stac-expr

Proposal for Expression Specification for Band Math across multiple STAC Feature Assets

specification

Bands can be specified in two ways: (1) asset key with band number or (2) common name.

asset key with band number

Bands can be specified by a combination of the unique asset key per the item-spec and the band index number (starting with zero). For example, RGB[2] refers to the RGB asset's 3rd band.

common name

The Electro-Optical Extension provides a list of common names for referring to bands here. For example, green when used with Landsat 8 would refer to Lansat 8's 3rd band, which is also the first (and only) band in the B3 asset file.

examples

NDVI for Landsat 8 using Asset Keys with Band Number

(B5[0] - B4[0])/(B5[0] + B4[0])

Because the Landsat 8 Asset GeoTIFFs only have one band, you can remove the [0] and the first band will be assumed:

(B5 - B4)/(B5 + B4)

NDVI for Landsat 8 using Common Names

(nir - red)/(nir + red)

multi-band output

In order to output RGB tiles, you must provided 3 expressions separated by a comma. For example, to specify a rgb output, you can enter: red,green,blue. If you wanted to subsitute Near-Infrared in for Red, you could do: nir,green,blue

case insensitivity

Asset variable names should not be case sensitive. In other words,

• (B5 - B4)/(B5 + B4) is equivalent to (b5 - b4)/(b5 + b4)
• (NIR - RED)/(NIR + RED) is equivalent to (nir - red)/(nir + red)

implementations

• Marblecutter Fork (partial implementation including references by asset key[band number], but not common name): https://github.com/DanielJDufour/marblecutter-virtual/blob/STAC/virtual/web.py#L156

resources

• https://github.com/developmentseed/stac-tiler

contribute

Post an issue here or submit a PR here!