Contributors: J. Wuite (ENVEO IT GmbH), T. Nagler (ENVEO IT GmbH)
Issued by: ENVEO/ Jan Wuite
Date: 17/08/2023
Ref: C3S2_312a_Lot4.WP2-FDDP-IS-v1_202212_IV_PQAR-v4_i1.1
Official reference number service contract: 2021/C3S_312a_Lot4_EODC/SC1
History of modifications
List of datasets covered by this document
Related documents
Acronyms
General definitions
Breakthrough (Global Climate Observing System (GCOS) requirement): An intermediate level between threshold and goal which, if achieved, would result in a significant improvement for the targeted application. The breakthrough value may also indicate the level at which specified uses within climate monitoring become possible. It may be appropriate to have different breakthrough values for different uses.
Climate Data Record (CDR): A time series of measurements of sufficient length, consistency, and continuity to determine climate variability and change.
Goal (GCOS requirement): An ideal requirement above which further improvements are not necessary.
Ice Velocity: Ice flow velocity describes the rate and direction of ice movement. It is a fundamental parameter to characterize the behaviour of a glacier or an ice sheet. Ice velocity and its spatial derivative, strain rate (which is a measure of the ice deformation rate), are required for estimating ice discharge and mass balance and are essential input for glacier models that try to quantify ice dynamical processes.
Threshold (GCOS requirement): The minimum requirement to be met to ensure that data are useful.
Uncertainty: An estimate of the error in a measurement, due to limitations in the measuring instrument or statistical fluctuations in the quantity being measured.
Validation: Comparison between two independent datasets to test their agreement.
Scope of the document
This document is the Product Quality Assessment Report (PQAR) for Ice Velocity (IV) as part of the Copernicus Ice Sheets and Ice Shelves service. It presents results of the quality assessment for the provided datasets and a discussion of how well Global Climate Observing System (GCOS) and user requirements have been met.
The service addresses three Essential Climate Variables (ECVs) by providing four separate products:
- Ice velocity is given for Greenland in product WP2-FDDP-IV-CDR
- Gravimetric mass balance is given for Greenland and Antarctica in product WP2-FDDP-GMB-CDR
- Surface elevation change is given for:
- Antarctica in product WP2-FDDP-SEC-CDR-AntIS
- Greenland in product WP2-FDDP-SEC-CDR-GrIS
The products are hosted on the Copernicus Climate Data Store1.
Executive summary
In this document, we provide the validation results for the Greenland Ice Sheet velocity datasets produced by the service. The assessments concern the annually averaged IV maps of Greenland derived from Sentinel-1 (S1) Synthetic Aperture Radar (SAR) data acquired from 2017-10-01 to 2018-09-30, 2018-10-01 to 2019-09-30 and 2019-10-01 to 2020-09-30 (combined in CDR v3) and updated with the IV map from 2020-10-01 to 2021-09-30 (CDR v4). For CDR v3 the validation has been updated from the previous version of this document to include validation datasets previously not available. For CDR v4 we use the latest up-to-date reference validation data sets.
The quality assessment for IV includes various procedures (see Chapter 1) including detailed validation with contemporaneous in-situ Global Positioning System (GPS) data, pixel-by-pixel basis intercomparisons against publicly available velocity products and checking of the performance of the algorithm in stable terrain. The validation results for each of these quality assessment procedures are described in detail in Chapter 2. The IV product and quality assessment comply with the 2022 GCOS requirements for Ice Velocity for measurement uncertainty and spatial/temporal resolution (see Chapter 4).
1. Product validation methodology
The ice velocity (IV) product assessment referred to in this document concerns the annually averaged IV maps of Greenland derived from Sentinel-1 (S1) Synthetic Aperture Radar (SAR) data acquired from 2017-10-01 to 2018-09-30, 2018-10-01 to 2019-09-30 and 2019-10-01 to 2020-09-30 (combined in CDR v3) and updated with the IV map from 2020-10-01 to 2021-09-30 (CDR v4). The quality assessment for IV includes detailed validation with contemporaneous in-situ Global Positioning System (GPS) data at various sites across the ice sheet (Figure 1) and acquired by field teams of the Danish Programme for Monitoring of the Greenland Ice Sheet operated by GEUS (Geological Survey of Denmark and Greenland) in collaboration with DTU (Technical University of Denmark) Space and Asiaq (PROMICE; How et al., 2022a, 2022b).
Figure 1: Locations of PROMICE GPS stations in Greenland used for validation.
The products are also evaluated, on a pixel-by-pixel basis, against publicly available products produced as part of the National Aeronautics and Space Administration (NASA) ‘Making Earth System Data Records for Use in Research Environments’ (MEaSUREs) program and derived from TerraSAR-X/TanDEM-X (TSX/TDX), Sentinel-1 and Landsat-8 (Joughin et al., 2021, Joughin, 2022). These intercomparisons provide a good level of quality assurance, in particular in areas where little change is to be expected. For the product intercomparison both the annually averaged maps as well as the individual (6/12-day repeat) ice velocity maps, on which the annual maps are based, are considered. The 6/12-day repeat maps are not provided as separate products in Copernicus Climate Change Service (C3S) but the intercomparison is included here as an extra quality assurance.
Additionally, we checked the performance of the algorithm in stable terrain, i.e. where no velocity is expected. This provides a good overall indication for the bias introduced by the end-to-end velocity retrieval including co-registration of images, template matching, geocoding etc.
Further details on the validation data sets and methodology can be found in the Product Quality Assurance Document (PQAD [RD2]).
2. Validation results
Figure 2 shows the results of the intercomparison between Sentinel-1 derived velocity and annually averaged in-situ GPS measurements for 2017/18, 2018/19, 2019/20 and 2020/21 (How et al., 2022a, 2022b). The scatterplots show a very good agreement between the GPS and Sentinel-1 velocity. For 2017/18, 17 stations could be used, the mean difference between the datasets is <0.01 m/d (Root Mean Square Error - RMSE 0.02 m/d); for 2018/19, 16 stations could be used with a mean difference of ~0.01 m/d (RMSE 0.02 m/d); for 2019/20, 13 stations could be used with a mean difference of ~0.02 m/d (RMSE 0.03 m/d) and for 2020/21, 22 stations could be used with a mean difference of ~0.01 m/d (RMSE 0.02 m/d). Differences between the ice velocity maps and the GPS data can partly be attributed to uncertainties inherent to both methods including for example differences in spatial sampling: GPS provides a point measurement, while feature tracking averages an area of which the size is based on the window size used for image correlation.
a) | b) |
c) | d) |
Figure 2: Scatter plots showing annually averaged GPS versus Sentinel-1 ice velocity for a) 2017/18, b) 2018/19, c) 2019/20, and d) 2020/21.
Figure 3 shows the intercomparison results of the S1 derived ice sheet wide velocity maps from C3S and MEaSUREs (Joughin, 2022). Based on a sample size greater than 33 million pixels, the overall mean bias between the data sets is generally <0.002 m/d with an RMSE varying between 0.024-0.028 m/d for both easting (vx) and northing (vy) components. Differences between the datasets are caused by, among others, different resolution of the SAR data (TSX vs. Sentinel-1), slightly different temporal range, different algorithm settings used for IV retrieval (e.g. matching window, correlation threshold), differences in post processing (e.g. outlier removal, gap filling), different land/ocean and lay-over masks or short term velocity fluctuations. In general, higher resolution satellite data captures velocity better, in particular in shear zones, where the velocity gradient is high. The drawback is that often much smaller regions are covered.
a) | b) |
c) | d) |
e) | f) |
g) | h) |
Figure 3: Histogram of easting (left, a, c, e, g) and northing (right, b, d, f, h) velocity residuals of the intercomparison between the MEaSUREs and the Greenland Ice Sheet velocity maps for 2017/18 (a, b) 2018/19 (c, d), 2019/20 (e, f) and 2020/21 (g, h).
Figure 4 shows the results of the intercomparison of the ice velocity maps, based on 6/12-day repeat-pass Sentinel-1 data, with ice velocity maps derived from TSX/TDX and covering major Greenland outlet glaciers (Joughin et al., 2021). Depicted are histograms of the residuals for the easting (vx) and northing (vy) components. Excluding the TSX/TDX derived IV maps that do not fall within the desired temporal range (max time difference 2 days in comparison to S1 IV maps) leaves a total number of respectively 277 (2017/18), 286 (2018/19), 333 (2019/20) and 321 (2020/21), usable TSX/TDX IV maps for the inter-comparison. For each map of TerraSAR-X derived IV, multiple intercomparisons are possible as an area can be overlapped by several S1 tracks within the 2-day time range. In total 977 (2017/18), 1084 (2018/19), 1353 (2019/20) and 1429 (2020/21) S1 maps fulfil this 2-day criterium and have geographic overlap with TSX/TDX. For these maps the residuals and their statistics are calculated. Based on a sample size of, combined, more than 11.5, 10.4, 13.2 and 12.8 million pixels (for 2017/18, 2018/19, 2019/20, 2020/21 respectively), the overall mean bias between the data sets is well below 1 cm/d (RMSE 0.2 m/d) for both vx and vy components, demonstrating the good agreement between the datasets (Table 1).
a) | b) |
c) | d) |
e) | f) |
g) | h) |
Figure 4: Histogram of easting (left, a, c, e, g) and northing (right, b, d, f, h) residuals of the intercomparison with MEaSUREs TerraSAR-X derived IV maps (selected outlet glaciers) acquired within 2 days of Sentinel-1 derived IV maps for 2017/18 (a, b), 2018/19 (c, d), 2019/20 (e, f) and 2020/21 (g, h).
Finally, Figure 5 shows the results of the stable terrain test. Depicted are histograms of easting and northing velocity on stable terrain for all annual maps. Based on more than 5 million pixels (per map), the outcome of the stable terrain test indicates for all periods mean velocities of <0.001 m/d with an RMSE varying between 0.009 m/d and 0.016 m/d for both easting and northing velocity components.
a) | b) |
c) | d) |
e) | f) |
g) | h) |
Figure 5: Histogram of easting (left, a, c, e, g) and northing (right, b, d, f, h) velocity in stable terrain for 2017/18 (a, b), 2018/19 (c, d), 2019/20 (e, f) and 2020/21 (g, h).
Table 1 provides a summary statistical overview of the intercomparison results for all IV maps. For further details, please see the related Product Quality Assurance Document (PQAD [RD2]).
Table 1: Summary of inter-comparison results (values in m/day; d = mean bias, RMSE = root mean square error, Mag = velocity magnitude, E= easting velocity, N= northing velocity).
Product | Reference/Test | Pixels | dMag | RMSEMag | dE | RMSEE | dN | RMSEN |
|---|---|---|---|---|---|---|---|---|
CDR v3 | In-situ GPS | 17 | 0.00 | 0.02 | - | - | - | - |
MEaSUREs TSX (outlet glaciers) | 11.5 M | - | - | 0.00 | 0.18 | 0.00 | 0.21 | |
MEaSUREs (ice sheet) | 33.6 M | - | - | 0.00 | 0.03 | 0.00 | 0.03 | |
Stable Terrain | 5.1 M | - | - | 0.00 | 0.02 | 0.00 | 0.01 | |
CDR v3 | In-situ GPS | 16 | 0.01 | 0.02 | - | - | - | - |
MEaSUREs TSX (outlet glaciers) | 10.4 M | - | - | 0.00 | 0.18 | 0.00 | 0.22 | |
MEaSUREs (ice sheet) | 33.6 M | - | - | 0.00 | 0.03 | 0.00 | 0.03 | |
Stable Terrain | 5.1 M | - | - | 0.00 | 0.02 | 0.00 | 0.01 | |
CDR v3 | In-situ GPS | 13 | 0.02 | 0.03 | - | - | - | - |
MEaSUREs TSX (outlet glaciers) | 13.2 M | - | - | 0.01 | 0.16 | 0.00 | 0.19 | |
MEaSUREs (ice sheet) | 33.6 M | - | - | 0.00 | 0.02 | 0.00 | 0.03 | |
Stable Terrain | 5.1 M | - | - | 0.00 | 0.02 | 0.00 | 0.01 | |
CDR v4 | In-situ GPS | 22 | 0.01 | 0.02 | - | - | - | - |
MEaSUREs TSX (outlet glaciers) | 12.9 M | - | - | 0.01 | 0.17 | -0.01 | 0.20 | |
MEaSUREs (ice sheet) | 33.6 M | - | - | 0.00 | 0.02 | 0.00 | 0.02 | |
Stable Terrain | 5.1 M | - | - | 0.00 | 0.01 | 0.00 | 0.01 |
3. Application(s) specific assessments
Not applicable.
4. Compliance with user requirements
The 2022 GCOS requirements for measurement uncertainty and spatial/temporal resolution of ice velocity are listed in Table 2. For the criteria in the GCOS Implementation Plan a goal, breakthrough and threshold value are defined as follows:
- Goal (G): an ideal requirement above which further improvements are not necessary.
- Breakthrough (B): an intermediate level between threshold and goal which, if achieved, would result in a significant improvement for the targeted application. The breakthrough value may also indicate the level at which specified uses within climate monitoring become possible. It may be appropriate to have different breakthrough values for different uses.
- Threshold (T): the minimum requirement to be met to ensure that data are useful.
The GCOS requirements are based on the User Requirements Document (URD) of the Ice Sheets CCI project (Hvidberg, et al, 2012), identified through an extensive user survey within the glaciology community. They list a minimum threshold measurement uncertainty of 100 m/y (0.27 m/d) with a breakthrough/goal accuracy of 10-30 m/y (0.03-0.08 m/d). The results of our quality assessments, all showing cell-averaged mean differences of less than 0.03 m/d, fall well within this optimum range. The C3S 250 m annual IV product also complies with the threshold requirements for temporal resolution (12 months) as well as for spatial resolution (1000 m). For further details related to the requirements, please see the Target Requirements and Gap Analysis Document (TRGAD [RD3]).
Table 2. GCOS target requirements for ice sheet velocity (source: GCOS, 2022: The 2022 ECVs Requirements; https://gcos.wmo.int/en/publications/gcos-implementation-plan2022). G: Goal, B: Breakthrough, T: Threshold.
Item needed | Unit | Metric | G/B/T | Value |
|---|---|---|---|---|
Horizontal Resolution | m | Grid cell size | G | 50 |
B | 100 | |||
T | 1000 | |||
Temporal Resolution | month | Time | G | 1 |
B | ||||
T | 12 | |||
Required Measurement Uncertainty | m y-1 | G | 10 | |
B | 30 | |||
T | 100 | |||
Standards and References | Hvidberg, C.S., et al., User Requirements Document for the Ice_Sheets_cci project of ESA's Climate Change Initiative, version 1.5, 03 Aug 2012. | |||
References
How, P.; Kenneth D. Mankoff; Patrick J. Wright; Baptiste Vandecrux; Andreas P. Ahlstrøm; Robert S. Fausto, 2022a, "AWS one boom tripod Edition 4", https://doi.org/10.22008/FK2/IW73UU (last viewed 12th June 2023), GEUS Dataverse, V2
How, P.; Kenneth D. Mankoff; Patrick J. Wright; Baptiste Vandecrux; Andreas P. Ahlstrøm; Robert S. Fausto, 2022b, "AWS two boom mast Edition 1", https://doi.org/10.22008/FK2/GNYFUK (last viewed 12th June 2023), GEUS Dataverse, V2
GCOS, 2022. The 2022 GCOS Implementation Plan. Geneva: World Meteorological Organization, 85. https://gcos.wmo.int/en/publications/gcos-implementation-plan2022 (last viewed 12th June 2023)
Hvidberg, C.S., et al., User Requirements Document for the Ice_Sheets_cci project of ESA's Climate Change Initiative, version 1.5, 03 Aug 2012. Available from: http://www.esa-icesheets-cci.org/ (last viewed 12th June 2023)
Joughin, I., I. Howat, B. Smith, and T. Scambos. (2021). MEaSUREs Greenland Ice Velocity: Selected Glacier Site Velocity Maps from InSAR, Version 4. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. https://doi.org/10.5067/GQZQY2M5507Z. (last viewed 12th June 2023).
Joughin, I. (2022). MEaSUREs Greenland Annual Ice Sheet Velocity Mosaics from SAR and Landsat, Version 4 [Data Set]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. https://doi.org/10.5067/RS8GFZ848ZU9. (last viewed 12th June 2023).
