Geographic raster data are commonly used for representing imagery, digital elevation, and other spatially distributed phenomena. A raster dataset can have a single or multiple bands (or layers) of data. A band is represented by a single matrix of cells organized into rows and columns, each with a value for the variable of interest such as elevation, temperature, or precipitation.
In HydroShare, the Geospatial Data Abstraction Library Virtual (GDAL VRT) format is used to hold geographic raster content. GDAL is a widely used open source code library for managing and accessing raster data. A GDAL VRT assembly must have one .vrt file and may have one or more raster files. To be recognized by HydroShare, rasters must be GeoTIFF files (.tif or .tiff). The .vrt file serves as an index to the component .tif or .tiff files and is saved in an XML format. With all files in a single folder, the .vrt file uses relative addressing to facilitate moving the assembly of files to different locations while retaining the connectivity of the assembly. VRT assemblies allow a large raster dataset to be built from smaller files, avoiding unwieldy single files to represent large datasets.
When a .vrt and .tif files are uploaded to HydroShare, they are automatically aggregated as a geographic raster content type. HydroShare also recognizes a single .tif file as a raster, in which case a .vrt file is automatically created.
Example 1 - Geographic Raster from single GeoTIFF file
A a single GeoTIFF file is added to a HydroShare resource (e.g., by dragging and dropping it onto the content area of the file browser):
HydroShare recognizes this as a GeoTIFF raster file and automatically converts it into geographic raster content, depicted by a folder with a geographic raster symbol. Metadata is read from the GeoTIFF file and displayed in the panel on the right of the file browser.
Metadata may also be added manually. In the example above, VariableName and VariableUnit were added manually. These are not part of common raster format specifications, but HydroShare allows users to add this information to describe the content of the rasters.
The geographic raster folder contains the original .tif file and a .vrt file automatically created by HydroShare. Together, these comprise the GDAL VRT object used to represent raster data in HydroShare.
Example 2 - Geographic Raster from existing VRT raster object
Five files comprising an existing raster represented as a VRT object are added to a HydroShare resource.
HydroShare recognizes these as a VRT Raster dataset. The files are aggregated into a folder depicted by a geographic raster symbol. Metadata is read from these files and displayed in the panel on the right of the file browser. Note that HydroShare arbitrarily assigns the folder name based on one of the rasters, and this can be renamed.
Inside the folder are the original .tif and .vrt files that comprise the GDAL Virtual Raster