بهبود تخمین ارتفاع جنگل به کمک بهینه سازی ماتریس پراکنش به روش تغییر پایه پلاریزاسیون مطالعه موردی: جنگل های شمالی سوئد

نوع مقاله: مقاله پژوهشی

نویسندگان

1 دانشجوی دکتری، دانشکده مهندسی نقشه برداری- دانشگاه صنعتی خواجه نصیرالدین طوسی

2 استاد،دانشکده مهندسی نقشه برداری- دانشگاه صنعتی خواجه نصیرالدین طوسی

3 استادیار،دانشکده مهندسی نقشه برداری- دانشگاه صنعتی خواجه نصیرالدین طوسی

چکیده

در دهههای اخیر توجه زیادی به تخمین زیست توده جنگلی شده است. تهیه نقشههای جامع و صحیح از زیست توده جنگلی جهت مدل کردن چرخه کربن جهانی و کاهش گازهای گلخانهای از اهمیت بسیار زیادی برخوردار است. روشهای قدیمی برای تخمین زیست توده براساس مقادیر بازپراکنشها به کمک آنالیزهای رگرسیون صورت میپذیرفت. مشکل اصلی این روشها، سطح اشباع پایین آنها در طول موجها و پلاریزاسیونهای مختلف بدلیل در نظر نگرفتن پارامترهای ساختاری بود. به کمک تکنیکهای اینترفرومتری، تحقیقات به سمت استخراج پارامترهای ساختاری سوق پیدا کرد. ارتفاع یکی از پارامترهای ساختاری میباشد که جهت تخمین زیست توده جنگلی میتواند استفاده شود. بهبود روشهای بازیابی ارتفاع درختان نقش بسیار مهمی در استخراج صحیح زیست توده جنگلی ایفا میکند. در این مقاله یک روش جدید به منظور بهینهسازی ماتریس پراکنش به کمک تغییر پایه پلاریزاسیون جهت تخمین ارتفاع معرفی شده است. به کمک تغییر ماتریس پراکنش در پایه پلاریزاسیونهای مختلف برای هر دو تصویر پایه و پیرو، پارامترهای همبستگی مختلف استخراج شده و با روشهای مختلف تخمین ارتفاع، ارتفاع درختان تخمین زده شده است. دادههای مورد بررسی، دادههای تمام پلاریمتری از سنجنده هوایی SETHI در باند P میباشد که در منطقه جنگلهای شمالی واقع در Remningstorp در جنوب کشور سوئد برداشت شده است. نتایج نشان میدهد که روشهایی که در آنها تغییر فاز وجود دارد در اثر تغییر پارامترهای هندسی بیضوی، بهبود چشمگیری داشتهاند بطوری که روشهای فاز حجم تصادفی برروی زمین با  76/0= R2 و76/3 = RMSE و تفاضلی مدل رقومی با 69/0-= R2 بهترین بهبود در نتایج را داشتهاند و روش وارونگی دامنه همدوسی که با مقدار کوهرنس ارتفاع را استخراج میکند، با 17/0= R2 بهبود چندانی در نتایج آن ملاحظه نشده است.

کلیدواژه‌ها


عنوان مقاله [English]

Improvement of Forest Height Estimation using Scattering Matrix Optimization by Altering Polarization Bases Case Study: Swedish Boreal forests

نویسندگان [English]

  • Samira Hosseini 1
  • Hamid Ebadi 2
  • Yasser Maghsoudi 3
1 PhD student of photogrammetry, Faculty of Geomatics Eng, K.N.T.University of technology
2 Professor, Faculty of Geomatics Eng, K.N.T.University of technology
3 Assistant Professor, Faculty of Geomatics Eng,K.N.T.University of technology
چکیده [English]

Extended Abstract
Introduction
Estimation of forest biomass has received much attention in recent decades including assessing the capability of different sensor data (e.g., optical, radar, and LiDAR)and the development of advanced techniques such as synthetic aperture radar (SAR),polarimetry and polarimetric SAR interferometry for forest biomass estimation. Accurate estimation of forest biomass is of vital importance to model global carbon cycle. Deforestation and forest degradation will result in the loss of forest biomass and consequently increases the greenhouse gases. Radar systems including SAR have a great potential to quantify biomass and structural diversity because of its penetration capability. These systemsare also independent of weather and external illumination condition and can be designed for different frequencies and resolutions.Moreover, SAR systems operating at lower frequencies such as L- and P-band have shown relatively good sensitivity to forest biomass. Regression analysis is among thecommon methods for evaluation forest biomass which have been investigated for many years on different areas. This analysis is based on the correlation between backscattering coefficient values and the forest biomass. However, previous studies demonstratedthat such approaches are very simple and they do not consider structural effects of different species. One of the restrictions and limitations of these methods is the low saturation level. The level of saturation is lower in higher frequencies and vice versa. Considering the structural parameters, researchers have tried to use the interferometry techniques.Forest canopy height is one of the important parameters that can be used to estimate Above Ground Biomass (AGB) using allometric equations.
 
Materials &Methods
Recentforest height retrieval methods rely on model based interferometric SAR analysis. The random volume over ground (RVOG) model is one of the most common algorithms. This method considers two layers, one for the ground under the vegetation and one for the volumetric canopy. This model has been investigated in different forest environments (e.g. tropical, temperate and boreal forests). Estimation of forest biomass based on forest height using allometric equations can overcome radar signal saturation to some extent.Improvement of Forest height estimation can play an important role to retrieve accurate forest biomass estimation. In this paper, a new method using scattering matrix optimization is introduced to extract forest height by changing polarization bases. Scattering matrices for slave and master images have been extracted by changing polarization bases. Then polarimetric interferometry coherences have been calculated and forest height was estimated by various forest height methods including DEM Difference, coherence amplitude inversion, RVOG Phase, Combined and RVOG.
 
 
Results& Discussion
P-band full Polarimetric synthetic aperture radar (SAR) images acquired by SETHI sensor over Remningstorp (a boreal forest in south of Sweden) were investigated for forest biomass estimation.Mean of Lidar height values which fall in each shapefile was used to check corresponding results with the heights of retrieval methods.
The results of tree height retrieval methods without changing polarization bases between PolInSAR tree height and LIDAR height show that three methods including coherence amplitude inversion, RVOG Phase and RVOG have low R2 value.  DEM Difference and combined methods yielded better results in comparison with the other three aforementioned methods; however the results are not satisfactory.DEM Difference method underestimated the tree height compared to that of LIDAR. This is perhaps due to the fact that volume phase center does not lie at the top of the tree.Temporal decorrelation decreases volume correlation, consequently small values in the SINC function lead to generate large values in results; therefore RMSE of coherence amplitude method is relatively high.New master and slave scattering matrices in arbitrary polarization basis were extracted by alteringandin transformation matrix.Results show that RVOG phase has the best result with R2=0.76 and RMSE=3.76. Following this method, DEM difference method shows R2=-0.69.It is likely that methods which include phase information by changing geometricalparameters, in transformation matrix (e.g. RVOG phase and DEM difference) significantly increase the tree height accuracy.sOn the other hand, methods that only apply magnitude of coherence such as coherence amplitude method do not show notable improvementfor retrieving tree height.
 
Conclusion
Robustness of forest height estimation using Scattering Matrix Optimization by changing Polarization Bases was studied in this paper.PolInSAR data was acquired by SETHI on Remningstorp, a boreal forest in south of Sweden. Results indicated that forest height retrieval methods which included phase parameter shows remarkable improvement by changing the geometrical parameters for height estimation.Therefore RVOG phase method with R2=0.76, RMSE=3.76m and DEM Difference method with R2=-0.69 gave the best results, whereas coherence amplitude method which only included magnitude of coherence with R2=0.17 showed the lowest correlation.
 

کلیدواژه‌ها [English]

  • PolInSAR
  • Height Estimation
  • Transformation Matrix
  • Optimization
  • Scattering Matrix

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