MapServer Support for Web Coverage Services

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MapServer Support for Web Coverage Services

• Stephen Lime - Minnesota DNR
• Dr. Thomas E. Burk - University of Minnesota
• MUM 2004 - Ottawa, Canada

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Presentation Overview

• Background
• Implementation
• Usage
• Demo Application
• Future Work

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MapServer

• Pre-dates OGC interoperability efforts
• Two primary objectives, via the Internet
• create maps and associated products (e.g.,
  scalebars, legends and reference maps)
• query spatial data
• Two primary components
• mapserv CGI provides out-of-box functionality
• MapScript, access to the C/C API from popular
  programming languages

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GDAL

• Geographic Data Abstraction Library
• Middleware for raster data access, similar in
  concept to ODBC
• Provides most MapServer raster input and some
  output capabilities
• Provides limited image processing support for
  operations such as projection and resampling

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MapServer OGC Support

• Web Mapping Service (WMS) - a perfect match for
  existing MapServer functionality
• Web Feature Service (WFS) - MapServer supports
  non-transactional elements, equates roughly to
  MapServer query support
• Web Coverage Service (WCS) - furthest spec from
  core MapServer functionality, must rely heavily
  on MapServer/GDAL interface

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Groundwork

• MODIS Download Facility early 2000
• First extension of MapServer that has several
  characteristics of recent WCS standard
• Lead to development of a SEEDS proposal for NASA
  funding
• Partners University of North Dakota and the Jet
  Propulsion Laboratory

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WCS Overview

• Like WFS, WCS is a natural progression from WMS
• Unfiltered access to a coverage, values or
  properties at a set of geographic locations
• WCS 1.0.0 is limited to simple, that is grid,
  coverages whew
• Supports simple queries in the form of spatial,
  temporal and/or other property subsets

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Design Assumptions

• Not a ground-up WCS implementation
• Will not support the full WCS specification
• Build upon functionality already available within
  MapServer
• Any extensions to MapServer must be made
  available via the core product, the CGI interface
  and MapScript
• Must integrate with OGC specification
  implementations supported by MapServer (e.g. WMS
  and WFS)
• Relies upon GDAL for data extraction,
  transformation (e.g. projection) and formatting

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Our ToDo List

• Enable ISO8601 extended date format parsing
• Enhance tiling to support
• filtering based on tile attributes
• specification of band/bands as an attribute
• Alter GDAL/MapServer interface to support key WCS
  features (multi-band raw mode)
• Update MapServer compilation process
• Developed interface to process WCS requests via
  the MapServer CGI program
• GetCapabilities/DescribeCoverage/GetCoverage

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Spatial/Temporal Tiling

• Mechanism to organize and present a large volume
  of data using a single MapServer layer definition
• Has utility beyond immediate WCS needs, for
  example for access control
• Simplifies development of client applications
• Potential performance benefits
• May be cumbersome to setup

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Tiling Example Multiple Images
1.img
2.img
3.img
4.img
5.img
6.img
7.img
8.img
9.img
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Tiling Example Single Image, Temporal Bands
NDVI_03.img
May
June
July
August
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Development Status

• Elements of WCS (e.g. tiling) is enabled in 4.2
  release, beta WCS support is now available in 4.3
  development version
• Requires GDAL and Proj.4
• Supports GetCapabilities, DescribeCoverage and
  GetCoverage

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Enabling WCS

• Very similar to enabling WMS and WFS
• Define WCS specific metadata elements
• GDAL can discover some values (e.g. resolution)
• Optionally, these values can be hardcoded
• Set the layer DUMP attribute
• If necessary, structure image data appropriately
  using new tiling capabilities

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An Example
LAYER NAME "ndvi" STATUS OFF TYPE
RASTER METADATA wcs_label "North Central
US MODIS-based NDVI Images for 2002" ows_srs
"EPSG26915 EPSG4269 EPSG4326"
ows_extent "159707 4597895 1400207 5501395"
wcs_resolution "500.0 500.0" wcs_formats
"GEOTIFF_INT16" wcs_nativeformat "raw
binary" wcs_timeposition "2002-001,2002-033,
,2002-193" wcs_timeitem "imgdate" END
DUMP TRUE TILEINDEX 'ndvi_idx' END
LAYER NAME "ndvi_idx" TYPE TILEINDEX DATA
"mod13" END
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Demo Application

• MODIS Download Facility
• Run the demo...

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Future Enhancements

• Non-temporal range subsetting
• Via TILEINDEX layers
• Via raster cell values
• Automatic metadata discovery
• XML encoded requests
• Support for multiple spatial interpolation
  methods
• True support of ISO8601 date formats
• WCS client support
• Oh yeah, need to write documentation

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Questions?