Using Multitemporal Sentinel-1 C-band Backscatter to Monitor Phenology and Classify Deciduous and Coniferous Forests in Northern Switzerland
"> Figure 1
<p>The five investigated main study areas are located within the yellow rectangles in the Cantons of Zurich (ZH) and Bern (BE) in northern Switzerland. For some analyses, the entire forested area within the Canton of Zurich, depicted in the green rectangle, was used. The background aerial images were provided by the Swiss Federal Office of Topography swisstopo [<a href="#B44-remotesensing-10-00055" class="html-bibr">44</a>]. Reproduced by permission of swisstopo (BA17116). Swiss map coordinates: CH1903+/LV95.</p> "> Figure 2
<p>Processing scheme of the SAR data from S-1 Ground Range Detected High Resolution (GRDH) images to geometrically terrain corrected (GTC) images and local resolution weighting (LRW) composites. GRDH images of both polarisations were processed in this manner.</p> "> Figure 3
<p>Processing chain for deriving the backscatter descriptors from the geometrically terrain corrected (GTC) images and local resolution weighting (LRW) composite time series for different forest types and species. The descriptors listed at the bottom right (defined in <a href="#sec2dot3dot4-remotesensing-10-00055" class="html-sec">Section 2.3.4</a> and <a href="#remotesensing-10-00055-t004" class="html-table">Table 4</a>) were derived from both polarisations.</p> "> Figure 4
<p>Classification scheme. This framework was used for both <span class="html-italic">forest type</span> and <span class="html-italic">species</span> classifications.</p> "> Figure 5
<p>Evolution of the number of acquisitions available for the LRW compositing. The number of acquisitions available varied significantly for the study areas 1–2 in ZH and 3–5 in BE. Especially in summer 2015 but also in summer 2016, few acquisitions were available. Generally, more acquisitions were available for the study areas in BE, and the number of available acquisitions increased towards the end of the investigated period in all study areas.</p> "> Figure 6
<p>RGB-overlays of (<b>a</b>) <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mi>E</mi> <mn>0</mn> </msubsup> </semantics> </math> images and (<b>b</b>) <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mrow> <mi>L</mi> <mi>R</mi> <mi>W</mi> </mrow> <mn>0</mn> </msubsup> </semantics> </math> composites. The assigned bands are Red and Blue = summer and leaf-on (<math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mi>E</mi> <mn>0</mn> </msubsup> </semantics> </math>: 6 June 2016, <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mrow> <mi>L</mi> <mi>R</mi> <mi>W</mi> </mrow> <mn>0</mn> </msubsup> </semantics> </math>: 24 May–16 June 2016) , Green = end of winter and leaf-off (<math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mi>E</mi> <mn>0</mn> </msubsup> </semantics> </math>: 16 March 2017, <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mrow> <mi>L</mi> <mi>R</mi> <mi>W</mi> </mrow> <mn>0</mn> </msubsup> </semantics> </math>: 2 March–25 March 2017). The range of the backscatter values (−20 to −10 dB) was set identically for all channels in both RGB-overlays. Non-forested regions are masked out. In the lower row, maps of the main influencing factors on the backscatter are depicted. The slope map (<b>c</b>) of the area is shown as well as (<b>d</b>) the forest type map with the classes ‘deciduous’, ‘coniferous’ and ‘not defined’, consisting of all non-forested areas and areas with less than 50% homogenous species (Data source: GIS-ZH). The top row images contain modified Copernicus Sentinel data [2016/2017]. Swiss map coordinates: CH1903+/LV95.</p> "> Figure 7
<p>Correlation density plots comparing end-of-winter and summer VH backscatter of (<b>a</b>) coniferous and (<b>b</b>) deciduous forest in the whole of the Canton of Zurich for the <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mrow> <mi>L</mi> <mi>R</mi> <mi>W</mi> </mrow> <mn>0</mn> </msubsup> </semantics> </math> composites. The number of pixels (n) was considerably higher for coniferous forests. A higher Pearson correlation coefficient (<math display="inline"> <semantics> <mi>ρ</mi> </semantics> </math>) was achieved for deciduous forest.</p> "> Figure 8
<p>Histograms of the difference between end-of-winter and summer VH backscatter of the two forest types in the whole of the Canton of Zurich for (<b>a</b>) the <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mi>E</mi> <mn>0</mn> </msubsup> </semantics> </math> images and (<b>b</b>) the <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mrow> <mi>L</mi> <mi>R</mi> <mi>W</mi> </mrow> <mn>0</mn> </msubsup> </semantics> </math> composites. <span class="html-italic">n</span> = 36,621 for deciduous forest, 152,930 for coniferous forest.</p> "> Figure 9
<p>Temporal plots of species backscatter from <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mrow> <mi>L</mi> <mi>R</mi> <mi>W</mi> </mrow> <mn>0</mn> </msubsup> </semantics> </math> composites for selected study areas. (<b>a</b>–<b>e</b>) Blue shows the median, green corresponds to the 25 and 75% percentiles. The mean temperature for the 24 day period corresponding to each <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mrow> <mi>L</mi> <mi>R</mi> <mi>W</mi> </mrow> <mn>0</mn> </msubsup> </semantics> </math> composite is shown in solid red. The horizontal dotted red lines mark the 0 °C line: forest backscatter was generally reduced when the temperature dipped below 0 °C. The solid vertical lines show phenological ground observations. Vertical green lines indicate leaf emergence for beech, and needle shoot for spruce. Red lines show the leaf fall date for beech. Dashed lines correspond to the backscatter derived break dates. The black bars on top of the figure display the periods used for the calculation of winter (1 December–15 March) and summer (1 June–15 September) medians.</p> "> Figure 10
<p>Map of the <span class="html-italic">species</span> classification for a subset of the study area in Rafz (ZH). It can be observed that the segmentation into the forest types ‘deciduous’ (Beech, Oak) and ‘coniferous’ (Spruce) is possible to a high accuracy. Confusion is seen between the two deciduous species shown in green tones. The background aerial image SWISSIMAGE was provided by the Swiss Federal Office of Topography swisstopo [<a href="#B44-remotesensing-10-00055" class="html-bibr">44</a>]. Reproduced by permission of swisstopo (BA17116). Swiss map coordinates: CH1903+/LV95.</p> "> Figure 11
<p>Strength of individual predictors in the two classifications. The loss in prediction accuracy for each predictor is shown when the values of that predictor are distributed randomly across the pixels [<a href="#B70-remotesensing-10-00055" class="html-bibr">70</a>]. The predictors are the total annual medians (<math display="inline"> <semantics> <mover accent="true"> <mi>x</mi> <mo stretchy="false">˜</mo> </mover> </semantics> </math>To) and <math display="inline"> <semantics> <mi>δ</mi> </semantics> </math>s of the two years for the two polarisations. The break dates (BD) extracted from the VH time series for both years complete the set of predictors.</p> "> Figure 12
<p>Map of the <span class="html-italic">species</span> classification for a subset of the study area in Rafz (ZH). The impact of logging activities on the classification result can be observed. A former logged spruce stand was mainly classified as beech or oak. The background aerial image SWISSIMAGE was provided by the Swiss Federal Office of Topography swisstopo [<a href="#B44-remotesensing-10-00055" class="html-bibr">44</a>]. Reproduced by permission of swisstopo (BA17116). Swiss map coordinates: CH1903+/LV95.</p> "> Figure 13
<p>Temporal plots of the per <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mrow> <mi>L</mi> <mi>R</mi> <mi>W</mi> </mrow> <mn>0</mn> </msubsup> </semantics> </math> composite calculated spatial descriptors median (<b>blue</b>), 25 and 75% percentiles (<b>green</b>) for decidous and coniferous forests of the Alpine region (Lat/Lon: 43.5–49° N/5.5–17.5° E). The temporal resolution of the <math display="inline"> <semantics> <msubsup> <mi>γ</mi> <mrow> <mi>L</mi> <mi>R</mi> <mi>W</mi> </mrow> <mn>0</mn> </msubsup> </semantics> </math> composites was 12 days and the time period depicted ranges from October 2014 to May 2017. The number of investigated pixels (n) was comparable for both forest types.</p> ">
Abstract
:1. Introduction
2. Materials and Methods
2.1. Study Areas
2.2. Data
2.3. Methods
2.3.1. SAR Data Processing
2.3.2. Species Mask Generation
2.3.3. Comparison of an End-of-Winter and Summer GTC Image () and LRW Composite ()
2.3.4. Time Series Analysis of LRW Composites ()
2.3.5. Classification of Species and Forest Type
3. Results
3.1. Comparison of the Seasonal GTC Images () and LRW Composites ()
3.2. Time Series Analysis of LRW Composites ()
3.2.1. Deciduous Species (Beech and Oak)
3.2.2. Spruce
3.3. Classification Performances
4. Discussion
4.1. Seasonal Signature of Deciduous Species
4.2. Break Date Extraction to Monitor Phenology
4.3. Seasonal Signature of Spruce
4.4. Differences between the Investigated Years
4.5. Comparisons to Other Studies
4.6. Classification of Forest Types and Species
4.7. Implications of the Findings
5. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
Abbreviations
BD | Break date |
BE | Canton of Bern (Switzerland) |
Ca | Canton |
CORINE | Coordination of Information on the Environment |
CUSUM | Cumulative sum |
DBH | Diameter at breast height |
DTM | Digital terrain model |
DLR | German aerospace centre |
ESA | European space agency |
GDAL | Geospatial data abstraction library |
GRDH | Ground Range Detected High Resolution (standard level 1 Sentinel-1 backscatter product) |
GTC | Geometrically terrain corrected |
HH | Horizontal transmitted, horizontal received (polarisation) |
IW | Interferometric Wide swath (Sentinel-1) |
LAI | Leaf area index |
LE | Leaf emergence |
LF | Leaf fall |
LRW | Local resolution weighting |
MODIS | Moderate Resolution Imaging Spectroradiometer |
NDVI | Normalised Difference Vegetation Index |
OA | Overall accuracy |
OLS | Ordinary least squares |
PA | Producer’s accuracy |
RF | Random forest |
RTC | Radiometrically terrain corrected |
SAR | Synthetic aperture radar |
SVM | Support vector machine |
S-1 | Sentinel-1 |
UA | User’s accuracy |
VH | Vertical transmitted, horizontal received (polarisation) |
VV | Vertical transmitted, vertical received (polarisation) |
ZH | Canton of Zurich (Switzerland) |
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No. | Study Area (Ca) | Species | Elevation (m a.s.l.) | Slope (°) | Core Extent (ha) | DBH (cm) |
---|---|---|---|---|---|---|
1 | Rafz (ZH) | Spruce | 498 | 6.2 | 23.0 | ≥51 |
Beech | 474 | 12.8 | 13.5 | 41–50 | ||
Oak | 466 | 11.5 | 14.9 | 41–50 | ||
2 | Teufen (ZH) | Spruce | 596 | 12 | 46.7 | 41–50 |
Beech | 602 | 15.5 | 15.7 | 41–50 | ||
3 | Galsberg (BE) | Beech | 507 | 15.4 | 7.6 | 41–50 |
4 | Feiberg (BE) | Beech | 509 | 16.5 | 12.7 | ≥51 |
5 | Büren a.d.A. (BE) | Oak | 496 | 2.3 | 6.5 | 31–40 |
Specification | Value |
---|---|
Swath width | 250 km |
GRDH sample distance | 10 m |
Nominal incident angle range | 31°–46° |
Radiometric stability | 0.5 dB (3) |
Radiometric accuracy | 1 dB (3) |
Process | Acronym | Backscatter Product | Variable |
---|---|---|---|
Geometric terrain correction [50,51] | GTC | Ellipsoid-referenced gamma nought | |
Radiometric terrain correction [52] | RTC | Terrain-flattened gamma nought | |
Local resolution weighting [53] | LRW | Composite terrain-flattened gamma nought |
Descriptors | Symbols | |
---|---|---|
Within single composite | Median, 25 and 75% percentiles | , Q1, Q3 |
Within time series | Median of and of Q1 & Q3 | To, Q1To, Q3To |
Median winter of , median summer of | , | |
(see Equation (1)), break dates | , BD1, BD2 |
Forest Type | End-of-Winter | Summer | |||||||
---|---|---|---|---|---|---|---|---|---|
Backscatter (dB) | Backscatter (dB) | ||||||||
Q1 | Q3 | Q1 | Q3 | ||||||
Deciduous forest | VH | −14.0 | −12.3 | −10.6 | −15.0 | −13.3 | −11.6 | ||
VV | −9.3 | −7.6 | −6.0 | −9.1 | −7.2 | −5.4 | |||
Coniferous forest | VH | −15.7 | −14.2 | −12.6 | −14.7 | −13.2 | −11.7 | ||
VV | −10.3 | −8.8 | −7.3 | −9.0 | −7.5 | −5.9 |
SA | S | N | To | Q1To | Q3To | 2015/16 | 2016/17 | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Sp | 72 | 2303 | −14.08 | −15.00 | −13.22 | −14.28 | −14.10 | −0.18 | −15.40 | −13.25 | −2.15 | |
2 | Sp | 72 | 4583 | −14.02 | −15.00 | −13.09 | −14.03 | −13.96 | −0.07 | −15.01 | −13.11 | −1.90 | |
1 | Be | 72 | 1348 | −13.05 | −13.87 | −12.24 | −12.53 | −14.14 | 1.61 | −13.37 | −12.88 | −0.49 | |
2 | Be | 72 | 1569 | −12.69 | −13.48 | −11.83 | −11.80 | −13.76 | 1.97 | −12.61 | −12.82 | 0.21 | |
3 | Be | 73 | 757 | −12.58 | −13.35 | −11.87 | −11.67 | −14.00 | 2.33 | −12.12 | −12.77 | 0.65 | |
4 | Be | 73 | 1271 | −12.54 | −13.23 | −11.82 | −11.56 | −13.87 | 2.31 | −12.23 | −12.54 | 0.30 | |
1 | Oa | 72 | 1485 | −12.97 | −13.62 | −12.26 | −11.98 | −13.94 | 1.96 | −12.84 | −13.23 | 0.38 | |
5 | Oa | 73 | 639 | −13.09 | −13.71 | −12.51 | −12.17 | −13.62 | 1.45 | −12.64 | −13.24 | 0.60 |
SA | S | N | To | Q1To | Q3To | 2015/16 | 2016/17 | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Sp | 72 | 2303 | −8.46 | −9.25 | −7.62 | −8.72 | −7.91 | −0.81 | −9.62 | −7.76 | −1.86 | |
2 | Sp | 72 | 4585 | −8.46 | −9.39 | −7.59 | −8.68 | −7.95 | −0.74 | −9.52 | −7.75 | −1.78 | |
1 | Be | 72 | 1348 | −7.75 | −8.54 | −6.98 | −7.73 | −8.23 | 0.51 | −8.43 | −7.49 | −0.93 | |
2 | Be | 72 | 1569 | −7.34 | −8.14 | −6.41 | −7.22 | −7.83 | 0.61 | −7.77 | −7.28 | −0.48 | |
3 | Be | 73 | 757 | −7.07 | −7.86 | −6.38 | −6.85 | −7.44 | 0.59 | −7.28 | −7.08 | −0.20 | |
4 | Be | 73 | 1271 | −7.11 | −7.83 | −6.34 | −6.92 | −7.37 | 0.44 | −7.28 | −6.85 | −0.43 | |
1 | Oa | 72 | 1485 | −7.45 | −8.13 | −6.71 | −7.13 | −7.89 | 0.76 | −7.98 | −7.60 | −0.38 | |
5 | Oa | 73 | 639 | −7.70 | −8.26 | −7.15 | −7.79 | −7.43 | −0.36 | −7.98 | −7.63 | −0.35 |
Study Area | Species | 2015 | 2016 | |||||||
---|---|---|---|---|---|---|---|---|---|---|
BD1 | LE | BD2 | LF | BD1 | LE | BD2 | LF | |||
Rafz (ZH) | Beech | 121 ± 12 | 110 | 301 ± 12 | 300 | 61 ± 12 | 104 | 109 ± 12 | 308 | |
Galsberg (BE) | Beech | 109 ± 12 | 106 | 301 ± 12 | 313 | 97 ± 12 | 111 | 301 ± 12 | 304 | |
Feiberg (BE) | Beech | 109 ± 12 | 106 | 301 ± 12 | 313 | 97 ± 12 | 111 | 301 ± 12 | 304 | |
Rafz (ZH) | Oak | 121 ± 12 | - | 301 ± 12 | - | 109 ± 12 | - | 301 ± 12 | - | |
Büren a.d.A. (BE) | Oak | 205 ± 12 | - | 313 ± 12 | - | 121 ± 12 | - | 313 ± 12 | - |
Reference | n Training | Confusion Matrix | ||||
---|---|---|---|---|---|---|
Classification | PA | OA | ||||
Coniferous | Deciduous | |||||
Coniferous | 19982 | 7675 | 1028 | 0.88 | ||
Deciduous | 20006 | 1415 | 7655 | 0.84 | ||
UA | 0.84 | 0.88 |
Reference | n Training | Confusion Matrix | |||||
---|---|---|---|---|---|---|---|
Classification | PA | OA | |||||
Spruce | Beech | Oak | |||||
Spruce | 2004 | 858 | 78 | 42 | 0.88 | ||
Beech | 2003 | 121 | 522 | 285 | 0.56 | ||
Oak | 1995 | 54 | 239 | 699 | 0.70 | ||
UA | 0.83 | 0.62 | 0.68 |
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Rüetschi, M.; Schaepman, M.E.; Small, D. Using Multitemporal Sentinel-1 C-band Backscatter to Monitor Phenology and Classify Deciduous and Coniferous Forests in Northern Switzerland. Remote Sens. 2018, 10, 55. https://doi.org/10.3390/rs10010055
Rüetschi M, Schaepman ME, Small D. Using Multitemporal Sentinel-1 C-band Backscatter to Monitor Phenology and Classify Deciduous and Coniferous Forests in Northern Switzerland. Remote Sensing. 2018; 10(1):55. https://doi.org/10.3390/rs10010055
Chicago/Turabian StyleRüetschi, Marius, Michael E. Schaepman, and David Small. 2018. "Using Multitemporal Sentinel-1 C-band Backscatter to Monitor Phenology and Classify Deciduous and Coniferous Forests in Northern Switzerland" Remote Sensing 10, no. 1: 55. https://doi.org/10.3390/rs10010055