Mapping tree cover and extent with Sentinel-1 and 2

Description

This is the GitHub repository for the Sentinel-1 and Sentinel-2 dataset Tropical Tree Cover, which is viewable on Google Earth Engine here. The asset is public as of May 2023 on Google Earth Engine here. The dataset is published in Remote Sensing of Environment.

This project maps tree extent at the ten-meter scale using open source artificial intelligence and satellite imagery. The data enables accurate reporting of tree cover in urban areas, tree cover on agricultural lands, and tree cover in open canopy and dry forest ecosystems.

This repository contains the source code for the project. A full description of the methodology can be found in the publication. The data product specifications can be accessed on the wiki page.

Citation

Brandt, J., Ertel, J., Spore, J., & Stolle, F. (2023). Wall-to-wall mapping of tree extent in the tropics with Sentinel-1 and Sentinel-2. Remote Sensing of Environment, 292, 113574. doi:10.1016/j.rse.2023.113574

Brandt, J. & Stolle, F. (2021) A global method to identify trees outside of closed-canopy forests with medium-resolution satellite imagery. International Journal of Remote Sensing, 42:5, 1713-1737, DOI: 10.1080/01431161.2020.1841324

Getting started

An overview Jupyter notebook walking through the creation of the data can be found here

A Google Earth Engine script to export Geotiffs of the extent data by country can be found here

Installation

Utilizing this repository to generate your own data requires:

Sentinel-Hub API key, see Sentinel-hub
Amazon Web Services API key (optional) with s3 read/write privileges

The API keys should be stored as config.yaml in the base directory with the structure:

key: "YOUR-SENTINEL-HUB-API-KEY"
awskey: "YOUR-AWS-API-KEY"
awssecret: "YOUR-AWS-API-SECRET"

The code can be utilized without AWS by setting --ul_flag False in download_and_predict_job.py. By default, the pipeline will output satellite imagery and predictions in 6 x 6 km tiles to the --s3_bucket bucket. NOTE: The specific layer configurations for Sentinel-Hub have not yet been released but are available on request.

With Docker

git clone https://github.com/wri/sentinel-tree-cover
cd sentinel-tree-cover/
touch config.yaml
vim config.yaml # insert your API keys here
docker build -t sentinel_tree_cover .
docker run -it --entrypoint /bin/bash sentinel_tree_cover:latest
cd src
python3 download_and_predict_job.py --country "country" --year year

Without docker

Clone repository
Install dependencies pip3 install -r requirements.txt
Install GDAL (different process for different operating systems, see https://gdal.org)
Download model python3 src/models/download_model.py
Start Jupyter notebook and navigate to notebooks/ folder

Usage

The notebooks/ folder contains ordered notebooks for downloading training and testing data and training the model, as follows:

1a-download-sentinel-2: downloads monthly mosaic 10 and 20 meter bands for training / testing plots
1b-download-sentinel-1: downloads monthly VV-VH db sigma Sentinel-1 imagery for training / testing plots
2-data-preprocessing: Combines satellite imagery for training / testing plots with labelled data from Collect Earth Online
3-feature-selection: Feature selection for remote sensing indices utilizing random forests
4-model: Trains and deploys tree cover model

The src/ folder contains the source code for the project, as well as the primary entrypoint for the Docker container, download_and_predict_job_fast.py

download_and_predict_job_fast.py can be used as follows, with additional optional arguments listed in the file: python3 download_and_predict_job_fast.py --country $COUNTRY --year $YEAR

Methodology

Model

This model uses a U-Net architecture with the following modifications:

Convolutional GRU encoder with group normalization to develop temporal features of monthly cloud-free mosaics
Concurrent spatial and channel squeeze excitation in both the encoder and decoder (https://arxiv.org/abs/1803.02579)
DropBlock and Zoneout for generalization in both the encoder and decoder
Group normalization and Swish activation in both the encoder and decoder
AdaBound optimizer with Stochastic Weight Averaging and Sharpness Aware Minimization
Binary cross entropy and boundary loss
Smoothed image predictions across moving windows with Gaussian filters
A much larger input (28x28) than output (14x14) at training time, with 182x182 and 168x168 input and output size in production, respectively

Data

This project uses Sentinel 1 and Sentinel 2 imagery. Monthly composites of Sentinel 1 VV-VH imagery are fused with the nearest Sentinel 2 10- and 20-meter bands. These images are preprocessed by:

Super-resolving 20m bands to 10m with DSen2
Calculating cloud cover and cloud shadow masks
Removing steps with >30% cloud cover, and linearly interpolating to remove clouds and shadows from <30% cloud cover images
Applying Whittaker smoothing (lambda = 100) to each time series for each pixel for each band to reduce noise
Calculating vegetation indices, including EVI, BI, and MSAVI2

The cloud / shadow removal and temporal mosaicing algorithm is summarized below:

Select all images with <30% cloud cover
Select up to two images per month with <30% cloud cover, closest to beginning and middle of month
Select least cloudy image if max CC > 15%, otherwise select the image closest to the middle of the month
Linearly interpolate clouds and cloud shadows with a rolling median
Smooth time series data with a rolling median
Linearly interpolate image stack to a 15 day timestep
Smooth time stack with Whittaker smoother

License

The code is released under the GNU General Public License v3.0.

Project Organization

├── LICENSE
├── Makefile           <- Makefile with commands like `make data` or `make train`
├── README.md          <- The top-level README for developers using this project.
├── docs               <- A default Sphinx project; see sphinx-doc.org for details
│
├── models             <- Trained and serialized models, model predictions, or model summaries
│
├── notebooks          <- Jupyter notebooks
│   └── baseline
│   └── replicate-paper
│   └── visualization
│
├── references         <- Data dictionaries, manuals, and all other explanatory materials.
│
├── requirements.txt   <- The requirements file for reproducing the analysis environment, e.g.
│                         generated with `pip freeze > requirements.txt`
│
├── setup.py           <- makes project pip installable (pip install -e .) so src can be imported
├── src                <- Source code for use in this project.
│   ├── __init__.py    <- Makes src a Python module
│   │
│   ├── data           <- Scripts to download or generate data
│   │   └── make_dataset.py
│   │
│   ├── features       <- Scripts to turn raw data into features for modeling
│   │   └── build_features.py
│   │
│   ├── models         <- Scripts to train models and then use trained models to make
│   │   │                 predictions
│   │   ├── predict_model.py
│   │   └── train_model.py
│   │
│   └── visualization  <- Scripts to create exploratory and results oriented visualizations
│       └── visualize.py
│
└── tox.ini            <- tox file with settings for running tox; see tox.testrun.org

wri/sentinel-tree-cover

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