What is Georeferencing?
The term refers to bringing a raster image into a GIS and matching the pixels in the image to specific geographic coordinates.
There are three generic ways to do this
| 1. A scanned image of a printed map may include either intersections of graticule lines or registration marks with known coordinates. |  |
| 2. A scanned image may have identifiable marks for which GPS coordinates have been ascertained in the field. |  |
| 3. An image file (aerial photo,satellite photo, or scanned map) can be referenced point by point to a known map layer. |  |
Basic Introduction
Georeference: to supplement an image with geospatial coordinates so that it can be displayed in a mapping system.
In the field we are accustomed to orienting a map - rotating it so that north on the map corresponds to north in the world - and our brains naturally stretch the map insofar as we appreciate that it IS a map of the physical world around us. Georeferencing is basically teaching the GIS to do the same thing.
The underlying operation is simple: we specify the spatial coordinates of several pixels in a raster image and the software comes up with a "mapping" or "transformation" that matches all pixels in the raster with spatial coordinates. This allows us to import the image into the GIS and display it as a layer alongside other geographic data.
RTM
Georeferencing in QGIS V 2.0 Manual
Gorey Details
Tecnobit, a software company based in Italy, has a nice set of web pages that provide a gentle introduction to some of the mathematical ideas behind geo-referencing.
Tutorials
Work through the following tutorials
- Tutorial: Georeferencing Topo Sheets, Topo Maps, Satellite Image or Scanned Maps in QGIS
- Tutorial: Advanced Georeferencing in QGIS using a Reference Layer
Homework
Georeference this aerial photo of the Mills campus (circa 1999) using Mills area parcels as your reference layer.
Create a new layer with several Mills buildings and save as an nicely designed print layout.
| File name | File type | Size | |
|---|---|---|---|
| MillsAerial1999_georef.zip | Zip archive data | 2.78 MB | Info |
Vocabulary and Concepts
world file
spline
(rubber sheeting) transformation algorithm
Trouble Shooting
Georeferencing is a plugin in most versions of QGIS. It is usually added at installation time, but it might not be (read here (2.0) and here (1.8) about issues). Here are some solutions folks have worked out:
- (JK) I used this download, restarted QGIS, and the plugin appeared. GSL_Framework-1.16-1
See Also
Georeferencer: Online Georeferencing Tool For Scanned Maps (watch the 4 minute video intro)
References
- QGis Tutorial I: how to georeference a map
- Tutorial: Digitizing in QGIS
- U. Mass. Georeferencing Lab (Uses QGIS v1.6)
- Jarrett Totton Georeferencing and Vectorization with Quantum GIS (video 7:00) (v1.7)
- National Library of Scotland. 2009. Georeferencing Using Quantum GIS (v1.5)
- ku.ca.zoi|nossey.sirhc#ku.ca.zoi|nossey.sirhc 2011. Georeferencing with QGIS
- StackExchange. How to install QGIS 1.8 plugin Georeferencer?
- AWF-Wiki (Goettingen). Georeferencing
- Visualizing Urban Geographies. Basic Georeferencing Workflow
- Eihan Shimizu and Takashi Fuse Rubber-Sheeting of Historical Maps in GIS and Its Application to Landscape Visualization of Old-Time Cities: Focusing on Tokyo of the Past