The colour of the Wadden Sea is measured by an optical sensor in a multispectral dataset, which allows to determine different surfaces of the intertidal flats. Here, different sediment types, algae coverage, seagrass meadows and mussel beds are the main categories.

Optical remote sensing

In optical remote sensing, the energy of the sun light that is reflected from a surface is measured by the sensor. Different surface types such as sand flats and seagrass meadows reflect the energy differently in the different wavelengths of the light spectrum. In addition to visible light, an optical sensor captures light in the near and mid infrared, that is not visible to the human eye but which does contain important information about the surfaces. The resulting reflectance spectrum is then used for performing the analyses.
 

Different colours of surfaces in the Wadden Sea

"Gray" sand

"Green" macro algae

"Brown" diatoms

In the Wadden Sea it is of high importance to be able to accurately capture the water cover (e.g. tidal channels or areas of residual water on the mudflats). Spectral bands in the mid-infrared range are very useful for this. Many sensors are not able to capture mid-infrared and are therefore only partly suitable for the classification.

Other aspects that complicate classification are the spectral similarity of different types of vegetation (e.g. seagrass and green algae), low density coverage of seagrass or mussels for example, and sometimes low spatial resolution of the available satellite data.

Data availability of optical satellite images of intertidal flats is severely limited by two further factors: Cloud cover and the tides. In order to be able to make optimum use of an image it should be cloud-free (at least on the mudflat areas) and should have been taken as close to low tide as possible. In combination with (in some cases) the long return rate of individual satellites, the acquisition of suitable optical satellite data is already a challenge in itself.  
 

Satellite sensors, SAMOWatt uses data from

SensorChannelsMedium infraredSpatial resolution (Pixelsize)Return rate
SPOT 4/55yes10x10 bis 20x20 m26 days
Landsat 78yes30x30 m16 days
Landsat 811yes30x30 m16 days
RapidEye5no6,5x6,5 mOn request
Meris15no300x300 m1-2 days

 

Landsat 8 shot on 15 August 2013

True colour

False colour

However, we are also investigating data with a higher temporal resolution by examining data from the MERIS sensor, which acquired images of the Wadden Sea every 1-2 days. Our intention is to investigate the development of the seagrass meadows over time more closely. The pay-off for the increased temporal resolution is however a lower spatial resolution. With a pixel size of 300 m, an accurate assessments of the seagrass areas cannot be made, but the seasonal development of the centre of a seagrass meadow can be assessed.