publishingplanetarymapsusingopenstandardsandopensourcesoftware · mapserver [5] is a popular open...

Publishing Planetary Maps using Open Standards and Open Source SoftwareS. H. G. Walter, G. Neukum, Freie Universitaet Berlin, Institute for Geological Sciences – Planetary Sciences andRemote Sensing, Malteserstr. 74-100, 12249 Berlin, Germany (sebastian.walter@fu-berlin.de).

Introduction

Map Servers based on standards defined by the OpenGeospatial Consortium (OGC) [1] are very common toserve planetary data sets over the internet. We willdemonstrate techniques to deliver HRSC and SRC imagedata as well as OMEGA footprint data based on OGC’sWeb Services (OWS). As many applications on the mar-ket are equipped with interfaces to these services, plentyof tools are already available that can be applied to thedata. By using open source software, we can focus ourefforts on providing data instead focussing on developingand maintaining software.

Motivation

Several existing services defined by the OGC are avail-able for planetary data delivery. Here we will examplar-ily focus on the following services which are well suitedfor a prototype HRSC/OMEGA data delivery and appli-cation system.

The Web Feature Service (WFS) defines an interfacefor specifying requests for retrieving geographic featuresas vector data across the web. We will be evaluating thisservice for delivering footprints (geometric outlines ofimage data projected on the ground) of HRSC, SRC andOMEGA data combined with additional metadata.

The Web Map Service (WMS) interface standardprovides a simple HTTP interface for requesting geo-registered map images from geospatial databases. AWMS request defines the geographic layer(s) and areaof interest to be processed. The response to the requestis one or more geo-registered map images (returned asJPEG, PNG, etc) that can be displayed in a browser ap-plication. The interface also supports the ability to spec-ify whether the returned images should be transparent (sothat layers from multiple servers can be combined) ornot. This service is evaluated for distributing the HRSCand SRC image data themselves.

The Web Coverage Service (WCS) standard definesinterface and operation that enable interoperable accessto geospatial coverages, such as satellite images, digitalelevation data and other gridded datasets. Because thecoverages are delivered together with detailed metadataand their original semantics, WCS delivered data is in-tended for further processing of the gridded datasets.

OGC Compliant Footprint Database

As the amount of HRSC/SRC image data is rising dur-ing the further extension of the mission, there is a fun-damental need for selecting, visualising and overlayingthe ground footprints of the images. Speed, handlingand extended capabilities are the reasons for using geo-databases to store and access these data types. Tech-niques for such a spatial database of image metadataare demonstrated using the Relational Database Manage-ment System (RDBMS) PostgreSQL [2], spatially en-abled by the PostGIS extension [3]. PostGIS followsthe OGC’s OpenGIS Simple Features Interface Stan-dard (SFS) as a well-defined and common way for ap-plications to store and access feature data in relationalor object-relational databases, so that the data can beused to support other applications through a commonfeature model, data store and information access inter-face. OpenGIS Simple Features are geospatial featuresdescribed using vector data elements such as points, linesand polygons.

As an example, footprints of the HRSC/SRC and theOMEGA instruments are generated and connected to at-tribute information such as orbit name, ground- and im-age resolution, solar constellation and illumination con-ditions. The footprint and label of HRSC/SRC sequencesare read out by the VICAR RTL developed at JPL, addi-tional meta information is calculated using SPICE soft-ware routines [4]. This process is included in the au-tomatic processing chain of HRSC images, so the foot-print database is always up to date. OMEGA footprintsand their attributes are generated using the IDL softwareprovided by the OMEGA team. The data are then trans-ferred into the database by PostgreSQL’s libq C libraryusing the Well-Known Text (WKT) notation for the geom-etry. PostgreSQL’s advanced features such as geometrytypes, rules, operators and functions allow complex spa-tial queries and on-the-fly processing of data on DBMSlevel e. g. generalisation of the outlines.

Global Mosaics of Image Data

As a very simple approach for the creation of a globalHRSC mosaic, preliminary map projected sequences ofHRSC and SRC images are queried from the footprintdatabase for each colour channel, are brought to a com-mon mapscale (here exemplary 200 m per pixel) and are

combined to mosaics by use of VICAR software routinesdeveloped at JPL and DLR. Without any further correc-tion, strong seams are visible due to different illumina-tion conditions of the images.

For the reason of speed and realtime availability, theplanet’s global surface is divided in to a limited num-ber of tiles connected by a geometric index. These tilesare referenced on the planetary body using world files tomake them accessible by spatially enabled applications.Additionally, internal and external overviews are calcu-lated, resulting in better display speeds for low resolution(global scale) as well as high resolution (image scale)map requests.

MapServer [5] is a popular open source projectwhose purpose is to display dynamic spatial mapsover the Internet. It connects directly to the Post-greSQL/PostGIS footprint database and serves the dataas WFS. To speed up drawing performance of the re-quest call, a spatial (GiST) index is built for the featuretable. For layer queries, an additional object id (oid) in-dex should be established on the table.

Image data of HRSC and SRC are served as WMS.MapServer enables the creation of a network of MapServers from which clients can build customised mapscontaining e. g. MOC, THEMIS and MOLA imagery(these data sets are already available as WMS). Defi-nition of the Spatial Reference System (SRS) is madeby the use of OGP Surveying and Positioning Commit-tee’s EPSG projection codes [6]. For extended supportof planetary reference systems, the implementation ofOGC’s Well Known Text (WKT) notation of the coordi-nate system would be favourable. MS RFC 37 describesa mechanism to establish more flexible SRS definitionsthan the actual capabilities of MapServer [7].

As connectors to WMS-compliant mapping applica-tions such as NASA World Wind or Google EarthTM arefreely available, the data is instantly available in well-funded 3D interpretation environments.

Outlook

The methods presented here are well suited for con-structing OGC Web Services without the additionaleffort to develop and maintain software. A sim-ple HRSC/OMEGA prototype application demonstrat-ing footprint representation and orbit selection based onthe open source CartoWeb framework [8] has been im-plemented (see figure 1). Getting updated during theHRSC processing chain, the footprint database is al-

ways up to date. For an officially released WMS ver-sion of the HRSC/SRC images, the resolution should beadjusted to the best possible image resolution. For thisto be achieved, the automated image mosaicking processmust be improved and further performance considera-tions have to be made. WMS-served elevation informa-tion will lead to promising 3-D viewing applications ofplanetary data.

Figure 1: A prototype mapserver application based onthe CartoWeb framework. The view shows footprint out-lines on top of a mapping of Martian valley networks.

References

[1] The Open Geospatial Consortium (OGC):http://www.opengeospatial.org

[2] PostgreSQL open source object-relational databasesystem: http://www.postgresql.org

[3] PostGIS extension for PostgreSQL:http://postgis.refractions.net/

[4] The NASA Planetary Science Division’s AncillaryInformation System (SPICE):http://naif.jpl.nasa.gov/naif/

[5] MapServer: http://mapserver.org/index.html[6] OGP Surveying and Positioning Commitee:

http://www.epsg.org[7] MapServer Request for Comments No. 37:

http://mapserver.org/development/rfc/ms-rfc-37[8] CartoWeb Web-GIS and framework:

http://www.cartoweb.org