Canada Center for Mapping and Earth Observation

Ottawa, Canada

Canada Center for Mapping and Earth Observation

Ottawa, Canada
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Brisco B.,Canada Center for Mapping and Earth Observation | Ahern F.,TerreVista Earth Imaging | Murnaghan K.,Canada Center for Mapping and Earth Observation | White L.,Environment Canada | And 2 more authors.
Remote Sensing | Year: 2017

Water is an essential natural resource, and information about surface water conditions can support a wide variety of applications, including urban planning, agronomy, hydrology, electrical power generation, disaster relief, ecology and preservation of natural areas. Synthetic Aperture Radar (SAR) is recognized as an important source of data for monitoring surface water, especially under inclement weather conditions, and is used operationally for flood mapping applications. The canopy penetration capability of the microwaves also allows for mapping of flooded vegetation as a result of enhanced backscatter from what is generally believed to be a double-bounce scattering mechanism between the water and emergent vegetation. Recent investigations have shown that, under certain conditions, the SAR response signal from flooded vegetation may remain coherent during repeat satellite over-passes, which can be exploited for interferometric SAR (InSAR) measurements to estimate changes in water levels and water topography. InSAR results also suggest that coherence change detection (CCD) might be applied to wetland monitoring applications. This study examines wetland vegetation characteristics that lead to coherence in RADARSAT-2 InSAR data of an area in eastern Canada with many small wetlands, and determines the annual variation in the coherence of these wetlands using multi-temporal radar data. The results for a three-year period demonstrate that most swamps and marshes maintain coherence throughout the ice-/snow-free time period for the 24-day repeat cycle of RADARSAT-2. However, open water areas without emergent aquatic vegetation generally do not have suitable coherence for CCD or InSAR water level estimation. We have found that wetlands with tree cover exhibit the highest coherence and the least variance; wetlands with herbaceous cover exhibit high coherence, but also high variability of coherence; and wetlands with shrub cover exhibit high coherence, but variability intermediate between treed and herbaceous wetlands. From this knowledge, we have developed a novel image product that combines information about the magnitude of coherence and its variability with radar brightness (backscatter intensity). This product clearly displays the multitude of small wetlands over a wide area. With an interpretation key we have also developed, it is possible to distinguish different wetland types and assess year-to-year changes. In the next few years, satellite SAR systems, such as the European Sentinel and the Canadian RADARSAT Constellation Mission (RCM), will provide rapid revisit capabilities and standard data collection modes, enhancing the operational application of SAR data for assessing wetland conditions and monitoring water levels using InSAR techniques. © 2017 by the authors.

White L.,Canada Center for Mapping and Earth Observation | Brisco B.,Canada Center for Mapping and Earth Observation | Pregitzer M.,Canada Center for Mapping and Earth Observation | Tedford B.,Ducks Unlimited Canada | Boychuk L.,Ducks Unlimited Canada
Canadian Journal of Remote Sensing | Year: 2014

Abstract: Synthetic Aperture Radar (SAR) is well known for its ability to map surface water. There are a number of SAR satellites providing data for this application including the Canadian RADARSAT-2 system. RADARSAT-2 has a wide range of beam modes and some users may be intimidated by the variety of choices and have a difficult time deciding on the most appropriate beam mode. This technical note addresses some issues behind beam mode and polarization selection for surface water mapping with RADARSAT-2 and the upcoming RADARSAT Constellation Mission (RCM). This includes the impacts of resolution, wind effects, and the best mode for flooded vegetation detection. The results show that high resolution modes like the single polarized Spotlight are best for accurately delineating the surface water edge and small patches of flooded terrain. The addition of the cross-polarization available in other beam modes can provide useful surface water information in windy or rough surface conditions because there is little effect on the RADAR backscatter compared to the HH single polarization. For accurately delineating flooded vegetation, a polarimetric or compact polarimetric mode is best because the phase is maintained, which allows the user to apply polarimetric decompositions models to help separate the RADAR backscatter. © 2014, Copyright © Crown Copyright 2014.

Ou C.,University of New Brunswick | LaRocque A.,University of New Brunswick | Leblon B.,University of New Brunswick | Zhang Y.,Canada Center for Mapping and Earth Observation | And 2 more authors.
International Journal of Remote Sensing | Year: 2016

Permafrost occurs mainly at high latitudes and is currently subjected to widespread thawing due to global warming, which has significant ecological and socio-economic impacts. One way to map permafrost condition and its change at high spatial resolution is to calculate soil temperatures using a process-based permafrost model, such as the Northern Ecosystem Soil Temperature (NEST) model, based on remote-sensing data. In a previous paper (Ou et al. 2015), we showed that the NEST model can be used to model soil temperatures, frozen depth, and active layer thickness in a warm and discontinuous permafrost area that is located in the Hudson Bay Lowlands in northern Ontario, Canada. In this article, we applied the model over the whole study area to map the distribution of permafrost conditions and their changes from the 1960s to the 2000s at high spatial resolution. Some of the model inputs were estimated based on land-cover and surficial material maps derived from Landsat-5 TM and Radarsat-2 SAR C-HH and C-HV images. By comparison to field observations, we were able to achieve an average mapping accuracy of 99% for both the land-cover and surficial material maps, when both Landsat and Radarsat-2 images were used. In addition, all the 49 field-observed treed and shrub permafrost landform sites found in the study area were correctly identified on the classified image. This spatial modelling study shows that permafrost currently underlies about 9.4% of the land in this area, and varies significantly among land-cover types. With the increases in mean annual air temperature (1.9°C) and precipitation (2.4%) from the 1960s to the 2000s, the model results show a deepening of active layer by 20.5%. However, the permafrost has persisted in most of the area due to the protection of thick peat. The modelled permafrost extent reduced only from 9.8% to 9.4%. © 2016 Taylor & Francis

Guindon L.,Natural Resources Canada | Bernier P.Y.,Natural Resources Canada | Beaudoin A.,Natural Resources Canada | Pouliot D.,Canada Center for Mapping and Earth Observation | And 4 more authors.
Canadian Journal of Forest Research | Year: 2014

Disturbances such as fire and harvesting shape forest dynamics and must be accounted for when modelling forest properties. However, acquiring timely disturbance information for all of Canada's large forest area has always been challenging. Therefore, we developed an approach to detect annual forest change resulting from fire, harvesting, or flooding using Moderate Resolution Imaging Spectroradiometer (MODIS) imagery at 250 m spatial resolution across Canada and to estimate the withinpixel fractional change (FC). When this approach was applied to the period from 2000 to 2011, the accuracy of detection of burnt, harvested, or flooded areas against our validation dataset was 82%, 80%, and 85%, respectively. With FC, 77% of the area burnt and 82% of the area harvested within the validation dataset were correctly identified. The methodology was optimized to reduce the commission error but tended to omit smaller disturbances as a result. For example, the omitted area for harvest blocks greater than 80 ha was less than 14% but increased to between 38% and 50% for harvest blocks of 20 to 30 ha. Detection of burnt and harvested areas in some regions was hindered by persistent haze or cloud cover or by insect outbreaks. All resulting data layers are available as supplementary material. © 2014, National Research Council of Canada. All rights reserved.

Brisco B.,Natural Resources Canada | Ahern F.,TerreVista Earth Imaging | Hong S.-H.,Korea Polar Research Institute | Hong S.-H.,University of Miami | And 4 more authors.
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing | Year: 2015

C-band SAR is well established as a useful sensor for water resources applications. It is commonly accepted that the backscatter from wetlands that consist of many emergent stems over open water (swamps and marshes) is dominated by a double-bounce scattering mechanism. However, recent observations with fully polarimetric data from Radarsat-2 over the extensive wetlands of the Everglades and numerous small wetlands in Ontario appear to be inconsistent with this interpretation of the backscatter physics. In this paper, we use several forms of polarimetric analysis and decomposition. All of these indicate that the backscatter from small marshes and swamps in Ontario is dominated by polarimetric characteristics normally attributed to the odd-bounce mechanism. This anomalous result might be explained as a consequence of changes in the double-bounce reflectance properties of vegetation as a function of the incidence angle. However, detailed electromagnetic backscatter modeling will be needed to provide a more complete and reliable understanding of the details of backscattering from wetlands with emergent vegetation. Additional observational and theoretical work will be required to document and understand the unusual results we report here. If these results are substantiated, the SAR community must re-interpret the generally accepted meanings of the popular decomposition variables, and introduce new terminology to describe them. This would lead to an improved understanding of the backscatter physics and better use of polarimetric SAR for wetland management applications. © 2015 IEEE.

Lacelle D.,University of Ottawa | Brooker A.,University of Ottawa | Fraser R.H.,Canada Center for Mapping and Earth Observation | Kokelj S.V.,Northwest Territories Geoscience Office
Geomorphology | Year: 2015

Retrogressive thaw slumps are one of the most active geomorphic features in permafrost terrain. This study investigated the distribution and growth of thaw slumps in the Richardson Mountains and Peel Plateau region, northwestern Canada, using Tasseled Cap (TC) trend analysis of a Landsat image stack. Based on the TC linear trend image, more than 212 thaw slumps were identified in the study area, of which 189 have been active since at least 1985. The surface area of the slumps ranges from 0.4 to 52ha, with 10 slumps exceeding 20ha. The thaw slumps in the region are all situated within the maximum westward extent of the Laurentide Ice Sheet. Based on relations between frequency distribution of slumps and that of terrain factors in the landscape, the slumps are more likely to occur on the ice-rich hummocky rolling moraines at elevations of 300-350m and 450-500m and along east-facing slopes (slope aspects of 15° to 180°) with gradients of 8° to 12°. Pixel-level trend analysis of the TC greenness transformation in the Landsat stack allowed calculating headwall retreat rates for 19 thaw slumps. The 20-year average retreat rates (1990-2010 period) for 19 slumps ranged from 7.2 to 26.7myr-1, with the largest slumps having higher retreat rates. At the regional scale, the 20-yr headwall retreat rates are mainly related to slope aspect, with south- and west-facing slopes exhibiting higher retreat rates, and large slumps appear to be generating feedbacks that allow them to maintain growth rates well above those of smaller slumps. Overall, the findings presented in this study allow highlighting of key sensitive landscapes and ecosystems that may be impacted by the presence and growth of thaw slumps in one of the most rapidly warming region in the Arctic. © 2015 published by Elsevier B.V. All rights reserved.

Short N.,Canada Center for Mapping and Earth Observation | LeBlanc A.-M.,Geological Survey of Canada | Sladen W.,Geological Survey of Canada | Oldenborger G.,Geological Survey of Canada | And 2 more authors.
Remote Sensing of Environment | Year: 2014

The use of Differential Interferometric Synthetic Aperture Radar (D-InSAR) is rapidly gaining acceptance as a source of ground displacement information for permafrost regions. The accuracy of the information however, is still not well established. This paper uses a stack of RADARSAT-2 D-InSAR data to map seasonal ground displacement at the Iqaluit Airport, Baffin Island, Canada. The accuracy and information content of the D-InSAR products are evaluated with a variety of ground truth data. The D-InSAR derived seasonal ground displacement patterns align well with surficial geology units and reflect the thaw settlement characteristics of the sediments; they also identify localised displacement patterns in the vicinity of features such as ice wedges and frost cracks. Comparisons with ground settlement measurements from in-situ thaw tubes indicate sub-centimetre agreement in dry areas. In low lying wet areas however, subject to saturation and flooding, the D-InSAR stack significantly underestimates the true settlement, detecting only 3.7. cm of the 8.5. cm thaw tube measurement. The combination of high phase gradients and poor coherence over intermittently flooded surfaces makes it challenging to preserve reliable phase measurements in these areas. Information from electromagnetic surveys shows that some surface displacement is related to sub-surface conditions (<. 6. m depth), possibly conditions related to the presence and movement of water in the active layer and to a lesser extent, unfrozen water content within the permafrost.© 2013 .

Mahdavi S.,Memorial University of Newfoundland | Salehi B.,Memorial University of Newfoundland | Moloney C.,Memorial University of Newfoundland | Huang W.,Memorial University of Newfoundland | Brisco B.,Canada Center for Mapping and Earth Observation
IOP Conference Series: Earth and Environmental Science | Year: 2016

Speckle degrades the radiometric quality of a Synthetic Aperture Radar (SAR) image and makes its visual interpretation difficult. The approaches proposed previously for speckle filtering of SAR images exploit a window of fixed size for this purpose. But a fixed size window is not sufficient as the size of objects may vary throughout the image. In this paper, a method is introduced by which each pixel in the image is filtered using a window size which is optimal for that pixel. Real and imaginary parts of a single-channel SAR image are used for the selection of the best window size for each pixel, and then intensity image is filtered by applying that window size. The Average and Minimum Mean Square Error (MMSE) filters are modified using the Adaptive Window Size method. This approach is implemented on the HH-channel of a RADARSAT-2 image acquired over the Avalon Peninsula near St. John's, Newfoundland, Canada. This filter can supress speckle effectively while retaining the details reasonably.

Ahern F.J.,TerreVista Earth Imaging | Brisco B.,Canada Center for Mapping and Earth Observation | Murnaghan K.,Canada Center for Mapping and Earth Observation | White L.,Canada Center for Mapping and Earth Observation | And 4 more authors.
International Geoscience and Remote Sensing Symposium (IGARSS) | Year: 2014

It is commonly-accepted that the enhanced backscatter from wetlands that consist of many emergent stems over open water (swamps and marshes) is dominated by double-bounce backscatter. Recent observations with fully polarimetric data from Radarsat-2 over the extensive wetlands of the Everglades and numerous small wetlands in Ontario are not consistent with this interpretation of the backscatter physics. In this paper we use several forms of polarimetric analysis and decomposition. All of these indicate that the backscatter from small marshes and swamps in Ontario has polarimetric characteristics normally attributed to the odd-bounce mechanism. A recently-proposed form of Bragg scattering provides a conceptual model to explain these observations. However, detailed electromagnetic backscatter modelling is still needed to provide a new and reliable understanding of backscattering from wetlands with emergent vegetation. © 2014 IEEE.

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