Image registration takes an image like an aerial
photograph or scanned map, and assigns coordinates to
some points so that the computer can calculate the coordinates for every point.
If the process is complicated, the program might
the data so that it has a simple geometry.
In the simplest case, the coordinates for three points will define a
translation and rotation. This is an
The following conditions will limit the effectiveness of
such registration, and
require more complicated algorithms:
- Lens distortion (poor optics), either in a camera or
- Off-nadir viewing geometry. This distorts the
view of the camera from what should be a rectangle, into a parallelogram.
- Terrain displacement in steep areas for imagery.
- For scanned historical maps, low quality of the original map.
- Paper distortion for old paper maps. Mapping
agencies use mylar sheets for archival purposes, and many maps and charts
use high quality paper which does not show the differential stretching of
The term "rubbersheeting" refers to a stretching or warping of the image to
bring it into the correct geometric location.
registration maps points on one
map to another, computes an optimal set of triangles, and then distorts the map
to obtain a correct geometry.
- The most robust solutions use
which you must have installed.
Database with image registration points
- Image coordinates came from the location
where the mouse was clicked. The origin (0,0) is the upper
left corner of the image, the computer graphics convention.
The user will not interact with these, other than not panic about
their reference system.
- Lat/long coordinates were entered from the
- UTM coordinates for the current zone,
converted from the lat/long values entered.
- The steps in both cases, and in others such as ArcGis, involve:
- Selecting points on the image, and giving the program their
coordinates. The can be via the keyboard, or by clicking on an
already registered map in the GIS. All point selection must be
done at the full resolution of the maps, for the maximum accuracy of the
- If the map has a grid or graticule
- The four corner points offer a good starting
point. For a large scale map, those four points will often
provide an adequate registration.
- If the map has either a full grid/graticule,
or with ticks in the center of the map, those points will allow a
more precise registation than allowed by just the corners. On
a common quadrangle map, there will be 16 points in the graticule on
- In optimal circumstances, two points will suffice, but in general
you need three and should select at least four.
- The registration points must be spread throughoug the map, and
cannot be collinear.
- Having the program register the image, which will create a new
- Verify the results.
Last revision 12/4/2017