STACK service specifications

Service Description

The Co-located Stacking (STACK) service computes the co-location of single-band assets having different map projections and spatial resolutions on a common grid. It performs resampling and warping of the secondary assets and the stacking of each secondary with the reference. The upsampling or/and downsampling of spatially overlapping assets is performed on a common area (intersection based on STAC asset geometry) and is based only on pixel coordinates. The service requires input geocoded assets, which are a prerequisite for the stacking service. As an example, input single-band assets can be taken from multi-sensor Optical or SAR Calibrated Datasets, Results of other on-demand processing services, and Auxiliary Datasets. For more details find the section about inputs here. It can also generate assets from the co-located stack using band arithmetic (via s-expressions). Co-location results depend on the level of accuracy of geopositioning of source images. The Co-located Stacking processor is built with the GDAL VRT method¹. It can also generate new assets from the co-located stack using band arithmetic with symbolic expression (S-expression).

Workflow

The service implements the workflow depicted below.

Input

Input of the STACK service can be geocoded images from supported SAR or optical sensors (e.g. reflectance or Sigma Nought single-band assets from Calibrated Datasets derived systematically in the calibration processing from supported Optical or SAR sensors). Co-location stacking is also possible if the input set of images is built by mixing single band assets from SAR and Optical Calibrated Datasets. Also, input single band assets in STACK can originates from Results of some on-demand processing services and from Auxiliary Datasets.

STACK supports single-band assets derived from the following services: PAN-Sharp, OPT-Index, BAS, IRIS, HOTSPOT, HASARD, and the two InSAR processors DInSAR, and SAR-COIN (for coherence asset only).

It also supports single band assets from the following Auxiliary Datasets:

1. Copernicus DEM,

2. ESA World Cover,

3. GSW Water Occurrence,

4. GSW Water Occurrence Change Intensity,

5. GSW Seasonality,

6. GSW Recurrence,

7. GSW Transitions,

8. GSW Water Extent,

9. World Settlement Footprint 2019,

10. WorldPop Population Counts ,

11. HAND.

Parameters

The STACK service requires a specified number of mandatory and optional parameters. All service parameters are listed in the below Table 1.

Table 1 - Service parameters for the STACK processor.

More information about the service parameters are given below.

Input single band assets

This first mandatory parameter is the list of input single band assets that are used to create the collocated stack. These assets can originate from:

• Optical or Radar Calibrated Datasets,

• Auxiliary datasets,

• Results from on-demand processing (e.g. from HASARD, or SAR-COIN).

Inputs are the path to the single band assets from the chosen Calibrated Datasets, Aux Dataset, or Results STAC items.

In the definition of each input single band asset the drag and drop of single-band asset/s is foreseen. This is possible by dragging and dropping one of the single-band assets included into a Calibrated Dataset, Aux Dataset or Results. These assets can be easily retrieved by showing the single band assets of a Dataset, Aux Dataset or Result features in the Results panel or in the features basket.

The drag and drop of the single band asset (e.g. nir) provides to the service a string in the format

input_dataset#single-band_asset

As an example, after the drag and drop the following string will be automatically inserted as a value for the parameter:

https://catalog.disasterscharter.org//charter/cat/[chartercalibrateddataset,{callid895}]/search?format=json&uid=call895_PT01N07_966794E007_8043502022080500000000MS00_GG003002001-calibrated#nir"

If multiple selected single band assets (e.g. red, nir) from a single dataset are defined via a single drag and drop interaction the string becomes:

input_dataset#asset1,asset2,assetN

As an example, after the drag and drop the following string will be automatically inserted as a value for the parameter:

https://catalog.disasterscharter.org//charter/cat/[chartercalibrateddataset,{callid895}]/search?format=json&uid=call895_PT01N07_966794E007_8043502022080500000000MS00_GG003002001-calibrated#red,nir"

In this way the STACK processor is able to get the path to the single band assets from the chosen Calibrated Datasets, Aux Dataset, or Results STAC items.

AOI (optional)

This second parameter (optional) may define the area of interest expressed as a Well-Known Text value.

S-expression (optional)

This third optional parameter allows generating new single-band assets derived from the collocated image stack (e.g. average, normalized difference etc.) using band arithmetic with symbolic expressions (S-expression).

How to specify input assets in symbolic expressions

After the drag and drop the STACK creates an indexing of all single band assets based on a prefix with the number of source Datasets or Result (1 for first dataset, 2 for the second one, etc.). Thus, to employ a single band asset in band arithmetic with STACK the user shall specify it as following:

dataset_or_result_number.single-band_asset

How to define a symbolic expression

In STACK band arithmetic, a new asset is defined with the name of the new output asset and it's associated s-expression separated by a colon : .

output_asset_name:(s-expression)

As an example to derive a new asset as the average between red bands from a pair of input single band assets (1.red and 2.red):

average:(/ (+ 1.red 2.red) 2)

This will add a new asset called average in the co-located stack with average values derived from 1.red and 2.red.

Arithmetic/logical operators and functions

S-expressions in the ESA Charter Mapper supports arithmetic (* + / -) and logical (< <= == != >= > & |) operators plus some pre-defined functions.

More information about supported functions can be found in Table 2.

Table 2 - Supported functions that can be used in s-expressions.

Examples of s-expressions which can be used to generate new bands from the STACK are listed in the below sections.

Compute sum

The following s-expression:

sum:(+ 1.pan 2.pan)

can be used to generate a band as the sum of 1.pan and 2.pan.

Compute difference

The following s-expression:

difference:(- 1.pan 2.pan)

can be used to generate a band as the difference of 1.pan and 2.pan.

Compute average

The following s-expression:

average:(/ (+ 1.pan 2.pan) 2)

can be used to generate a band as the average between the values of 1.pan and 2.pan.

Compute difference from average

The following s-expression:

diff_from_avg:(- 1.pan (mean 1.pan))

can be used to estimate a band of difference from the average value of 1.pan.

Compute normalized difference

The following s-expression:

ndvi:(norm_diff 1.nir 1.red)

can be used to derive multiple spectral indexes defined as normalized difference (e.g. NDVI, NDWI, NDBI, etc.).

Interpolate or rescale

The following s-expression:

rescaled:(interp 1.red (asarray 0 10000) (asarray 0 1))

can be used to interpolate the rescaled TOA reflectance into its original [0,1] range. Here (asarray 0 10000) returns [0, 10000] and is used to specify input range to be used for the interpolation.

Binarization

The following s-expression:

opt_water_mask:(where (>= (norm_diff 1.green 1.nir) 0.3) 1 0)

can be used to derive a water mask from the binarization of a spectral index. Here (norm_diff 1.green 1.nir) is used to derive the NDWI index. The value 0.3 represents the threshold as TOA reflectance. Similar s-expressions can be made also for SAR such as:

sar_water_mask:(where (<= 1.s0_db_c_hh -23) 1 0)

in which 1.s0_db_c_hh is the asset and the value -23 represents the threshold as Sigma Nought in dB.

Combine Datasets, Results and Auxiliary Datasets

STACK can be used also to post-process an asset from a Result of another processor. For instance, this makes it possible to co-locate a binary flood map from the Result of the HASARD on-demand service with the Land Cover Auxiliary Dataset to extract flooded pixels over only a certain LC class. As an example the following s-expression:

flooded_cropland:(where (== 2.worldcover 40) 1.flood-mask 0)

can be used to derive a flood mask over only crop fields. In this case the s-expression (where (== 2.worldcover 40) 1.flood-mask 0)) is used to generate the flooded_cropland asset from the flood-mask one using the condition (== 2.worldcover 40). The value 40 represents the pixel value of the LC class cropland for the single-band asset worldcover offered in the ESA World Cover Auxiliary Dataset.

Figure 1 - Extraction of flooded cropland using the STACK service from a co-location of an HASARD`s binary flood map and the ESA world Cover.

Output

The output of STACK is a multi-mission and multitemporal co-located stack with N single band assets in COG format. The output STAC item includes as many assets as provided in input plus the ones generated with s-expressions.

STACK Product specifications can be found in the table below.