Mowing Marker
Basic info
The mowing marker identifies all observations where mowing events lead to a drop and subsequent recovery in the time series of vegetation index signals, such as the Normalized Difference Vegetation Index (NDVI). The algorithm works on any signal where mowing events introduce detectable changes in vegetation patterns.
Further info
Mowing causes vegetation indices like NDVI to drop, as can be seen in the figure below showing NDVI time series around a labeled mowing event averaged over close to ten thousand events. For producing this image, NDVI time series of each FOI was shifted in time so that the observation with the minimal NDVI value during the mowing event is at day zero. The figure shows that NDVI drops on average by around 0.15 and that it takes around 20 days on average for NDVI to recover to values prior to the mowing event. The mowing marker algorithm searches for such drops and recoveries in NDVI time series.
Two versions of the marker are available:
FOI-level marker: Mowing detection is executed on a time series of a vegetation index, such as NDVI, aggregated over all pixels within a FOI.
Pixel-level marker: Mowing detection algorithm is executed on a time series of a vegetation index, such as NDVI, of each pixel within a FOI. Pixel-level mowing marker results are then aggregated over all pixels within a FOI into quantities like area of FOI that has been mowed at least once, largest connected area that has been mowed, etc. Such FOI-level information can later be used in the decision-making process.
FOIs with detected FOI-level mowing event
FOI = 92275267 is declared as permanent meadow. The mowing marker detected one mowing event indicated with a vertical green band in the Signal component of the shown Area Monitoring App screenshot. True color ARPS images from the time period around the detected mowing event confirm that the FOI has been mowed.
FOIs with detected pixel-level mowing event
This FOI is declared as permanent meadow. The FOI-level mowing event detection finds no events. However, the pixel-level mowing marker finds that the eastern part of the FOI was mowed. Visual inspection shows that the western part of the FOI was also mowed during a second partial mowing event towards the end of the season.
FOIs without a detected mowing event
There can be many reasons why no mowing event is detected. For example:
Grazing instead of mowing: A FOI is not mowed but is grazed. Grazing doesn't introduce such large sudden drops in the time series of vegetation indices, so grazing is typically difficult to detect.
Insufficient valid observations: There are not enough valid observations available around the time the mowing event occurred. If Sentinel-2 observations are not available for a longer period due to cloud cover, then mowing events can't be detected with Sentinel-2. When this happens we can use alternative sources, such as Analysis Ready Planet Scope or Sentinel-1 to make an independent check if an area of a FOI was mowed or not.
Cloud obstruction: An observation where evidence of mowing events is seen but partially obscured by a cloud or cloud shadow can be identified as invalid. The mowing marker algorithm can thus miss detecting such an event; however, an expert can correctly interpret the imagery and confirm that a FOI has been mowed.
Threshold limitations: NDVI drop or recovery is just below the threshold applied by the algorithm. An expert can correctly interpret the imagery and confirm that a FOI has been mowed.