Development of a Prototype Web GIS Server for HDF-EOS Data based on OGC

Web Map Specifications

Liping Di, Zhangshi Yin, Wenli Yang, Meixia Deng, and R. SureshNASA/RITSS, 4500 Forbes Blvd, Lanham, MD 20706

lpd@rattler.gsfc.nasa.gov

Ken McDonaldNASA Goddard Space Flight Center, Code 423

Greenbelt, MD 20771, USA

Introduction• HDF-EOS is the standard data format for

NASA EOSDIS. Huge amount of remote sensing data will be produced in this format.

• Make HDF-EOS data easily accessible to GIS users will greatly increase the utilization of EOSDIS data.

• There are three data models in HDF-EOS– Grid--Georectified raster data

– Swath--potential georeferable raster data

– Point-- point measurement

Open GIS Consortium(OGC) • A membership organization consisting of 220

members from GIS industries, government, and universities.

• Promotion of the interoperability of GIS systems, data, and services in the network environment.

• The main work of OGC is to develop interface specifications for GIS interoperability though– Regular standard development procedure

– Testbed activities

Web Mapping Testbed(WMT)• Phase I (WMT I):

– Finished in September 1999 and produced:• Web Mapping Specification Version 1.0

• A set of prototype interoperable clients, middleware, and servers developed by participating members.

• Phase II (WMT II)– Started in July 2000, and will finish by the end of

this year.– Extensive set of new specifications (details later)– We are one of the OGC-paid participants

OGC Web Mapping Specification V1.0

• Define web interfaces for interoperability of map assembly and delivery middleware within a heterogeneous distributed computing environment over Internet.

• Three interfaces for Web Mapping– GetMap

– GetCapbilities

– GetFeatureInfo

• XML DTD for the capabilities description• Tested in Phase I WMT.

WMT-2• Web Map Server: provide interface for accepting multi-

dimensional query requests and responding with symbolized graphics

• Portrayal: provide interface for associating symbols with geographic feature.

• General service model: provide interface for representing service metadata, service capabilities, and service interfaces using XML.

• Web Feature Server, Filter, ID: Provide interface for accepting spatially enabled SQL-like query request and responding with GML feature collections, include spatially enabled filter grammar.

WMT-2, Cont’d• Web Coverage Server: Provide interface for accepting multi-

dimensional query requests and responding with real, gridded data (OGC coverages)

• Geography Markup Language (GML): Provide XML encoding for geographic features, including XML schema definition to structure encodings and temporal extensions.

• Imagery and Coverage Markup Language: Provide XML encoding for images, gridded data, and textual and graphical annotation on extensions.

• Security and E-Commerce: Demonstrate application of commercial security technologies.

• OGC catalog server: test the catalog specification in WMT 2.

Objectives• To develop a NASA Web GIS server

(NWGISS) that serves HDF-EOS data to GIS clients according to OGC Web Mapping Specifications.– OGC Map Server, coverage server, and catalog

server

• The server should be able to serve data in all three types of HDF-EOS data files.

• Easy-to-use and easy-to-setup.

OGC Web Mapping Interfaces To Be Implemented in NWGISS

• Meet WMS version 1.0 for all HDF-EOS 3 data models in the map server– GetCapabilities

– GetMap

– GetFeatureInfo

• WMT II Specifications– Web coverage server specifications

– Map server enhancements

• Web Catalog Server (Planned if funding available)

NWGISS Top Level Architecture

Modified DIAL Data Server

HDF-EOS Files

NWGISS OGC Server

data access

WMT GIS Clients

Modified DIAL catalog server

Data Catalog

Map/Coverage/Feature

OGC Map Services

Capabilities

Catalog

catalog access

CreateCapabilities

XML capability files

Current Status • All three interfaces of the map server are

implemented, tested through WMT I, and will be released to users by the end of September.– Free to NASA customers.

• An alpha version of the coverage server is developed and released to WMT II for testing.

• Plan to release the coverage server with map server enhancement by end of the year

GetCapabilities

• The server responses to the client request by sending the XML capabilities file.

• Capabilities file contains information about the server’s capabilities as well as data served by the server (catalog).

• CreateCapabilities reads HDF-EOS structure information, converts it to layer structure, and encode it in XML.

GetMap• The server sends a map back to the client in the

exact geographic area, size, projection, style, and encoding specified by the client.

• HDF-EOS grid– Subset the grid by the geographic area specified by the

client.

– Resampling the subset grid to the specified size.

– Convert data to 8-bit image with specified style and encode it with specified encoding (e.g. gif)

– No reprojection is done at NWGISS but a cascading server in WMT I can do the job.

GetMap, Cont’d• HDF-EOS Swath

– Not georectified but OGC map server requires data to be georectified (e.g., the HDF-EOS grid).

– Geo-rectify swath data on-the-fly based on the client’s specifications and georeferencing information associated with a swath.

• This step performs both subsetting and resampling.

– Convert data to 8-bit image based on style required by the client

– Encode the image (e.g. gif)

GetMap, Cont’d• HDF-EOS Point

– Subset the point data based on the geographic area specified by the client.

– Rasterize the point data by plotting the point on 8-bit raster with the style specified by the client.

– Encode the 8-bit raster map.

GetFeatureInfo• The server returns to client the information

about a geographic location specified by the geographic coordinate from the client.

• NWGISS returns the real data values encoded in XML for the coordinate.

NWGISS Coverage Server• Coverage server returns the real data and

associated information to the client with client specified geographic location, parameters, and encoding.

• Current specification selected HDF-EOS, GeoTiff, GML (for vector data) as the encoding method.– WMT II is developing XML-based Image

Markup Language to encode metadata for raster data and has external pointer to either HDF-EOS or GeoTiff.

Coverage Server, Cont’d• HDF-EOS Grid and Point

– Subset and subsampling data based on client’s specifications.

– Repack the subset data in HDF-EOS.– Copy the original core metadata with a note of the

subsetting--too difficult to change coremeta.– Multi-dimensional subsetting for grid– No reprojection is implemented yet.

Coverage Server, Cont’d• Swath

– Use the georeferencing information to determine the maximum rectangle that covers the geographic bounding box specified by the client and the box’s projection in the swath coordinate, which usually are not a rectangle.

– Retrieve the swath scanlines, truncate the scanlines by the maximum rectangle, fill the fill-value between the maxium rectangle and the bouding’s box’s projection.

– Package the subset data in HDF-EOS with the geolocation info for the data from the original file.

– Plan to provide georectified data as an option in the future.

Setup NWGISS at Your Machine• If you have data in HDF-EOS, it is really easy:

– Setup a web server if you don’t have– Obtain NWGISS from us.– Copy executables to cgi-bin directory of the web

server.– Run CreateCapabilities program on your HDF-

EOS files to create a capabilities file.– Register your server through OGC registry – Done.

NWGISS Map Server Demo

• A demo map server has been setup.– With sample MODIS level 1b, TOMS ozone, etc,

and more data will be added.– Part of WMT I testbed, accessible through Cubewerx

client. http://209.217.120.146/wmt/cubeview/cubeview.cgi

– There are 19 servers in the testbed accessible through the client, our one is called NASA-EOSDIS