Good day, folks! My name is Khalid, and I am a member of the Geodata Raster Unit at ESRI Support Services. This post is the first of a series that examines basic concepts revolving around raster data. Gaining familiarity with the terminology associated with a new subject matter is arguably the most challenging hurdle in mastering any new content: a notion that is not lost on GIS professionals inaugurating themselves into the world of imagery, scanned documents, and other manifestations of raster data. The hodgepodge of formats and properties intrinsic to rasters may cause confusion with understanding the processes needed for preparing the data so that it may be properly utilized in a GIS application. With this in mind, we will explore some of the more common topics associated with raster data by taking a look at georeferencing: what it is and when it is needed.
Before we get into georeferencing, it is important to develop an understanding of raster data, so please take the time to review the linked article. In short, if you have a scanned paper map, it is a raster. If you have an aerial photograph, it is a raster. If you took a picture from your digital camera, it is a raster. A raster can come in many formats; this list summarizes the formats supported by ArcGIS Desktop. The proper geographical placement of the raster, as with any other spatial data, is critically important for use in a GIS application. The idea of georeferencing a raster is done to ensure that the data essentially falls in the right place on the map. When it comes to rasters, satisfying this requirement involves evaluating a series of questions noted in this article on gathering raster data information:
- Where did the raster come from?
- Was there any special processing used to create the raster?
- What has been done to it?
Understanding where the raster came from and what has been done to it plays a large part in determining whether the raster needs to be georeferenced. For example, a paper map that you might have scanned a few moments ago will certainly need to be georeferenced, whereas a DEM downloaded from the USGS Web site will not. If you are unsure about the details of the raster and whether it may or may not need georeferencing, examine its extent values and determine whether they appear to reflect real world coordinates in a projected or geographic coordinate system. If the data's extent appears to use real-world coordinates, review this article on identifying an unknown coordinate system. Hint: Projected coordinates typically have a minimum of 4 units for each extent, whereas geographic coordinates have 2 units for the top/bottom and up to 3 units for left/right. Also, be sure to check out the "Georeferencing a raster dataset" Web help article; it provides detailed information on this topic and has a neat video demonstrating the process.
- Khalid D., Support Analyst, Geodata Raster Unit, ESRI Support Services