Adapted VHI: the base of ExoSilva

A deep understanding of the VHI is the key to get the best out of ExoSilva. The essence of the VHI stands in the calculation of a relative NDVI instead of an absolute value. This simple but crucial step makes all the difference for the user but also for the producer of the statistics. While calculating NDVI is relatively simple on a single scene, the compilation of a robust reference usable over decades of data is a real challenge.

In summary, VHI is composed by two components, the VCI, being the relative NDVI w.r.t the long term minimum and maximum, and the TCI, being the relative land surface temperature w.r.t the long term minimum and maximum. ExoSilva has two particularities compared to a standard VHI. First, the ExoSilva VHI is calculated based on a constellation of satellite with very different characteristics. It leads to a more complex calculation of the VHI with coarse-resolution imagery treated in a differential space instead of providing direct observations. Also, the VHI provided to the user in ExoSilva is shifted by 0.5 towards the negatives, leading to a value of VHI zero being in the middle of the reference data, positive value expressing a better health, and negative values expressing a degradation of health.

Other providers of VHI data exists, such as the NOAA STAR.

Current State Map

The main map of ExoSilva exhibit the current condition of the forest with respect to the history. For every single location, we look at the variation in two key quantities, the NDVI and the land surface temperature, with respect to what happened in the last decades at this exact location.

Layer: Data

The layer data represent a quantity calculated in a similar way to the scientifically studied quantity called VHI for Vegetation Health Index. This layer does not represent exactly VHI as we spiced up its computation with some magical tools developed in the ExoLabs laboratories.

Layer: Regions

The regional layer is, simply, a aggregation of the data layer, described above, at the communal level. For every region, we also provide a precision estimation, also calculated on the top of the data layer. The precision estimate can be read in the following way: 100% means that data are trustfull, certainly very accurate, while 0% means that the data layer suffer from its low quality, leading to unreliable information.

Layer: WSL forest mask

This layer correspond to the forest mask of very high quality produced by the WSL in the state of early 2020. It serves mainly a contextualization purpose in the map view, while also used for the computation of data layer in the background.

Archive: time series

The map view on the top of the page allows you to request archive data for a given location. Once you click on a location, you can press the "fetch" button to view the archive data in the bottom graphics. Don't forget to check the API if you want to get a archive data for a wide location! The archive graphics display many information. The various graphics can be explored using the toolbar 1.
A quick focus to the early years is triggered by the upper button 2.
The legends 3 can be clicked to hide or show the different datasets.
The X-axis A at the bottom of the graphs shows the date of observations.
The Y-axis B are specific to each graphics.
The top one B1 represents the adapted VHI dataset. The axis is unitless.
The middle one B2 represents different NDVI time series. The axis is unitless.
The bottom one B3 represents time series of land surface temperature. The units are Celsius degrees.

Registration to notifications

It is possible to register for weekly update on the vegetation development through a mailing service. The registration to this notification service is realized in the "notification" page. Regions of interest can be clicked in the map. The current selection 1 will be updated with newly clicked regions. To remove a region from the notification system, click on the region in the map or on the "delete" option in the selection menu 1.
It is possible to look for a commune in the search box 2 on top left.
Once the regions of interest selected, a validation is necessary to get the notification enabled by pressing the top right button 3.
A test notification can be sent directly using the bottom button 4. The test notification will contain status of the current selection. Once the test notification sent, the page will reload itself with the current registered regions selected by default.
The info box 5 on the bottom right on the map helps for the choice of regions.


The notification depicts the current state of the registered regions. For each region, a leaf is depicted, either upwards-green, horizontal-yellow, or downgrads-red, corresponding to good conditions, degraded conditions or strongly degraded conditions. The overall VHI value for the region is also given, along with the product date of reference.


The dedicated page on the API contains all information to make an extensive use of ExoSilva.