4.1. Raster datasets - formats and finding them on the Internet

Raster datasets are important in Web-based mapping. In this section, we will discuss two aspects related to Raster datasets: some common formats, and where you might go to find some already “ready made” for use in WebGIS projects.

4.1.1. Raster Data Formats

If you search Wikipedia [1] for GIS Data formats, you find the following definition under the subheading “Raster”:

“A raster data type is, in essence, any type of digital image represented by reducible and enlargeable grids. Anyone who is familiar with digital photography will recognize the Raster graphics pixel as the smallest individual grid unit building block of an image, usually not readily identified as an artifact shape until an image is produced on a very large scale. A combination of the pixels making up an image color formation scheme will compose details of an image, as is distinct from the commonly used points, lines, and polygon area location symbols of scalable vector graphics as the basis of the vector model of area attribute rendering. While a digital image is concerned with its output blending together its grid based details as an identifiable representation of reality, in a photograph or art image transferred into a computer, the raster data type will reflect a digitized abstraction of reality dealt with by grid populating tones or objects, quantities, cojoined or open boundaries, and map relief schemas. Aerial photos are one commonly used form of raster data, with one primary purpose in mind: to display a detailed image on a map area, or for the purposes of rendering its identifiable objects by digitization. Additional raster data sets used by a GIS will contain information regarding elevation, a digital elevation model, or reflectance of a particular wavelength of light, Landsat, or other electromagnetic spectrum indicators.

Raster data type consists of rows and columns of cells, with each cell storing a single value. Raster data can be images (raster images) with each pixel (or cell) containing a color value. Additional values recorded for each cell may be a discrete value, such as land use, a continuous value, such as temperature, or a null value if no data is available. While a raster cell stores a single value, it can be extended by using raster bands to represent RGB (red, green, blue) colors, colormaps (a mapping between a thematic code and RGB value), or an extended attribute table with one row for each unique cell value. The resolution of the raster data set is its cell width in ground units.”

As you search for data on the Internet, or if you create your own raster data by using a digital camera (e.g., such as a camera on an airplane or on a quadcopter to capture landcover imagery), you need to be familiar and be able to recognize, particular raster data formats. There is a large list of different formats. Take a moment to see the full list that the GDAL translation and utility software supports. Visit:

http://www.gdal.org/formats_list.html

There is a lot of raster data formats, isn’t there!

Understanding and recognizing at least the commonly used formats is an important skill to gain. Below, we provide some links to Internet information that describes some of the common formats you might encounter. Take a moment to familiarize yourself with these.

ESRI Grid. This is a proprietary raster data storage format that is native to the ArcGIS software package by ESRI. These are made up of a number of different files and are stored in a file directory or folder, not an individual file.

ESRI ASCII raster format. This is a non-proprieary raster format that is useful for transferring or importing ESRI GRID data into other raster software packages. These have an extension name of “.asc”.

GeoTIFF is a collaborative efforts by many different companies and organizations to establish an agreed upon TIFF-based interchange format for georeferenced raster imagery. The coordinate system information can be stored in the GeoTIFF file itself, or sometimes is accompanied by a Tiff World File (.tfw).

GIF. The Graphical Interchange Format is a raster image format that supports up to 8 bits per pixel per image, allowing a single raster image to be able to display up to 256 different colors. It also supports animations. Extension name: .gif.

IMG. While there are several uses of .img, in raster processing we often encounter .img files that are a proprietary raster format developed for the raster software ERDAS Imagine. For example, this format is one of the raster data formats provided by the National Map Viewer for the National Elevation dataset.

JPG. This stands for Joint Photographic Experts Group and is a commons standard for representing one band (greyscale) or three band (Red, Green, Blue or RGB) datasets. File extension name is .jpg.

JP2 (JPEG 2000) is an image compression standard that provides some increased compression performance compared to its predecessor JPG. File extension name is .jp2.

KML. Keyhole Markup Language. This is is a data format for storing place locations, polygons, images and other things for use with Google Earth, formally called Keyhole Earth Viewer. KML format is an accepted open standard by the OGC. KML files are often distributed in a .KMZ file, which is a zipped, compressed file format.

PNG. Portable Network Graphics. This is a raster graphics file format that supports “lossless data compression”, which means that the digital information is compressed by an algorithm that allows the original data to be completely reconstructed from the compressed data. This differs from “lossy compression” that reconstructs only an approximation of the original compressed data. PNG was invented as a replacement for another graphics format - GIF. Extension name .png.

TIF or TIFF. This Tagged Image File Format is a raster file format that has been around for some time, often used to store “raster graphics” images that were used in the publishing industry and by photographers. The extension name is either .tif or .tiff.

There is even a raster format for PostGIS, called PostGIS Raster (formerly called WKTRaster).

In short, for WebGIS, raster files are a critical component. You’ve already seen Web Mapping Services which transmit raster files. Also, soon we will be discussing the concept of “Tiling” or “Tiled Maps” which use raster images.

References

[1] https://en.wikipedia.org/wiki/GIS_file_formats#Raster_formats

4.1.2. Searching for Raster Data on the Internet

It is really incredible just how much available data these days. There are a variety of web-database repositories “out there” that provides access to GIS data (vector, raster) in general, and raster data specifically. You may have a lot of experience looking for GIS data on the Internet already. But if you don’t, take a few moments to visit some of the GIS data repositories in the table below, just to see the kinds of data that are available, and note that the table below is just a listing of some of the Internet data repositories available.

If you are looking to try out a couple of these sites to see what you can find for an area of your own interest, we’d recommend you try the National Map Viewer or the USGS Earth Explorer.