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Added information on resampling algorithms for overviews #509

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7 changes: 7 additions & 0 deletions docs/data/product/dea-land-cover-landsat/_access.md
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Expand Up @@ -33,6 +33,13 @@ For detailed instructions, please visit the [Continental Cloud-Optimised GeoTIFF
<figcaption>The animation above shows the DEA Land Cover COG mosaic moving from the continental-scale down to the 30m x 30m pixel level.</figcaption>
</figure>

:::{admonition} Note
:class: note

The overviews (pyramid layers) embedded in the DEA Land Cover COG mosaics were generated using the **MODE** resampling algorithm. This means that, when viewing the data at coarser zoom levels, each overview pixel is assigned the most common land cover class within its corresponding area.

:::

:::{dropdown} How to integrate DEA Land Cover continental mosaics into your own Python workflow

You can seamlessly open a Land Cover mosaic, such as Level 4 for year 2024, using Python and the `rioxarray` library. For example:
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12 changes: 9 additions & 3 deletions docs/guides/continental-cogs-geotiff-mosaics.rst
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Expand Up @@ -7,11 +7,17 @@ Some Digital Earth Australia (DEA) products are provided as **continental-scale

These datasets are made available as **Cloud-Optimised GeoTIFFs (COGs)**, a format that enables users to efficiently *stream* raster data directly from the cloud without downloading the files. This provides a fast and convenient way to access full-continental coverage from tools like QGIS or ArcGIS Pro, especially when working with large datasets.

COGs include overviews, which are lower-resolution pyramid layers that allow fast rendering at different zoom levels. When zooming out, GIS software displays a coarser overview to improve performance. These layers are generated using a **resampling algorithm**, which determines how pixel values are aggregated when reducing spatial resolution. For DEA products, the choice of resampling algorithm depends on the nature of the data:

- **MODE** – used for **categorical data**; assigns the most frequent value within the extent of an overview pixel.
- **BILINEAR** – used for **continuous data** with minimal missing values; interpolates pixels to produce a smooth gradient.
- **NEAREST** – used for **continuous or categorical data with many missing values**; each overview pixel takes the value of the higher-resolution pixel closest to its centroid. This algorithm appears to maximise the visualisation of products where valid pixels are surrounded by many no-data pixels.
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@GL-S Could "appears to maximise" be replaced by "optimises" ?

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I think I need assistance from @robbibt or @vnewey here. Because the source I have found say that NEAREST uses the nearest pixels, valid or invalid. So I am not sure it always optimises/maximises the visualisation, or if just happens to work for those products we applied it to.

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@robbibt robbibt Oct 1, 2025 •
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I'll do some quick testing today, but essentially "nearest" was the only method that wouldn't come out empty if an overview pixel overlapped with a high-res single nodata pixel. So using "nearest" was less about maximising data coverage, but more to ensure our overviews contained at least some data.

I think we can probably go with

- **NEAREST** – used for **continuous or categorical data with many missing values**; each overview pixel takes the value of the higher-resolution pixel closest to its centroid. This algorithm enhances the visualisation of products where valid pixels are surrounded by many no-data pixels.

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Yeah ok, I just wanted to double check this. thanks for confirming!
If you observed it working better to show data, then the statement you suggest is correct. I think it's ok to just describe the effect of applying the algorithm without including too many details on how the algorithm works, considering there is a bit of confusion around it


Not all DEA products provide continental-scale COGs. Currently, this access method is only available for selected products:

- `DEA Land Cover </data/product/dea-land-cover-landsat/>`_
- `DEA Intertidal </data/product/dea-intertidal/>`_
- `DEA Tidal Composites </data/product/dea-tidal-composites/>`_
- `DEA Land Cover </data/product/dea-land-cover-landsat/>`_ - uses `MODE` as the overview resampling algorithm
- `DEA Intertidal </data/product/dea-intertidal/>`_ - uses `NEAREST` as the overview resampling algorithm
- `DEA Tidal Composites </data/product/dea-tidal-composites/>`_ - uses `NEAREST` as the overview resampling algorithm

**VRT (Virtual Raster) files** are provided alongside the ``.tif`` mosaics. These files serve as lightweight wrappers around the main data and can be used to open data in GIS software with visual settings already applied. We use VRTs to provide:

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