Ensuring Long Term Access to Remotely Sensed HDF4 Data

with Layout MapsMike Folks, The HDF Group

Ruth Duerr, NSIDC

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Background and basic concept

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HDF4 is

FLEXIBLE

EXTENSIBLE

SELF-DESCRIBING

I’m Plastic Man!I’m Plastic Man!

ButThere’s a cost…

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Complexity!

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How do we save HDF users from having to deal with all of

the complexity under the hood?

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Through the HDF software libraries, either by using the

HDF APIs directly or by using HDF tools that depend on the

HDF libraries.

But what about the future…

• There is a risk in depending solely on the HDF libraries to access HDF-formatted data over the long term.

• It is possible, especially in the distant future, that the libraries may not be available.

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Really smart people and software?

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Maybe future data users and their computers will be so smart that the HDF4 format will be a piece of cake.

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Maybe not.

We need an “easy” button

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“If only we could read HDF data with an read HDF data with an independent program that does not rely on independent program that does not rely on

the HDF API… the HDF API… A possible approach [would be to] extend

hdfls to print a hierarchical map of a data file, [and] write ncdump/hdp-like utilities to find,

assemble and write out SDSes and vdatas.”

“Leveraging HDF Utilities”Christopher LynnesHDF Workshop X.

The project

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HDF4 mapping

• Problem The complex internal byte layout of HDF files

requires one to use the API to access HDF data. This makes long-term readability of HDF data

dependent on long-term allocation of resources to support HDF software.

• Proposed solution Create a map of the layout of data objects in an

HDF file, allowing a simple reader to be written to access the data.

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HDF4 mapping project activities

1. Assess and categorize HDF4 data held by NASA To determine what types of objects to map. To get an idea of the magnitude of the project.

2. Develop prototype for proof of concept Develop markup-language based layout

specification. Develop tool to produce layout for an HDF4 file. Develop and test two independent tools to read

HDF4 data based solely on the map files.

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Project activities (continued)

3. Assess results and plan next steps Present results and options for proceeding to the

community. Assess the likely usefulness of this approach, as

well as any desirable modifications Evaluate the effort required for a full solution that

best meets community needs Submit a proposal for the work needed to provide

a full solution

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1. Assess and categorize

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How many NASA HDF4 products?

Data Center HDF4 Products

ASF 0

GES-DISC 236

GHRC 54

ASDC 63

LP-DAAC 67

NSIDC 47

ORNL-DAAC 2

PO.DAAC 22

SDAC 0

MrDC 95

Total 586

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Data characteristics

• Product Identification Product Name Data Level Archive Location Product Version

• Whether the product was multi-file• For HDF-EOS products

HDF-EOS version For point data

• Number of point data sets• Maximum number of levels

For swath data• Number of swaths• Maximum number of dimensions• Organized by time, space, both, or other• Whether dimension maps were used

For gridded data• Number of grids• Max number of dimensions in a grid• Number of projections used• Whether any grids were indexed

• HDF Version

• For raster data Number of 8-bit rasters Number of 24-bit rasters Number of general rasters Whether any rasters had attributes Whether any rasters were compressed Whether any rasters were chunked Whether there were any palettes

• For SDS data Number of SDSs Maximum number of dimensions Did any SDS have attributes Was any SDS annotated Were dimension scales used Was compression used and if so what kind Was chunking used

• For Vdata Number of Vdata structures Did any Vdata have attributes Did any Vdata fields have attributes Was compression used and if so what kind Was chunking used

Product Characteristics Examined

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Other results

• Slightly more than half of the HDF4 products are in HDF-EOS 2 format

• Grids are the most common HDF-EOS data structures in use

• No products use a combination of grid, swath, and point data structures

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2. Prototype and proof of concept

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HDF4 mapping prototype workflow

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HDF4 File “H4.hdf”

HDF4 File “H4.hdf”

HDF4 Mapping File (XML document)“H4.hdf.map.xml”

HDF4 Mapping File (XML document)“H4.hdf.map.xml”

hmaplinked with HDF4 library

hmaplinked with HDF4 library

Reader 1(C program)

Object DataObject Data Groups, Data Objects, Structural and Application

Metadata; Locations of Object Data

Reader 2(Perl Script)Reader 2

(Perl Script)

Proof-of-concept results

• The HDF Group created prototype map generation software and a draft map specification

• Map generator was tested on a wide variety of data products

• GES-DISC and NSIDC independently wrote software that uses maps to read data files in NSIDC’s and GES-DISC’s archives

• Summary - the concept is feasible!

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Example map fragment

<?xml version="1.0" encoding="utf-8"?><hdf4:HDFMap xmlns:hdf4="http://www.hdfgroup.org/HDF4/HDF4Map"> <hdf4:RootGroup> <hdf4:SDS objName="data1" objPath="/" objID="xid-DFTAG_NDG-2"> <hdf4:Attribute name="data range" ntDesc="32-bit signed integer"> 0 255 </hdf4:Attribute> <hdf4:Datatype dtypeClass="INT" dtypeSize="4" byteOrder="BE" /> <hdf4:Dataspace ndims="2"> 10 100 </hdf4:Dataspace> <hdf4:Datablock nblocks="1"> <hdf4:BlockOffset> 2502 </hdf4:BlockOffset> <hdf4:BlockNbytes> 4000 </hdf4:BlockNbytes> </hdf4:Datablock> </hdf4:SDS> </hdf4:RootGroup></hdf4:HDFMap>

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Next steps

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Effort for full implementation

• Finalize map file xml specification compatibility with existing standards NCML, XFDU,

PREMIS, ESML, DFDL

• Implement production quality mapping tool and API

• Possibly do similar assessment for HDF5 maps.

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Implementation Processes

• Generate maps for existing archives GES-DISC approach: append the map XML to the XML

files already kept for each file in their archive NSIDC non-ECS data implementation: add an XML file

for each data file in same directory ROM to add capability to NASA ECS systems in

process Other NASA systems TBD

• Generate maps for new data Add map generation as a step in the ingest process

using stand alone tool Request product generation systems to use new API

calls that generate maps

How you can help

• Consider what it might take to implement this for your archive

• Review the materials on the wiki and elsewhere - comment heavily! Wiki page added to NASA’s ESDC wiki Project page at The HDF Group website:

• http://www.hdfgroup.org/projects/hdf4mapping/

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Thank you.This report is based upon work supported in part

by a Cooperative Agreement with the National Aeronautics and Space Administration (NASA)

under NASA Award NNX06AC83A. Any opinions, findings, and conclusions or

recommendations expressed in this material are those of the author(s) and do not necessarily

reflect the views of the National Aeronautics and Space Administration.

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