What is Image Georeferencing?
Georeferencing is the process of assigning geographic information to an image.  Knowing where an image is located in the world allows information about features contained in that image to be determined.  This information includes location, size and distance.  Blue Earth Imaging can process imagery to various levels, depending on project requirements.

Level 1: Un-georeferenced Imagery

Level 2: Georeferenced Imagery

Level 3: Orthorectified imagery


Level 1: Un-georeferenced Imagery


An image can be thought of as a grid of colored dots called pixels.  if information on the size of a pixel is known, then objects in the image can be measured.  The location of a feature can only be determined relative to another feature.  The position of an object in the real world is not known with Un-Georeferenced imagery. 

*In the example below, if the size of each grid element (pixel) is assumed to be 5 inches square

Simple-Grid_2assumeSimple-Grid_2a

If this concept is extended to an aerial photograph with a known scale, real objects can be measured.  The following image is photographed at a resolution of 5 inches or 1/8 meter.  Each pixel has a dimension of 5 X 5 inches.  Using this information, length and area can be measured.  Compass direction and geographic position, however are unknown. 

Non-Georeference_House

 

Level 2: Georeferenced Imagery


The Earth has been divided into a grid with lines of Longitude and Latitude.  When an aerial photograph is aligned to this grid, it becomes "georeferenced."  A georeferenced image allows the following:

  • Distances can be measured
  • Areas can be measured
  • Directions can be determined
  • Exact position of any pixel can be determined 

  • Imagery can be used in Blue Earth Imaging's online map viewing application

  • Imagery can be accessed via the iPhone 

     

world

To georeference an image, the positions of known points, called "control points," are determined.  These control points can be collected in the following ways:

Method of Collection
 Accuracy
Engineering survey or
Differential GPS Receiver
 1/32 - 1 meter
Handheld GPS Receiver 1 - 5 meters

Georeferenced
Base Map

 5 - 20 meter

Control points should be collected from immovable objects that will be easy to identify in the aerial photograph.  At least three control points are required for georeferencing.  Additional control points add redundancy and help to increase accuracy.  Larger areas, and higher resolutions can require many more control points to maintain accuracy. 

House_coordinates

 When the control points are plotted on a grid of latitude and longitude, the position and orientation of the aerial photograph can be determined. 
The aerial photograph is then moved and rotated until it fits the control points.  Software creates a statistical "best fit" of the image to the control points and then "re-samples" the image.

Grid_with_coords_SM  House_on_grid_SM

Once the image has been georeferenced, each pixel will have a coordinate associated to it.  That means the location of any object in the image can now be determined (green coordinates above).  With knowledge of where the map is in the world, it can be made available in Blue Earth Imaging's online map viewing application. 

Level 3: Orthorectified Imagery

 

Additional accuracy can be obtained by accounting for distortions caused by camera position and terrain.  This process is called orthorectification and is best suited for customers who will be making precise measurements, or when hilly terrain is expected to cause unacceptable distortions.

An Orthophoto is an aerial photograph with a uniform scale and is usually required if maps are to be generated from the aerial photograph.  To create an orthophoto, an aerial photograph is projected on to a digital elevation model (DEM).

3D_DTM

Having a DEM allows for distortions caused by terrain to be eliminated.  Digital elevation models are available from the USGS or can be generated by collecting stereoscopic images of the terrain to be modeled.  For mostly flat terrain, orthorectification of images does not serve to increase accuracy significantly.

For more information on how orthophotos are generated, click on this link. 

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