GeoTrellis: Create an Attribute Store for a Cloud-Optimized GeoTIFF hosted outside of GeoTrellis

Question

The strategic performance advantage of the Cloud-Optimized GeoTiff is the ability to retrieve raster data for a given extent while only pulling the overviews and a byte range from a remote resource.

In Python, the vsicurl and gdal.Warp abstractions make it possible to do this with just a URL an an extent:

vsicurl_url = '/vsicurl/' + url_to_cog

gdal.Warp(output_file, 
    vsicurl_url, 
    dstSRS = 'EPSG:4326', 
    cutlineDSName = jsonFileSliceAoi, 
    cropToCutline = True)

The newly-minted COG Spark Examples explain how to arrive at a Raster[Tile] using an AttributeStore created as a result of tiling an RDD in a previous step:

//tiling an RDD and writing out the catalog
...

// Create the reader instance to query tiles stored as a Structured COG Layer
val reader = FileCOGLayerReader(attributeStore)

// Read layer at the max persisted zoom level
// Actually it can be any zoom level in this case from the [0; zoom] values range
val layer: TileLayerRDD[SpatialKey] = reader.read[SpatialKey, Tile](LayerId("example_cog_layer", zoom))

// Let's stitch the layer into tile
val raster: Raster[Tile] = layer.stitch

The examples, release notes, and docs for COG support in GeoTrellis all confirm that there is support for tiling data and making it available for clients to consume as a COG. Does GeoTrellis also support the ability to act as a client?

How do you create a FileCOGLayerReader if you don't have a pre-existing catalog, but do have a URL that supports range requests?

Answer 1

We have two COG related concepts at the moment:

• The first one is a GeoTrellis COG Layer - a very similar to a GeoTrellis avro layer thing, but instead of storing separate avro tiles we store tiles as a segment of a tiff(s). To prepare such catalog you'd have to go through an ingest process to structure data and to reformat it to the GeoTrellis ready format. That would be a layer that consists of partial pyramids, where each partial pyramid is represented by a set of COGS.
• The second one is a GeoTrellis Unstructured COG Layer: https://github.com/locationtech/geotrellis/blob/master/doc-examples/src/main/scala/geotrellis/doc/examples/spark/COGSparkExamples.scala#L114

The last one allows you just to collect metadata somehow about your dataset into an (extent, URI) tuples and to provide an interface to query it. Get familiar with the example I posted you and let me know if that works for you.

Btw, RasterFoundry uses unstructured COG layers for their tile server.