Data sampling strategy#
To create a balanced dataset for model training, we used a sampling strategy based on land cover classes from the ESA WorldCover layer.
Our unit of analysis for sampling was the MGRS tile, the global tiling scheme that is used for distributing Sentinel-2 imagery. For each MGRS tile, we computed landcover statistics for all the classes in the WorldCover layer. To speed up processing, we used the third level overview in the WorldCover layer, which has a spatial resolution of 80 meters.
The goal of the landcover sampling was to ensure coverage of each class at a reasonable level. For each class, we selected a number of random MGRS tiles out of the subset of MGRS tiles with the highest fraction of that class present.
As an example, for “Wetlands” we selected 50 random ones out of the MGRS tiles with the highest wetland fraction globally. For the Built-up class on the other hand we selected the 400 most urban MGRS tiles.
In addition to the landcover classes, we also added diversity by selecting 500 tiles out of the 3000 tiles with the highest count of land cover classes present in the tile.
After selecting MGRS tiles for each of these criteria, we removed duplicates.
The following table summarizes the selection criteria for each class.
For model version v0.1#
Class |
Nr of Tiles |
From highest |
|---|---|---|
Diversity |
500 |
3000 |
Built-up |
400 |
400 |
Herbaceous wetland |
50 |
500 |
Mangroves |
50 |
500 |
Moss and lichen |
50 |
500 |
Cropland |
100 |
500 |
Tree cover |
100 |
500 |
Shrubland |
50 |
500 |
Grassland |
50 |
500 |
Bare / sparse vegetation |
50 |
500 |
Snow and Ice |
50 |
500 |
Permanent water bodies |
100 |
1000 |
This resulted in a sample of 1517 MGRS tiles total in our sample.
The resulting sample file can be downloaded from the following link
For model version v0.2#
Class |
Nr of Tiles |
From highest |
|---|---|---|
Diversity |
400 |
2000 |
Built-up |
300 |
300 |
Built-up |
1000 |
1500 |
Herbaceous wetland |
50 |
500 |
Mangroves |
50 |
500 |
Moss and lichen |
50 |
500 |
Cropland |
800 |
3600 |
Tree cover |
150 |
750 |
Shrubland |
100 |
500 |
Grassland |
200 |
500 |
Bare / sparse vegetation |
50 |
500 |
Snow and Ice |
25 |
500 |
Permanent water bodies |
50 |
1000 |
This resulted in a sample of 2728 MGRS tiles total in our sample.
The resulting sample file can be downloaded from the following link
https://clay-mgrs-samples.s3.amazonaws.com/mgrs_sample_v02.fgb