Publication Date: 2019-02-07

Approval Date: 2018-12-13

Submission Date: 2018-11-08

Reference number of this document: OGC 18-047r3

Reference URL for this document: http://www.opengis.net/doc/PER/t14-D007

Category: Public Engineering Report

Editor: Eugene Genong Yu, Liping Di

Title: OGC Testbed-14: Swath Coverage Engineering Report

OGC Engineering Report

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Copyright (c) 2019 Open Geospatial Consortium. To obtain additional rights of use, visit http://www.opengeospatial.org/

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This document is not an OGC Standard. This document is an OGC Public Engineering Report created as a deliverable in an OGC Interoperability Initiative and is not an official position of the OGC membership. It is distributed for review and comment. It is subject to change without notice and may not be referred to as an OGC Standard. Further, any OGC Engineering Report should not be referenced as required or mandatory technology in procurements. However, the discussions in this document could very well lead to the definition of an OGC Standard.

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Table of Contents

1. Summary

This Engineering Report (ER) presents a summary, description and findings of the Swath Coverage task conducted by the OGC Testbed-14 initiative.

1.1. Requirements & Research Motivation

The goal of the Swath Coverage task was to improve access to National Aeronautics and Space Administration (NASA) data in Swath structure, specifically for those Earth Observations in Level 1 and 2.

The objectives of the Swath Coverage task were as follows:

  • Modeling Swath data with Climate Forecast Convention

  • Encodings in GeoTiff, NetCDF, and GeoPackage

  • Serve typical swath data in WCS service

  • Demonstrate the use of swath data through clients

1.2. Prior-After Comparison

The Swath Coverage has not been modeled and supported with existing standards and specifications. This OGC Testbed-14 task provided solutions through adapting the data models and services to better support access and visualization of swath coverages.

Achievements of the Swath Coverage Task

Items Prior After

Swath Coverage Model

Not modeled

Modeled by (1) Climate Forecast Convention and (2) CS 1.1

Swath Coverage Encoding

No standard encoding

Encodings: GeoTiff-Swath, NetCDF-Swath, GeoPackage-Swath

Swath Coverage Service

No support by WCS

WCS Swath Coverage Profile, WCS Swath Coverage REST API

Swath Coverage Client

Ad hoc

Standard interaction with WCS for data access and visualization

1.3. Recommendations for Future Work

The Swath Coverage is relevant to the standardization work of the Web Coverage Service (WCS) Standards Working Group (SWG). The task contributes to the development of open specifications on swath coverage as follows.

  • WCS Swath Coverage Profile to adapt and specialize WCS and EO-WCS to model swath coverages

  • Encoding of swath data in GeoTiff, NetCDF, and GeoPackage as supported formats under WCS Swath Coverage

  • WCS Representational State Transfer (REST) Application Programming Interface (API) (OpenAPI 3.0.1) to facilitate stub code automatic generation for server and client

The ER makes the following recommendations for future testbed work:

  • If the Geospatial Data Abstraction Library (GDAL) is still considered as the library to work on different swath coverage, the GPKG driver of GDAL needs to be enhanced to support non-tiled data. This could be done within an OGC testbed and then provided to the GDAL developer community as open source.

  • Future testbeds could also look into how the Sensor Model Registry that was initiated in 2018 by the OGC Technical Committee could support the modeling of Swath Coverages. The Sensor Model Registry will be managed by the OGC Naming Authority.

1.4. Document contributor contact points

All questions regarding this document should be directed to the editor or the contributors:

Contacts

Name Organization

Eugene G. Yu

George Mason University

Liping Di

George Mason University

Stephan Meissl

EOX

Sergio Ferraresi

MEEO

Sizhe Wang

Arizona State University

Wenwen Li

Arizona State University

Ingo Simonis

OGC

1.5. Foreword

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. The Open Geospatial Consortium shall not be held responsible for identifying any or all such patent rights.

Recipients of this document are requested to submit, with their comments, notification of any relevant patent claims or other intellectual property rights of which they may be aware that might be infringed by any implementation of the standard set forth in this document, and to provide supporting documentation.

2. References

The following normative documents are referenced in this document.

NOTE: Only normative standards are referenced here, e.g. OGC, ISO or other SDO standards. All other references are listed in the bibliography. Example:

3. Terms and definitions

For the purposes of this report, the definitions specified in Clause 4 of the OWS Common Implementation Standard OGC 06-121r9 shall apply. In addition, the following terms and definitions apply.

  • swath coverage

    unreferenced Earth Observation data that has the following characteristics of being time-ordered, having geolocation files with longitude and latitude at least, and having definite mapping relationship between geolocations and data fields.

  • scanline|across-track axis

    one of the two axes that a swath data has. In general, it is across the track of sensor movement.

  • track|along-track axis

    one of the two axes that a swath data has. In general, it is along the track of sensor movement.

  • time

    one of the dimensions often occurred in swath coverage. The common units are expressed in seconds, microseconds, or nanoseconds for sensor recordings. It can be explicitly expressed in Coordinated Universal Time (abbreviated to UTC) and International Atomic Time (TAI). UTC and TAI are different because UTC has leap seconds while TAI is continuous. UTC is based on Gregorian Calendar, the most commonly used calendar. In this context, if "gregorian" is the calendar, the time should be in UTC which has leap second. In computer, Unix epoch (or Unix time or POSIX time or Unix timestamp) may often be used which is the number of seconds that have elapsed since January 1, 1970 (midnight UTC/GMT), not counting leap seconds (in ISO 8601: 1970-01-01T00:00:00Z). Proper library should be used to convert them into UTC or TAI.

3.1. Abbreviated terms

NOTE: The abbreviated terms clause gives a list of the abbreviated terms and the symbols necessary for understanding this document. All symbols should be listed in alphabetical order. Some more frequently used abbreviated terms are provided below as examples.

  • CALIOP Cloud-Aerosol Lidar with Orthogonal Polarization

  • CALIPSO Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations

  • CIS Coverage Implementation Schema

  • COM Component Object Model

  • CORBA Common Object Request Broker Architecture

  • COTS Commercial Off The Shelf

  • DCE Distributed Computing Environment

  • DCOM Distributed Component Object Model

  • GDAL Geospatial Data Abstraction Library

  • GIS Geographic Information System

  • GMT Greenwich Mean Time

  • IDL Interface Definition Language

  • ISO International Organization for Standardization

  • MODIS Moderate Resolution Imaging Spectroradiometer

  • NASA National Aeronautics and Space Administration

  • NOAA National Oceanic and Atmospheric Administration

  • OGC Open Geospatial Consortium

  • POSIX Portable Operating System Interface

  • TAI International Atomic Time

  • UTC Coordinated Universal Time

  • VIIRS Visible Infrared Imaging Radiometer Suite

4. Overview

Note
Highlights

This Chapter gives an overview of the complete document that helps the reader to better understand the various sections of the ER.

Section 5 introduces the problem of swath coverage access. It describes the situation prior to the testbed and discusses the requirements set by the sponsors.

Section 6 discusses the model using Climate Forecast Conventions. It provides recommendations on modeling typical types of swath coverages.

Section 7 discusses the model using OGC Coverage Implementation Schema. It provides recommendations on modeling typical types of swath coverages.

Section 8 presents other encodings or data models for Swath Coverage developed in this testbed in addition to netCDF and GML coverage.

Section 9 presents the Swath Coverage Profile under WCS. The Swath Coverage Profile is developed on CIS-1.1, WCS 1.1, and EO-WCS 1.1.

Section 10 presents the WCS REST API for Swath Coverage Profile. The version for OpenAPI is 3.0.1. The automatic generation of service and client stubs is supported with the full development of swath coverages under OpenAPI. The section only covers the design principles and major paths. Full details are included Appendix B.

Section 11 presents example implementations of Swath Coverage WCS services and clients, and typical use cases of Swath Coverages.

Section 12 provides a summary of the main findings and discusses links to other tasks such as WFS 3.0.

Annex A provides code snippets and examples that illustrate the functionality of the WCS Swath Coverage Service in requests and responses.

Annex B includes the complete draft specification for WCS REST API that follows OpenAPI 3.0.1. This is full document that is briefed in Section 10.

5. Swath Coverage

Note
Highlights

This section explains some basic concepts for swath and swath coverage. Typical swath data are also described. General requirements are reviewed with the functional expectation of visualization and analytics.

5.1. Swath Coverage

In this context, a swath coverage is generally defined as the Earth Observation data that has the following characteristics [1] [2] [3]:

  • They are time-ordered.

  • There are geolocation files with longitude and latitude at least.

  • There are definite mapping relationships between geolocations and data fields.

5.2. Typical Types of Swath Coverages

Swath coverages can be classified into different categories depending on the criteria. Based on the data fields, they may be grouped into two categories: radiometric swath data (Level 1) and geophysical swath data (Level 2) [4]. Based on observing geometry, they may be grouped into three categories: scanning (scan line), profiling (vertical), and scanning-profiling (curtain-like) [1], [4]. Depending on the mapping relationship between data fields and geolocations, they can be grouped into three: swath with time/Location, swath with Analytic fit, and swath with Attitude/Ephemeris data for spacecraft or airborne platforms [1].

5.2.1. Scanning Swath Coverage

A typical scanline swath coverage would look like the simplified sketch as shown in Figure 1.

scanning
Figure 1. Scanning swath

In this testbed, the Level 1 and Level 2 products from the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument are examples of scanning swath coverage datasets. The products of VIIRS are radiance at Level 1 and reflectance at Level 2 data. The product number is VNP09 for Suomi-NPP VIIRS produced by the Land Processes Distributed Active Archive Center (LP DAAC) of NASA. This is a typical scanline swath data. For the three I-Bands, the cross-track scan is expanded to 6400. For the M-Bands, the cross-track scan is expanded to 3200. The sensor has a constant scanning angular resolution while the footprint on the target Earth ground shows a "bow-tie" effect. To save the bandwidth in sending back the sensed data, certain tracks are skipped towards the tails along scanlines. Geolocation products are numbered VNP03 where VNP03IMGLL for I-Bands and VNP03MODLL for M-Bands. Three geolocation attributes are calculated for each pixel. They are latitude, longitude, and height. The mapping between the geolocation fields and data fields are one-to-one, i.e. each pixel has a unique geolocation value.

The operational products served at National Oceanic and Atmospheric Administration (NOAA) are numbered as SVI and SVM respectively. Each file has both radiance and reflectance data in one file. Geolocation files contain solar position (solar azimuth and solar zenith) and satellite position (satellite azimuth, satellite zenith, and satellite range) in addition to geometrical locations of latitude, longitude, and height. These parameters are useful in geometrical rectification and radiometric correction.

5.2.2. Profiling Swath Coverage

A typical profiling swath would look like these, as simply depicted in Figure 2

profiling
Figure 2. Profiling swath

The Level 1B and Level 2 backscatter products of Cloud-Aerosol LiDAR and Infrared Pathfinder Satellite Observations (CALIPSO) and the backscatter products of CloudSat are typical profiling swath coverages. The Level 1B of CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) - the sensor of CALIPSO is a profiling swath coverage. The aggregation and averaging algorithm on board of the satellite convert the raw measurements into uniformly 583 elements vertically at each point. There is no across-track scan. The resolution along the vertical direction varies from height to height due to the averaging scheme. The spatial resolution at each point is different from pixel to pixel due to its location at different record element (or altitude).

The level 1B and Level 2 product of CloudSat are also primarily backscatters of Radar and derived Cloud Geometric Profile respectively. They are also typical profiling swath coverage. At each point along the track, CloudSat has uniformly 125 bins which represents 240-meter vertical resolution. Each pixel has different spatial resolution due to their difference along vertical direction.

5.2.3. Scanning-profiling Swath Coverage

The scanning-profiling swath has both scan expansion with vertical profiles. To be general, this category of swath data includes those point clouds. It captures a volumetric description of the features.

A scanning-profiling product is the atmospheric profiling product from Moderate Resolution Imaging Spectroradiometer (MODIS). The product numbers are MOD07 and MYD07 for data collected on Terra and Aqua respectively. The MODIS Atmospheric Profile product consists of several parameters: they are total-ozone burden, atmospheric stability, temperature and moisture profiles, and atmospheric water vapor. All of these parameters are produced day and night for Level 2 at 5x5 1-km pixel resolution when at least 9 FOVs are cloud free. The MODIS Atmosphere Profile data product files are typical of scanning-profiling swath which profile data at different altitudes.

The MODIS total-ozone burden is an estimate of the total-column tropospheric and stratospheric ozone content. The MODIS atmospheric stability consists of three daily Level 2 atmospheric stability indices. The Total Totals (TT), the Lifted Index (LI), and the K index (K) are each computed using the infrared temperature- and moisture-profile data, also derived as part of MOD07.

The MODIS temperature and moisture profiles are produced at 20 vertical levels. A clear sky synthetic regression retrieval algorithm is used, where regression coefficients are derived by using a fast-radiative transfer model with atmospheric characteristics taken from a dataset of global (radiosonde and model) profiles. The MODIS atmospheric water-vapor product is an estimate of the total column water vapor made from integrated MODIS infrared retrievals of atmospheric moisture profiles in clear scenes.

5.3. Functional Requirements of Swath Coverage

The goal of this testbed effort was to better serve the swath coverage through a standard Web service that helps in extensively using swath data. To achieve the goal, the specific tasks included (1) modeling Swath data with Climate Forecast Convention, (2) encodings in GeoTiff, NetCDF, and GeoPackage, (3) serving typical swath data through a WCS service, and (4) demonstrating typical use cases of swath data through clients.

Two important aspects of using swath data are visualization and analytics. Visualization of the swath coverage in common GIS software packages allows visual exploration from different viewpoints or together with other features and properties in aligned geographical reference systems. Analytics include the use of actual data which puts a specific requirement on accessing the raw data as well as geospatially mapping to other properties and features.

5.3.1. Visualization

The visualization of the swath coverage may include a general 2D view, time series view, 3D view on a globe, or an analytical view.

5.3.2. General 2D views

The view of the swath coverage in 2D view may be shown by itself from different viewports or projections to a geographical reference system along with other geographical features. The view may be the result of sub-setting or slicing on a time dimension or (vertical) profile. Example views of CloudSat 2B-GEOPROF Level 2 (Swath) data are shown in Figure 3. The top shows the top view of the swath data over geographical regions in georeferenced projections. The lower view is a 2D view with axes of time and height (profile) along the track. The latter may not be projected in a standard spatial reference system, but the sensor coordinate system. Another example of a 2D profile view in a time-height coordinate system is shown in Figure 4.