GET TO KNOW THE STEPS OF THE MAPBIOMAS PAMPA TRINACIONAL METHODOLOGY
Here we detail the Mapbiomas Pampa Trinacional methodology step by step. For each class, time series and country there are specific peculiarities and characteristics that can be checked in detail in the ATBD in its appendices.
It all starts with Landsat satellite imagery, with 30-meter resolution, available for free on the Google Earth Engine platform and with a time series of more than 30 years. It takes 74 Landsat images to cover the Pampa Sudamericano Biome, each with tens of millions of pixels. Images can contain clouds, smoke, and other artifacts that can “dirty” them. To produce a clean image, the cloudless pixels are selected from the available images for the selected period. For each of these pixels are extracted metrics that explain the behavior of the pixel in that year. This is done with each of the 7 satellite spectral bands as well as for the calculated spectral fractions and indices. For example, for Band 1 the median of band values in the period, the maximum value, the minimum value in the year and the amplitude of variation is collected. At the end each pixel for one year carries up to 91 layers of information.
For each year a mosaic covering Pampa biome is set up representing the behavior of each pixel through 91 metrics or layers of information. This mosaic set is saved as a collection of data (Asset) within the Google Earth Engine platform. These mosaics will be used in two main ways. First as source parameters for the algorithm to produce classification (see next step). It is also from this mosaic that RGB composition is derived allowing visualization of the background image in the platform MapBiomas. This composition is also used for the collection of training samples and samples for assessment of accuracy by visual interpretation.
From the image mosaics, the land cover and use map (Natural forest, Grassland, Farming and silviculture, Non vegetated area, Water, etc.) is made. To do so, MapBiomas Pampa Trinacional analysts use an automatic classifier called “random forest”, which runs on Google Earth Engine. This system is based on machine learning: for each topic to be classified, the machines are “trained” with samples of the targets to be classified. These samples are obtained by generation of maps of stable classes of the previous series of MapBiomas Pampa Trinacional and by direct collection by visual interpretation of the Landsat images.
Finally, the maps are subjected to different types of filters. Among them, the most general ones applied to the entire series of maps are GAP, spatial and temporal. The GAP filter solves the problems due to missing data in the tiles. The spatial filter aims to increase the spatial consistency of the data by eliminating isolated or border pixels. Neighborhood rules are defined that can lead to a change in pixel classification. For example, a pixel that has less than two out of the nine neighboring pixels in the same class will be reclassified to the predominant class in the neighborhood. The filter applies to all classes and years in the collection.
Then a temporal filter is applied, in order to reduce temporal inconsistencies and to correct failures due to cloud overflow. In particular changes in coverage and use that are impossible or not permitted (eg Natural Forest > Non-Forest > Natural Forest). Each biome, theme or region may have specific temporal filter rules. The temporal filter is applied to each pixel analyzing all the years of the Collection (e.g. Collection 2 are 36 years).
In order to understand changes in land cover and land use, maps are produced with class transitions between different pairs of selected years. It is thus possible to visualize the dynamism of the territory, and answer questions such as how much of the forest has turned farming from one year to another, for example, among other changes in the landscape. Transition maps are produced pixel by pixel and after finalized also pass through a spatial filter to eliminate isolated transition or border pixels. From these maps are constructed the transition matrices for each administrative unit, available on the MapBiomas Pampa Trinacional platform.