all sites with maps offering access to their data via Internet, may be regarded as web-services. The integral part of a web-service is the application program interface (API) enabling the client application to communicate with the web-service. If the information system is based on the client/server architecture, the web-service receiving data through the database management system is a tool to integrate different parts into server applications that send final replies to client application on the user computer – for example, drawing the selection of vector objects in the browser or spooling metadata from the space imagery archive. Basically, web-services enable to create a distributed data access system, which components are interacting under a unified standard and exchange protocol.
Web-resources pretending to be a cartographic web-service and having key elements for geo-information data integration and display based on maps, are named geoportals.
For example, Google Maps – is a geoportal.
GIS developers and geo-information services in Internet
Techniques, offered by such GIS software developers as ESRI, also enable to create geoportals providing for the geographic information search and for its display on the map, being drawn “on-the-fly”. For example, such a technique was used to create a US geoportal ). When an efficient geodata storage method was found, GIS software products became a tool for the creation of a centralized multi-user system, where many users can operate in one information space simultaneously. In this way, GIS moved to the category of client/server information systems.
Does this look like a geoportal? On the one hand – it does and the use of such GIS capable of negotiating with the browser and designed for package solutions is very convenient from the standpoint of dispersed data integration and of multi-user support. For example, for major corporations such GIS is a well-promoted and supported software product having certain apparent advantages. On the other hand, the web-interface of such a system, built on patterns and frameworks, will never be associated with the original web-project for most Internet users. Why? Because it will not be such even for the project developers, since the Internet-GIS model is built on the software GIS model, which is originally is desktop application for specialists having a required set of functions. And it is not easy to breathe life into such a model and to customize it to the standards of Internet-startups. Here the interface developers face the problem: desktop look & feel or web look & feel – two approaches that are very similar in contemporary web-sites. Technologies are being integrated and nowadays the sites have to comprise more and more functions that formerly remained beyond the application.
To grasp the different in these two approaches when building the model, please recall the Google Maps site that gradually takes over functional possibilities of Google Earth as the application similar to GIS. Developing Google Maps API, i.e. building the interaction chain between the add-on projects (Mash-ups ? Mapplets) inside its own geosearch space, this project becomes a cartographic web-service that was built on the initial model of web-site with interactive online-map.
The French geoportal (https://www.geoportail.fr) is arranged as a social geo-information resource as well. Even if it is inferior to the US geoportal in geo-information search possibilities, visually it looks much better. Mostly this is thanks to an interactive map based on satellite imagery mosaics of France created on this geoportal. On the other hand, why do the US need a geoportal if they have Google Maps and a competition with Miscrosoft Virtual Earth? Actually, the Internet environment is important for the development of GIS software products as the area of application of distributed systems and of searching for standardized approach in the data models, exchange formats and most frequently used GIS functions.
However, the search for forms of cartographic information presentation in the web for the maximum possible range of the users is executed de facto by Internet-portals starting to create web-services with interactive maps, such as Google, for example. At this stage of geoweb the GIS developers unite in consortiums that bring formats to standards. Open Geospatial Consortium is a good example to illustrate how the potential for the unified search area is created, where all geo-coding information, submitted in available format, can be found, transmitted and displayed in the unified coordinate system. KML language, developed in Google Maps API, becomes one of the standards as well.
Unlike GIS products, web-services on Internet-portals have a general geographic basis, a geospatial model that uses satellite imagery mosaics displaying all additional geographical information.
Satellite imagery mosaics
The advantage of such a single spatial model is in allowing the user to have a most convenient geo-information search interface and a most handy tool for search results visualization. Besides, such a spectacular interface serves to attract new Internet users who are potential customers of geo-spatial information.
Satellite images turn out to be a useful, let alone spectacular, background for online-maps, as they may totally substitute cartographic content on many scale levels. On smaller scales of such basic maps, the topographic map layers are used mostly to outline the borders, settlements and highways in a “hybrid” mode. All the rest (rivers, lakes, forests, fields and sometimes hill shading) is clearly seen on the satellite images of middle resolution (5-6 m). Toponymical layers and addresses of houses are added on more detailed scales. A more detailed imagery (1-2m) can be used for mosaicking of the objects of town infrastructure.
Space imagery archives, which data enable to stitch mosaics and to create complete coverage to large territories, are used at the first stage of such mosaics preparations. A system of operational satellite data collection (reception complexes and automatic data processing systems) is applied to present most updated information. All this has a lot to do with cartographic web-services or geoportals, because such space imagery data have to be archived, catalogued and used as geographic data with wider access to their metadata.
Such an access can be provided by web-service used to perform the data search in metadata catalogs (imaging time, clouds conditions, etc.). After that the web-service generates an xml-reply, containing object contours, besides metadata, which is a telling example for visual search on the map. A hyperlink to the image preview can be incorporated in this reply. A single mosaic in this case can be used as the geographic basis for the integration of different data, i.e. in combination with the API platform and a set of mandatory and optional geo-information user services, as the basic tool of handling geoportal functions.
Therefore, the functions of interactive image preview, metadata search, preview of detected geographical data and imaging data display in hybrid mode, can be applied inside a single web-service. Same concerns automatic data ordering functions. Online-services of receiving, processing and forwarding the orders are also a part of automatic data ordering procedure. Fore example, online purchase of GPS-maps or space images.
It’s worth outlining the following as a summary of the above essay:
First – geoportals, meant both for mass users and for specialists, should be created based on a single spatial pattern. Seamless mosaics of satellite-based images is the most up-to-date and colorful option.
Second – a web-interface of a geoportal designed for Internet users should be created based on the web-site model with an interactive map (not a GIS-application), gradually adding new functions in it. A geoportal web-interface should have user-friendly tools for visualization and quick search for geodata on a territory.
Third – basic criteria of geo-information data search: data relevancy for the search query.
Fourth – a geoportal web-service should provide for the interaction of different systems, applied to geodata: data publications, search, processing, display on the geographic basis and automatic ordering.
Fifth – the geo-information data search limits in the geoportal should be as much extended as possible and data formats standardized and open in order to become popular among the geo-spatial data users. It’s worth narrowing down the search criteria by data types, increasing its quality by creating special data infrastructures: space imagery data or navigation or topographic maps data. However, the search criteria by manufacturing company should never be narrowed.
In addition to the abovementioned and popular online-maps web-services this article presents the new fledgling Russian web-site – Kosmosnimki ((https://new.kosmosnimki.ru), which is one of the ScanEx R&D Center projects, as an example of creating a single satellite imaging mosaic of the Russian territory.
Besides the Kosmosnimki site, the mosaics of Moscow and Moscow Region, prepared by ScanEx R&D Center, have been used in the map project of the Russian Yandex search engine (https://maps.yandex.ru/moscow_sputnik).