Geographic raster data are commonly used for representing and managing imagery, digital elevation, and other phenomena which is widely used in hydrological research. A raster dataset can have a single or multiple bands (or layers) of data. A band is represented by a single matrix of cell organized into rows and columns where each contains a value representing information such as watershed elevation, temperature or precipitation.
In HydroShare, the GDAL Virtual (VRT) format is used to hold Geographic Raster content. GDAL (Geospatial Data Abstraction Library) is a widely used open source library used by geographic information systems for raster data. A GDAL Virtual (VRT) assembly must have one .vrt file and may have one or more raster files. In HydroShare we restrict the raster files to GeoTIFF files and put all the files in a single folder. The Virtual (.vrt) file is a description of the virtual dataset saved in an XML format that serves as an index to the component .tif or .tiff files. The GeoTIFF files are the actual tile raster data. With all the files in a single folder the .vrt file can then use relative addressing facilitating moving of the assembly of files to different locations, while retaining the connectivity of the assembly. VRT assembly’s allow a large raster dataset to be built up of smaller files, and our choice of this format was to avoid having to have large and sometimes unwieldy single files to represent large datasets.
You may create a raster by uploading a single.tif file, in which case HydroShare creates the .vrt file for you, or by uploading a .vrt file and associated .tif files.
Geographic Raster from single geotiff file
A single geotiff file is added to a composite resource by dragging into the content area
HydroShare recognizes this as a geotiff raster file and automatically converts it into Geographic Raster Content stored in a folder depicted using a geographic raster symbol. Metadata is read from the geotiff file and displayed in the panel on the right, where you may add additional metadata.
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 indicate the content of the rasters.
Inside the folder there is the original .tif file and a .vrt file. Together these comprise the GDAL Virtual Raster object used in HydroShare to represent raster data.
Geographic Raster from existing VRT raster object
Five files comprising an existing raster represented as a VRT object are added to a composite resource.
HydroShare recognizes these as a VRT Raster and automatically converts it into Geographic Raster Content stored in a folder depicted using a geographic raster symbol. Metadata is read from these files and displayed in the panel on the right.
Inside the folder are the original .tif and .vrt files that comprise the GDAL Virtual Raster