COMBI service specifications
The Multi-Sensor Asset Composite (COMBI) is a processing service that performs multi-sensor, multi-temporal bands combinations. The processor selects as input a series of three Assets (such as bands that may come from different sensors) to generate a product combining them as a full resolution RGB composite, alongside with the three Assets resampled at the highest resolution and reprojected to the most frequent projection system. The resampled Assets are also provided as geophysical products derived from the original data product.
In this RGB combination COMBI makes use of pre-defined s-expressions to normalize the inputs and create RGB channels with values between 0 and 255.
The COMBI service supports optical and SAR data products. It can also take as inputs the outputs of other processing services of Third Party geospatial product provided those are single band GeoTIFFs.
Outputs of COMBI are: resampled and reprojected Geophysical Assets from a co-location and a full resolution RGB composite in COG format.
Input of the service are multiple types of physical meaning products derived in the processing environment from multiple Optical and SAR missions. The processor requires as inputs at least the calibrated datasets produced systematically by the optical and SAR calibration processors. In the ESA Charter Mapper, TOA Reflectances are encoded as uint16 with a scaling factor of 0.0001. Thus in the creation of a RGB composite, COMBI automatically performs a rescaling of this physical quantity during the normalization of each CBN. More information about the CBN schema employed in the ESA Charter Mapper can be found here.
Concerning the use of COMBI with Radar images, SAR calibrated datasets in the ESA Charter Mapper are given as Sigma Nought which are expressed in decibels. In the creation of RGB composites, COMBI handles Sigma Nought values for different SAR-bands and polarizations by using predefined intervals. Valid ranges expressed in dB can be found in Table 4 of this section.
When available, the processor can also take as inputs the outputs with physical meaning from all the other processing services provided those are single band raster in GeoTIFF format. In the ESA Charter Mapper multiple types of physical meaning Products can be derived from both systematic and on-demand processing services. The COMBI service supports the Physical Meaning Products. The list of physical meaning products available in the ESA Charter Mapper can be found here.
The COMBI service requires a specified number of mandatory and optional parameters. All service parameters are listed in the below Table 1.
|Red channel||Single band asset to be used in the RGB combination for the RED channel||YES|
|Green channel||Single band asset to be used in the RGB combination for the GREEN channel||YES|
|Blue channel||Single band asset to be used in the RGB combination for the BLUE channel||YES|
|Area of Interest||Area of interest expressed in WKT||NO|
|Color operations||Color formula based on Rio-color expressions||NO|
Table 1 - Service parameters for the COMBI processor.
In the definition of the input raster to be employed in red, green and blue channels, the drag and drop of the single-band asset (see Figure 1) is possible from the geobrowser results list or feature basket. This provides to the service an input in the format:
and this is the only type of input accepted. As an example, via the drag and drop feature in the geobrowser the following string will automatically set as parameter for one of the Red / Green or Blue channels:
Figure 1 - Example of list showing multiple assets for each calibrated dataset.
This parameter (optional) may define the area of interest expressed as a Well-Known Text value.
In the definition of “Area of interest as Well Known Text” it is possible to apply as AOI the drawn polygon defined with the area filter. To do so, click on the Magic tool wizard :fontawesome-solid-magic: button in the left side of the "Area of interest expressed as Well-known text" box and select the option AOI from the list. The platform will automatically fill the parameter value with the rectangular bounding box taken from current search area in WKT format.
Color operations (optional)
The remaining optional parameter to be filled in is the one dedicated to the color formula. The color formula expression is composed by three parameters: Gamma, Sigmoidal, and Saturation which need to be manually inserted by the user. More information about the formula syntax is described in this section which describes the TiTiler tool of Geobrowser.
For intrasensor RGB composites from optical EO data (e.g. TRC or CIV) the following color formula can be inserted:
Gamma RGB 1.5 Sigmoidal RGB 10 0.3 Saturation 1
Further information can be also found by looking at the application of color formulas in the ESA Charter Mapper which is described here
The output product of this service is a 4-band full resolution RGBA composite given as GeoTIFF in Cloud Optimized Geotiff (COG) format.
COMBI Product Specifications can be found below.
|Attribute||Value / description|
|Long Name||Basic Band Combination RGBA color composite|
|Description||RGBA color composite with a combination of 3 selected single-band assets plus an alpha band that acts as a transparency mask|
|Processing level||L1 / L2 (according to input)|
|Data Type||UnSigned 8-bit Integer|
|Projection||EPSG:4326 - WGS84|