Contributors: Lin Gilbert (University of Leeds), Sebastian B. Simonsen (Technical University of Denmark), Jan Wuite (ENVEO IT GmbH)
Issued by: University of Leeds / Lin Gilbert
Date: 25/06/2021
Ref: C3S_312b_Lot4_D2.IS.2-v3.0_211502_Product_Quality_Assessment_Report_IV_i1.0
Official reference number service contract: 2018/C3S_312b_Lot4_EODC/SC2
History of modifications
List of datasets covered by this document
Related documents
Acronyms
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.
Executive summary
The service addresses three essential climate variables (ECVs) by providing four separate products.
- Ice velocity is given for Greenland in product D3.IS.4
- Gravimetric mass balance is given for Greenland and Antarctica in product D3.IS.5
- Surface elevation change is given for
- Antarctica in product D3.IS.6.1
- Greenland in product D3.IS.6.2
We provide here the quality assessment results for Ice Velocity CDR v3.
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) SAR data acquired from 2017-10-01 to 2018-09-30, 2018-10-01 to 2019-09-30 (reprocessed from CDR v2) and 2019-10-01 to 2020-09-30 (combined in CDR v3). The quality assessment for IV includes detailed validation with contemporaneous in-situ 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 in collaboration with DTU Space and Asiaq (PROMICE; Fausto and Van As, 2019).
Figure 1: Greenland Ice Sheet velocity showing the locations of PROMICE GPS stations used for validation.
The products are also evaluated, on a pixel-by-pixel basis, against publicly available products produced as part of the 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, 2020; Joughin et al., 2020). 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 C3S but are 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.
It should be noted that the validation data (data set ID: NSIDC-0725; Joughin, 2020) is only occasionally updated, and at the time of this assessment the validation data for 2019 2020 is not yet fully available. Further details on the validation data sets and methodology can be found in the Product Quality Assurance Document (PQAD , D2.IS.1-v3.0).
2. Validation results
Figure 2 shows the results of the intercomparison between Sentinel-1 derived velocity (CDR v3) and annually averaged in-situ GPS measurements for 2017/18, 2018/19 and 2019/20 (Fausto and Van As, 2019; updated until July 2020). 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 <1 cm/d (RMSE 2 cm/d); for 2018/19, 16 stations could be used with a mean difference of ~1 cm/d (RMSE 2 cm/d); and for 2019/20, 13 stations could be used with a mean difference of ~2 cm/d (RMSE 3 cm/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)
|
Figure 2: Scatter plots showing annually averaged GPS versus Sentinel-1 ice velocity for a) 2017/18, b) 2018/19, and c) 2019/20.
Figure 3 shows the intercomparison results of the S1 derived ice sheet wide velocity maps from C3S (2017/18 and 2018/19) and MEaSUREs. Based on a sample size of in total >33 million pixels, the overall mean bias between the two data sets is <2 mm/d with an RMSE of <3 cm/d for both vx and vy. Differences between the two datasets are caused by, among others, different resolution of the SAR data (TSX vs. Sentinel-1), different temporal range, different 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 actual 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.
|
|
|
|
Figure 3: Histogram of easting (left) and northing (right) velocity residuals of the intercomparison between the MEaSUREs and the Greenland Ice Sheet velocity maps (CDR v3) for 2017/18 (top) and 2018/19 (bottom). For 2019-2020 validation data are not yet available.
|
|
|
|
|
|
Figure 4: Histogram of easting (left) and northing velocity (right) 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 (top), 2018/19 (middle) and 2019/20 (bottom). For 2019-2020 validation data is only available until January 2020.
Figure 4 visualizes the results of the intercomparison of the CDR v3 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., 2020; updated until Jan 2020). 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) and 62 (2019/20), usable TSX/TDX IV maps for the inter-comparison. For each map of TerraSAR-X derived IV, multiple intercomparisons are possible as the area can be overlapped by several S1 tracks within the 2-day time range. In total 977 (2017/18), 1084 (2018/19) and 284 (2019/20) S1 maps fulfill 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 and 3.5 million pixels (for 2017/18, 2018/19 and 2019/20 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).
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 (CDR v3). Based on more than 5.1 million pixels (per map), the outcome of the stable terrain test indicates for all three periods mean velocities of <1 mm/d with an RMSE of 2 cm/d and 1 cm/d for the easting and northing velocity components respectively.
Table 1 provides a statistical overview of the intercomparison results for all three maps included in CDR v3. For further details, please see the related Product Quality Assurance Document (PQAR, D2.IS.1-v3.0).
Table 1: Summary of inter-comparison results for CDR v3 (values in m/d).
Product | Reference/Test | Pixels | dMag | RMSEMag | dE | RMSEE | dN | RMSEN |
|---|---|---|---|---|---|---|---|---|
2017/18 | GPS | 17 | 0.00 | 0.02 | - | - | - | - |
MEaSUREs TSX/TDX (outlet glaciers only) | 11.5 M | - | - | 0.00 | 0.18 | 0.00 | 0.21 | |
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 | |
2018/19 | GPS | 16 | 0.01 | 0.02 | - | - | - | - |
MEaSUREs TSX /TDX (outlet glaciers only) | 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 | |
2019/20 | GPS | 13 | 0.02 | 0.03 | - | - | - | - |
MEaSUREs TSX/TDX (outlet glaciers only) | 3.5 M | - | - | 0.00 | 0.17 | 0.00 | 0.20 | |
Stable Terrain | 5.1 M | - | - | 0.00 | 0.02 | 0.00 | 0.01 |
|
|
|
|
|
|
Figure 5: Top: Histogram of easting (left) and northing (right) velocity in stable terrain for 2017/18 (top), 2018/19 (middle), and 2019/20 (bottom).
3. Application(s) specific assessments
Not applicable.
4. Compliance with user requirements
The GCOS requirements for accuracy/uncertainty of ice velocity are listed in Table 2. The uncertainty and stability requirements are both 0.1 m/y, well below 1 mm/day, which is unrealistic with present day technology. The accuracy requirements for IV, as described in 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, lists a minimum accuracy of 30-100 m/y (0.08-0.27 m/d) with an optimum 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 3 cm/d, fall well within this, more realistic, range.
Table 2: GCOS target requirements for ice sheet velocity (source: GCOS Implementation Plan, 2016)
Product | Frequency | Resolution | Measurement uncertainty | Stability |
|---|---|---|---|---|
Ice Velocity | 30 days | Horizontal 100 m | 0.1 m/y | 0.1 m/ye |
References
Fausto, R.S. and van As, D., (2019). Programme for monitoring of the Greenland ice sheet (PROMICE): Automatic weather station data. Version: v03, Dataset published via Geological Survey of Denmark and Greenland. DOI: https://doi.org/10.22008/promice/data/aws
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/
Joughin, I., I. Howat, B. Smith, and T. Scambos. 2020. MEaSUREs Greenland Ice Velocity: Selected Glacier Site Velocity Maps from InSAR, Version 3. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/YXMJRME5OUNC. [Date Accessed: Jan 2021]
Joughin, I. 2020. MEaSUREs Greenland Annual Ice Sheet Velocity Mosaics from SAR and Landsat, Version 2. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/TZZDYD94IMJB. [Date Accessed: Jan 2021].
This document has been produced in the context of the Copernicus Climate Change Service (C3S).
The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement signed on 11/11/2014). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.
The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.
