Using landsat spectral indices in time-series to assess wildfire disturbance and recovery

Satellite earth observation is being increasingly used to monitor forests across the world. Freely available Landsat data stretching back four decades, coupled with advances in computer processing capabilities, has enabled new time-series techniques for analyzing forest change. Typically, these methods track individual pixel values over time, through the use of various spectral indices. This study examines the utility of eight spectral indices for characterizing fire disturbance and recovery in sclerophyll forests, in order to determine their relative merits in the context of Landsat time-series. Although existing research into Landsat indices is comprehensive, this study presents a new approach, by comparing the distributions of pre and post-fire pixels using Glass's delta, for evaluating indices without the need of detailed field information. Our results show that in the sclerophyll forests of southeast Australia, common indices, such as the Normalized Difference Vegetation Index (NDVI) and the Normalized Burn Ratio (NBR), both accurately capture wildfire disturbance in a pixel-based time-series approach, especially if images from soon after the disturbance are available. However, for tracking forest regrowth and recovery, indices, such as NDVI, which typically capture chlorophyll concentration or canopy 'greenness', are not as reliable, with values returning to pre-fire levels in 3-5 years. In comparison, indices that are more sensitive to forest moisture and structure, such as NBR, indicate much longer (8-10 years) recovery timeframes. This finding is consistent with studies that were conducted in other forest types. We also demonstrate that additional information regarding forest condition, particularly in relation to recovery, can be extracted from less well known indices, such as NBR2, as well as textural indices incorporating spatial variance. With Landsat time-series gaining in popularity in recent years, it is critical to understand the advantages and lim

Detecção Automática de Alterações de Coberto Vegetal em Áreas de Interface Urbano-Rural

Para fazer face aos incêndios florestais o SDFCI estabelece faixas de gestão de combustíveis como forma de auxiliar o combate e mitigação deste problema. O objetivo do trabalho foi elaborar um modelo capaz de identificar a remoção da vegetação nestas faixas através da análise do NDVI em séries temporais de imagens Sentinel 2. O modelo busca diferenças estatisticamente significantes, através do Welch t-test, nas informações contidas nas imagens. O modelo foi aplicado no concelho de Figueiró dos Vinhos e os resultados mostraram-se promissores na identificação de áreas onde não foi feita a gestão, ou seja, áreas de infração a legislação.info:eu-repo/semantics/publishedVersio

Detecção automática de alterações de coberto vegetal em áreas de interface urbano-rural

Para fazer face aos incêndios florestais o Sistema de Defesa da Floresta contra Incêndios estabelece faixas de gestão de combustíveis como forma de auxiliar o combate e mitigação deste problema. O objetivo do trabalho foi elaborar um modelo capaz de identificar o controlo da biomassa nestas faixas através da análise do NDVI em séries temporais de imagens Sentinel 2. O modelo busca diferenças estatisticamente significativas, através do Welch t-test, nas informações contidas nas imagens. O modelo foi aplicado no concelho de Figueiró dos Vinhos e os resultados mostraram-se promissores na identificação de áreas onde não foi feita a gestão, ou seja, áreas de infração à legislação.To deal with forest fires, the Forest Defense System Against Fire establishes fuel management buffers as a way to help combat and mitigate this problem. The objective of the study was to develop a model capable of identifying the fuel management in these areas by analysing the NDVI in time series of Sentinel 2 images. The model seeks statistically significant differences, through the Welch t-test, in the information contained in the images. The model was applied in the municipality of Figueiró dos Vinhos and the results were promising in identifying areas where management was not carried out, i.e., areas of infringement of legislation.info:eu-repo/semantics/publishedVersio

Assessing post-fire forest structure recovery by combining LiDAR data and Landsat time series in Mediterranean pine forests

Understanding post-fire recovery dynamics is critical for effective management that enhance forest resilience to fire. Mediterranean pine forests have been largely affected by wildfires, but the impacts of both changes in land use and climate endanger their capacity to naturally recover. Multispectral imagery is commonly used to estimate post-fire recovery, yet changes in forest structure must be considered for a comprehensive evaluation of forest recovery. In this research, we combine Light Detection And Ranging (LiDAR) with Landsat imagery to extrapolate forest structure variables over a 30-year period (1990?2020) to provide insights on how forest structure has recovered after fire in Mediterranean pine forests. Forest recovery was evaluated attending to vegetation cover (VC), tree cover (TC), mean height (MH) and heterogeneity (CVH). Structure variables were derived from two LiDAR acquisitions from 2016 and 2009, for calibration and independent spatial and temporal validation. A Support Vector Regression model (SVR) was calibrated to extrapolate LiDAR-derived variables using a series of Landsat imagery, achieving an R2 of 0.78, 0.64, 0.70 and 0.63, and a relative RMSE of 24.4%, 30.2%, 36.5% and 27.4% for VC, TC, MH and CVH, respectively. Models showed to be consistent in the temporal validation, although a wider variability was observed, with R2 ranging from 0.51 to 0.74. A different response to fire was revealed attending to forest cover and height since vegetation cover recovered to a pre-fire state but mean height did not 26-years after fire. Less than 50% of the area completely recovered to the pre-fire structure within 26 years, and the area subjected to fire recurrence showed signs of greater difficulty in initiating the recovery. Our results provide valuable information on forest structure recovery, which can support the implementation of mitigation and adaptation strategies that enhance fire resilience.Comunidad de Madri

Tracking Rates of Forest Disturbance and Associated Carbon Loss in Areas of Illegal Amber Mining in Ukraine Using Landsat Time Series

Mapping forest disturbance is crucial for many applications related to decision-making for sustainable forest management. This study identified the effect of illegal amber mining on forest change and accumulated carbon stock across a study area of 8125.5 ha in northern Ukraine. Our method relies on the Google Earth Engine (GEE) implementation of the Landsat-based Detection of Trends in Disturbance and Recovery (LandTrendr) temporal segmentation algorithm of Landsat time-series (LTS) to derive yearly maps of forest disturbance and recovery in areas affected by amber extraction operations. We used virtual reality (VR) 360 interactive panoramic images taken from the sites to attribute four levels of forest disturbance associated with the delta normalized burn ratio (dNBR) and then calculated the carbon loss. We revealed that illegal amber extraction in Ukraine has been occurring since the middle of the 1990s, yielding 3260 ha of total disturbed area up to 2019. This study indicated that the area of forest disturbance increased dramatically during 2013–2014, and illegal amber operations persist. As a result, regrowth processes were mapped on only 375 ha of total disturbed area. The results were integrated into the Forest Stewardship Council® (FSC®) quality management system in the region to categorize Forest Management Units (FMUs) conforming to different disturbance rates and taking actions related to their certification status. Moreover, carbon loss evaluation allows the responsible forest management systems to be streamlined and to endorse ecosystem service assessment

Mapeamento Mensal do Estado da Vegetação no Âmbito das Políticas Públicas de Dados Abertos do projeto SMOS: Mapas Intra-Anuais do Estado da Vegetação direcionados para o público em geral e produzidos com imagens Sentinel-2

Dissertation presented as the partial requirement for obtaining a Master's degree in Geographic Information Systems and ScienceEste trabalho concentra num documento as partes principais da pesquisa, construção e justificação de um dos novos produtos do projeto SMOS, os Mapas Intra-Anuais do Estado da Vegetação. A DGT, dentro das suas competências, já produz uma Carta de Uso e Ocupação do Solo (COS) com uma periodicidade de 3 anos, e uma COS Simplificada (COSc) com periodicidade de 1 ano. Estes novos produtos pretendem preencher a necessidade da avaliação intra-anual (mensal) do estado da vegetação, num produto capaz de captar a variabilidade da paisagem. Fundamentalmente, este trabalho pergunta: “Como avaliar intra-anualmente o estado da vegetação?” de Portugal continental, usando imagens de satélite e atendendo a critérios como a maximização da especificidade espacial, da precisão e independência dos dados e minimizando os custos e a redundância dos produtos. A metodologia é pragmática, interessada maioritariamente nos melhores resultados possíveis, e passa pela utilização de imagens do satélite Sentinel-2, recolhidas do servidor da THEIA, as quais são utilizadas para produzir compósitos mensais que por sua vez são a base dos MIAEV. Os produtos resultantes são mensuravelmente capazes de detetar e avaliar qualitativamente o estado da vegetação, provando-se úteis no acompanhamento do estado de culturas intra-anuais, no destacar de mudanças bruscas no estado da vegetação e na deteção de anomalias em relação a um período homólogo. Estas contribuições, nomeadamente a disponibilização de uma série temporal de imagens desde 2017 até 2022 atualizada em tempo quase real, estabelecem uma base sólida e factual de produtos capazes de avaliar intra-anualmente o estado da vegetação, permitindo a um qualquer indivíduo ou instituição explorar e apresentar estudos mais específicos.This document is a summary of the all the work, research, and reasoning behind the new SMOS products, the MIAEV. DGT already produces products like the “Carta de Uso e Ocupação do Solo” (COS), every 3 years, and “COS Simplificada” (COSc), every year. These new products (MIAEV) aim to fill the need of a more frequent evaluation of the vegetation status, more specifically on a month-to-month basis. At its core, this document asks: “How can vegetation status be measured intra-annually?” within the spatial scope of the entire country, using satellite imagery and taking into account the maximization of spatial resolution, of the precision and independence of data while at the same time minimizing production costs and any redundancy that might arise. Methodology follows a pragmatic viewpoint, aiming to produce the best possible results, and begins in the collection of Sentinel-2 imagery from THEIA’s server, imagery that is then used to produce monthly composites that become the foundation of the MIAEV. The resulting products are measurably capable of detecting and evaluating the status of vegetation, presenting as particularly useful for the tracking of the status of intra-annual cultures, the highlighting of sudden changes in vegetation status and the detection of vegetation status anomalies when compared to an homologous period. These contributions, in particular the availability of a temporal series of satellite imagery from 2017 to 2022 that is updated in close to real-time, establish a solid and factual foundation of products capable of evaluating the vegetation status on a monthly basis, that in itself provides freedom for the exploration of more specific studies by any individual or institution

Landcover and crop type classification with intra-annual times series of sentinel-2 and machine learning at central Portugal

Dissertation submitted in partial fulfilment of the requirements for the degree of Master of Science in Geospatial TechnologiesLand cover and crop type mapping have benefited from a daily revisiting period of sensors such as MODIS, SPOT-VGT, NOAA-AVHRR that contains long time-series archive. However, they have low accuracy in an Area of Interest (ROI) due to their coarse spatial resolution (i.e., pixel size > 250m). The Copernicus Sentinel-2 mission from the European Spatial Agency (ESA) provides free data access for Sentinel 2-A(S2a) and B (S2b). This satellite constellation guarantees a high temporal (5-day revisit cycle) and high spatial resolution (10m), allowing frequent updates on land cover products through supervised classification. Nevertheless, this requires training samples that are traditionally collected manually via fieldwork or image interpretation. This thesis aims to implement an automatic workflow to classify land cover and crop types at 10m resolution in central Portugal using existing databases, intra-annual time series of S2a and S2b, and Random Forest, a supervised machine learning algorithm. The agricultural classes such as temporary and permanent crops as well as agricultural grasslands were extracted from the Portuguese Land Parcel Identification System (LPIS) of the Instituto de Financiamento da Agricultura e Pescas (IFAP); land cover classes like urban, forest and water were trained from the Carta de Ocupação do Solo (COS) that is the national Land Use and Land Cover (LULC) map of Portugal; and lastly, the burned areas are identified from the corresponding national map of the Instituto da Conservação da Natureza e das Florestas (ICNF). Also, a set of preprocessing steps were defined based on the implementation of ancillary data allowing to avoid the inclusion of mislabeled pixels to the classifier. Mislabeling of pixels can occur due to errors in digitalization, generalization, and differences in the Minimum Mapping Unit (MMU) between datasets. An inner buffer was applied to all datasets to reduce border overlap among classes; the mask from the ICNF was applied to remove burned areas, and NDVI rule based on Landsat 8 allowed to erase recent clear-cuts in the forest. Also, the Copernicus High-Resolution Layers (HRL) datasets from 2015 (latest available), namely Dominant Leaf Type (DLT) and Tree Cover Density (TCD) are used to distinguish between forest with more than 60% coverage (coniferous and broadleaf) such as Holm Oak and Stone Pine and between 10 and 60% (coniferous) for instance Open Maritime Pine. Next, temporal gap-filled monthly composites were created for the agricultural period in Portugal, ranging from October 2017 till September 2018. The composites provided data free of missing values in opposition to single date acquisition images. Finally, a pixel-based approach classification was carried out in the “Tejo and Sado” region of Portugal using Random Forest (RF). The resulting map achieves a 76% overall accuracy for 31 classes (17 land cover and 14 crop types). The RF algorithm captured the most relevant features for the classification from the cloud-free composites, mainly during the spring and summer and in the bands on the Red Edge, NIR and SWIR. Overall, the classification was more successful on the irrigated temporary crops whereas the grasslands presented the most complexity to classify as they were confused with other rainfed crops and burned areas

A Contribution to land cover and land use mapping: in Portugal with multi-temporal Sentinel-2 data and supervised classification

Dissertation presented as the partial requirement for obtaining a Master's degree in Geographic Information Systems and ScienceRemote sensing techniques have been widely employed to map and monitor land cover and land use, important elements for the description of the environment. The current land cover and land use mapping paradigm takes advantage of a variety of data options with proper spatial, spectral and temporal resolutions along with advances in technology. This enabled the creation of automated data processing workflows integrated with classification algorithms to accurately map large areas with multi-temporal data. In Portugal, the General Directorate for Territory (DGT) is developing an operational Land Cover Monitoring System (SMOS), which includes an annual land cover cartography product (COSsim) based on an automatic process using supervised classification of multi-temporal Sentinel-2 data. In this context, a range of experiments are being conducted to improve map accuracy and classification efficiency. This study provides a contribution to DGT’s work. A classification of the biogeographic region of Trás-os-Montes in the North of Portugal was performed for the agricultural year of 2018 using Random Forest and an intra-annual multi-temporal Sentinel-2 dataset, with stratification of the study area and a combination of manually and automatically extracted training samples, with the latter being based on existing reference datasets. This classification was compared to a benchmark classification, conducted without stratification and with training data collected automatically only. In addition, an assessment of the influence of training sample size in classification accuracy was conducted. The main focus of this study was to investigate whether the use of vi classification uncertainty to create an improved training dataset could increase classification accuracy. A process of extracting additional training samples from areas of high classification uncertainty was conducted, then a new classification was performed and the results were compared. Classification accuracy assessment for all proposed experiments was conducted using the overall accuracy, precision, recall and F1-score. The use of stratification and combination of training strategies resulted in a classification accuracy of 66.7%, in contrast to 60.2% in the case of the benchmark classification. Despite the difference being considered not statistically significant, visual inspection of both maps indicated that stratification and introduction of manual training contributed to map land cover more accurately in some areas. Regarding the influence of sample size in classification accuracy, the results indicated a small difference, considered not statistically significant, in accuracy even after a reduction of over 90% in the sample size. This supports the findings of other studies which suggested that Random Forest has low sensitivity to variations in training sample size. However, the results might have been influenced by the training strategy employed, which uses spectral subclasses, thus creating spectral diversity in the samples independently of their size. With respect to the use of classification uncertainty to improve training sample, a slight increase of approximately 1% was observed, which was considered not statistically significant. This result could have been affected by limitations in the process of collecting additional sampling units for some classes, which resulted in a lack of additional training for some classes (eg. agriculture) and an overall imbalanced training dataset. Additionally, some classes had their additional training sampling units collected from a limited number of polygons, which could limit the spectral diversity of new samples. Nevertheless, visual inspection of the map suggested that the new training contributed to reduce confusion between some classes, improving map agreement with ground truth. Further investigation can be conducted to explore more deeply the potential of classification uncertainty, especially focusing on addressing problems related to the collection of the additional samples

Using Landsat Spectral Indices in Time-Series to Assess Wildfire Disturbance and Recovery

Satellite earth observation is being increasingly used to monitor forests across the world. Freely available Landsat data stretching back four decades, coupled with advances in computer processing capabilities, has enabled new time-series techniques for analyzing forest change. Typically, these methods track individual pixel values over time, through the use of various spectral indices. This study examines the utility of eight spectral indices for characterizing fire disturbance and recovery in sclerophyll forests, in order to determine their relative merits in the context of Landsat time-series. Although existing research into Landsat indices is comprehensive, this study presents a new approach, by comparing the distributions of pre and post-fire pixels using Glass’s delta, for evaluating indices without the need of detailed field information. Our results show that in the sclerophyll forests of southeast Australia, common indices, such as the Normalized Difference Vegetation Index (NDVI) and the Normalized Burn Ratio (NBR), both accurately capture wildfire disturbance in a pixel-based time-series approach, especially if images from soon after the disturbance are available. However, for tracking forest regrowth and recovery, indices, such as NDVI, which typically capture chlorophyll concentration or canopy ‘greenness’, are not as reliable, with values returning to pre-fire levels in 3–5 years. In comparison, indices that are more sensitive to forest moisture and structure, such as NBR, indicate much longer (8–10 years) recovery timeframes. This finding is consistent with studies that were conducted in other forest types. We also demonstrate that additional information regarding forest condition, particularly in relation to recovery, can be extracted from less well known indices, such as NBR2, as well as textural indices incorporating spatial variance. With Landsat time-series gaining in popularity in recent years, it is critical to understand the advantages and limitations of the various indices that these methods rely on

Using earth observation satellites to explore forest dynamics across large areas

A third of the land on earth is covered by forests. Forests provide valuable resources and essential ecosystem services, including filtering air and water, harbouring biodiversity and managing the carbon cycle. Regular monitoring and reporting across various indicators is necessary to manage forests sustainably. Due to the vastness of forests, satellite Earth observation is one of the most practical and cost-effective ways to monitor forests. The regular and consistent measurements provided from space enable time series analysis, which can reveal trends over time. The temporal, spatial and radiometric depth of the Landsat archive, which extends back to 1972 in some cases, is one of the most useful resources for monitoring forest dynamics across large areas. Analysing forest disturbance and recovery trends using Landsat has recently become widespread, particularly since the opening of the image archive in 2008. However, deriving useful information from the data is challenging on many fronts, including overcoming cloud-cover, differentiating true changes from noise and relating spectral measurements to meaningful outputs. In addition, large data volumes create hurdles for processing and storage. This study presents new techniques for exploiting the Landsat archive in relation to monitoring and measuring forest disturbance and recovery across large areas. Landsat data were processed through a series of steps, analysed in time series, and combined with other data sources to produce mapped outputs and statistical summaries, which can be interpreted by non-experts. The spatial extent of the analysis expands across multiple scales - from local and regional to global (temperate and boreal forests). Firstly, eight Landsat spectral indices were assessed to determine their sensitivity to forest disturbance (caused by wildfire) and recovery in southeast Australian forests. Results indicated that indices making use of the shortwave infrared wavelengths were more reliable indicators of forest disturbance and recovery than indices using only the red and near-infrared wavelengths. Following this exploratory analysis, three indices and two change detection algorithms were evaluated in terms of their ability to detect forest disturbance. Results showed that the LandTrendr algorithm with the Normalised Burn Ratio (NBR) was the most accurate single algorithm/index combination (overall error 21%). However, results were greatly improved by using an ensemble approach. A Random Forests model combining several Landsat-derived metrics with multiple indices, trained with human interpreted reference data, had an overall error of 7%. A notable finding was that priming the training data with confusing cases (commission errors from the change detection algorithms) led to increased accuracy. One Random Forests model was used to create annual forest disturbance maps (1989-2017) across the state of Victoria, Australia. These maps, in conjunction with each pixel's temporal trajectory, were used to extract metrics for spectral disturbance magnitude and recovery length across 2 million ha of burned forest in southeast Australia. The association between disturbance magnitude and forest recovery length, as measured spectrally, was then explored. A novel patch-based technique was used to isolate the disturbance-recovery relationship from confounding factors such as climate, elevation and soil type. The results showed statistically significant differences across bioregions and forest types. The patch-based method demonstrated how Landsat time series can be harnessed to explore ecological changes. The methods developed above were then employed over a much larger area, to investigate trends in fire disturbance and forest recovery in temperate and boreal forests worldwide. This work used both MODIS and Landsat data, through the Google Earth Engine platform, to look at trends in burned area, fire severity and forest recovery across almost 2 billion ha of forests, over the last 18 years. Burned area results showed significant increasing trends in two cases: coniferous forests in Canada and Mediterranean forests in Chile. A significant decreasing trend was found in temperate mixed forests in China. An assessment of fire severity, as measured by Landsat spectral change, highlighted possible trends in a few cases; most notably, the Russian taiga, where increasing severity was observed. An analysis of forest recovery, based on Landsat time series, indicated recovery times were accelerating in many regions. However, given the relatively short time-period analysed, these results should be interpreted with caution. The results presented in this thesis demonstrate the power of Earth observation satellites in monitoring forests at the landscape scale. Although forests are complex systems that are influenced by a myriad of factors, the regular and consistent measurements provided by satellites can be analysed in time series to provide inter-comparable results across large areas. This can broaden our understanding of the dynamic nature of forests, and in doing so, help progress towards their sustainable management