Integrating the temporal dimension into remote sensing image analyses allows to answer relevant and exciting geographical questions. Several satellite-based optical sensors allow for tracking the Earth´s surface over several decades by means of long and dense times series. With long and dense we refer to time series representing multiple years (i.e. multi-annual) with satellite observations of high temporal density within each year (i.e., intra-annual).
The figure below shows a selection of satellite missions and sensors and their characteristics regrading image acquisition of long and dense time series. AVHRR and MODIS provide near-daily global coverage at coarse spatial resolution, while Landsat and Sentinel-2 provide images of higher spatial resolution but with lower revisit frequencies. Differences between these sensors also arise in terms of the temporal coverage.
AVHRR for instance captures the globe sice 1979, while the first MODIS sensor was launched in 1999. The Landsat family of sensors is operational since 1982 (or 1972 but with the MSS sensor which has lower spatial resolution and less spectral bands). Data acquired from the European Sentinel sensors are available only for recent years, limiting their use in long time series analyses.
For analyzing long and dense time series, the multi-spectral signal is commonly reduced to a desired target indicator, such as a vegetation index. Analyzing, for instance, a time series of NDVI facilitates interpretation and analysis as compared to analyzing time series of multiple spectral bands.
MOD13Q1 is a analysis-ready product providing NDVI (and EVI) measurements at 16-day intervals and in 250m (or 500m) spatial resolution for any location on the globe. It also comes with a quality layer indicating if an observation represents good data (clear sky, not quality issues), marginal data (some quality issues, may be of limited use), or if a pixel is covered by snow, ice, or clouds. The below video shows a global NDVI time series derived from the MODIS product MOD13Q1 (Version 6).
We can also get a different perspective on such time series by looking at a single pixel. Below, you look at a dense time series of Landsat NDVI values for three consecutive years for a deciduous forest in the Wuhlheide. The dots represent the actual NDVI values, whereas the gray line represents an interpolation between these values.
This graph reveals the seasonality of the forests, with increasing NDVI values in the green-up phase in spring, high NDVI values (>0.75) during the peak phenological phase in late spring until early autumn and decreasing NDVI values during the senescent phase in late autumn. Remember that we are looking at a 30x30m pixel, which may contain the signal from several tree canopies but also understory, such as shrubs or herbaceous vegetation in the openings of the canopy or under the trees when leaves are shed. Scientists in the remote sensing community refer to this mix of signal from various surface types as land surface phenology.
The seasonal component of long and dense time series was illustrated in the previous Wulheide example, representing the land surface phenology of a intact broadleaf forest stand. Many ecosystem are, however, subject to land surface change processes that are additionally captured by the time series. The ability to quantify these land surface changes largely depends on the lengths of the time-series back in time, and thus on the temporal coverage of the underlying satellite/sensor system. For example, pioneer deforestation patterns or land degradation processes can be traced back to the 70ties with the Landsat system, while latest disturbance induced changes like fires or logging can also be analyzed with Sentinel-2 data.
For analyzing land surface change processes, long and dense time series (but also only long time series with a single yearly observation) lead to characteristic trajectories, which are schematically represented in the figure below.
For a given target indicator such as the NDVI, we see a gradual decrease (a), a gradual increase (b), an abrupt decrease from high to low (c), an abrupt decrease from high to low with subsequent gradual increase (d), and a cyclic pattern (d). In the given context, these trajectories can be related to specific change processes such as a loss of green cover due to land degradation (a), an increase in green cover due to vegetation encroachment (b), replacement of forest by urban land (c), logging of forest with secondary forest regrowth (d). The cyclic pattern (e) represents a special case as it resembles the seasonal component and therefore rather represents land surface phenology. However, there are also cyclic land surface change processes that stem from regular management practices, e.g. removal of woody-vegetation to reduce fire risk.
In summary, long and dense time series contain multiple components that need to be considered during their interpretation and analysis:
All these components merge into a single time series signal. We need a sufficient temporal resolution of remote sensing observations to mitigate cloud cover and to match the phenology and the change processes to be monitored. Long and dense time series provide an analytic basis for such monitoring. Often, the different components can be well identified visually. Today, we want to interpret NDVI time series from selected locations around the world.
Download the session materials from our shared repository.
Try to draw a schematic representation of an NDVI time-series for the following situations:
What are the key differences?
We prepared three examples which we would like to discuss together. Please open Google Earth and visit the following sites in order to facilitate your interpretation:
The graphs below show the corresponding NDVI time series of these locations (250x250m) at 16-day intervals:
The goal of this assignment is to download and interpret individual pixel time series of MODIS-derived NDVI.
|1||-6.6197||-55.1835||Show good quality|
|2||30.0908||31.1917||Show good quality|
|5||50.8883||6.5388||Show good quality|
|6||-31.1007||152.4433||Show good quality|
Click on explore and view the contents of your request. You can visit the different sites, decide if you want to only visualize good quality observations (no clouds, snow, ice, water, radiometric saturation or other uncertainties). For some points, you will need to restrict to good quality observations, for others it does not make sense.
Please use the following table template to interpret the time series of each location.
|Point ID||Location||Land cover||Land surface phenology||Land surface change||Data availability|
With regard to your visual assessment of each point location in Google Earth, answer the following questions:
Now focus on the results of your AppEEARS request. You can visit the different sites, decide if you want to only visualize good quality observations (no clouds, snow, ice, water, radiometric saturation or other uncertainties). For some points, you will need to restrict to good quality observations, for others it does not make sense.
Interpret the time series from a geographical perspective and answer the following questions:
Note that you may use additional sources (Google Earth historic images, Wikipedia, literature) to better understand and describe the processes you are looking at
Please upload your interpretation (table) and screenshots of the time-series graphs as a pdf to moodle.
General submission notes: Submission deadline for the weekly assignment is always the following Monday at 10am. Please use the naming convention indicating session number and family name of all students in the respective team, e.g. ‘s01_surname1_surname2_surname3_surname4.pdf’. Each team member has to upload the assignment individually. Provide single file submissions, in case you have to submit multiple files, create a *.zip archive.
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