Ingesting Imagery with GeoTrellis¶

A common problem faced by those with large amounts of data is how it can be stored so that it can quickly queried without reading the entire dataset. The process of collecting data, transforming it into rasters of a desirable format, and storage for later querying, we refer to simply as an ‘ingest’. Over the years, the GeoTrellis maintainers have played around with a few different strategies for defining and executing them. Long time GeoTrellis users might recall the (now deprecated and removed) spark-etl package (which allowed users to build monolithic Extract/Transform/Load programs with a few formatting and projection options) and the more modular pipeline approach, borrowed from PDAL.

As often happens yesterday’s wisdom has fallen out of favor and the former ETL project is now seen as overly abstracted and unable to conveniently deal with the specificity and subtlty of real life ingests. If you’re looking for the former ETL project, it has been archived at https://github.com/geotrellis/etl.

While the pipeline approach remains viable, our recent work has tended to abandon generic, one-size-fits-all approaches in favor of directly writing programs with a few lower-level constructs. This approach has allowed us to handle edge cases with a minimum of hair-pulling and to focus on creating an API that conveniently expresses the ways that a program might interact with some source of imagery.

To this end, we’ve introduced the RasterSource which lazily represents some source of imagery. The format might be TIFFs or PNGs, images could be backed by S3 or your local hard drive - the RasterSource interface is abstract with respect to such details.

Below is an example ingest using RasterSources. Note that none of this code is ingest-specific; that all of it can (and is) used outside of ETL workflows.

A Sample Ingest with RasterSources¶

import geotrellis.contrib.performance.conf.{GDALEnabled, IngestVersion}
import geotrellis.contrib.vlm._
import geotrellis.contrib.vlm.avro._
import geotrellis.contrib.vlm.spark.{RasterSourceRDD, RasterSummary, SpatialPartitioner}
import geotrellis.proj4._
import geotrellis.raster.{DoubleCellType, MultibandTile}
import geotrellis.raster.resample.Bilinear
import geotrellis.spark._
import geotrellis.spark.io._
import geotrellis.spark.io.s3._
import geotrellis.spark.io.index.ZCurveKeyIndexMethod
import geotrellis.spark.tiling.{LayoutLevel, ZoomedLayoutScheme}

import org.apache.spark.{SparkConf, SparkContext}
import org.apache.spark.rdd.RDD
import cats.syntax.option._

implicit val sc: SparkContext = createSparkContext("IngestRasterSource", new SparkConf(true))

/** Some constants for us to refer back to */

// a name for the layer to be ingested
val layerName = "my-rastersource-ingest"
// the projection we'd like things to be tiled in
val targetCRS = WebMercator
// an interpolation method
val interp = Bilinear
// the scheme for generating tile layouts
val layoutScheme = ZoomedLayoutScheme(targetCRS, tileSize = 256)

// Here, we parallelize a list of URIs and then turn them each into RasterSources
// Note that reprojection/celltype manipulation is something that RasterSources allow us to do directly
val sourceRDD: RDD[RasterSource] =
sc.parallelize(paths, paths.length)
.map(uri => getRasterSource(uri, gdalEnabled).reproject(targetCRS, interp).convert(DoubleCellType): RasterSource)
.cache()

// A RasterSummary is necessary for writing metadata; we can get one from an RDD
val summary = RasterSummary.fromRDD[RasterSource, Long](sourceRDD)

// levelFor gives us the zoom and LayoutDefinition which most closely matches the computed Summary's Extent and CellSize
val LayoutLevel(zoom, layout) = summary.levelFor(layoutScheme)

// This is the actual in (spark, distributed) memory layer
val contextRDD = RasterSourceRDD.tiledLayerRDD(sourceRDD, layout, rasterSummary = summary.some)

// A reference to the attribute store for this layer
val attributeStore = S3AttributeStore(catalogURI.getBucket, catalogURI.getKey)

// Build a layer writer
val writer = S3LayerWriter(attributeStore)

// Actually write out the RDD constructed and transformed above
writer.write(LayerId(layerName, zoom), contextRDD, ZCurveKeyIndexMethod)