1 October 2003
. There are two commonly used rectification methods for projecting data. Image
data can be rectified by registering the data to another image that has been projected or
by assigning coordinates to the unprojected image from a paper or digital map. The fol-
lowing sections detail these methods. A third method uses newly collected GIS refer-
ence points or in-house GIS data such as road, river, or other Civil Works GIS informa-
5-15 Image to Map Rectification. Unprojected images can be warped into projec-
tions by creating a mathematical relationship between select features on an image and
the same feature on a map (a USGS map for instance). The mathematical relationship is
then applied to all remaining pixels, which warps the image into a projection.
5-16 Ground Control Points (GCPs). The procedure requires the use of prominent
features that exist on both the map and the image. These features are commonly referred
to as ground control points or GCPs. GCPs are well-defined features such as sharp
bends in a river or intersections in roads or airports. Figure 5-4 illustrates the selection
of GCPs in the image-to-image rectification process; this process is similar to that used
in image to map rectification. The minimum number of GCPs necessary to calculate the
transformation depends upon the order of the transformation. The order of transforma-
tion can be set within the software as 1st, 2nd, or 3rd order polynomial transformation.
The following equation (5-1) identifies the number of GCPs required to calculate the
transformation. If the minimum number is not met, an error message should inform the
user to select additional points. Using more that the minimum number of GCPs is rec-
(t + 1)(t + 2) = minimum number of GCPs
where t = order of transformation (1st, 2nd, or 3rd ).
a. To begin the procedure, locate and record the coordinate position of 10 to 12 fea-
tures found on the map and in the image. Bringing a digital map into the software pro-
gram will simplify coordinate determination with the use of a coordinate value tool.
When using a paper map, measure feature positions as accurately as possible, and note
the map coordinate system used. The type of coordinate system used must be entered
into the software; this will be the projection that will be applied to the image. Once pro-
jected, the image can be easily projected into a different map projection.
b. After locating a sufficient number of features (and GCPs) on the map, find the
same feature on the image and assign the coordinate value to that pixel. Zooming in to
choose the precise location (pixel) will lower the error. When selecting GCPs, it is best
to choose points from across the image, balancing the distribution as much as possible;
this will increase the positional accuracy. Once the GCP pixels have been selected and
given a coordinate value, the software will interpolate and transform the remaining pix-
els into position.
5-17 Positional Error. The program generates a least squares or "Root Mean
Square" (RMS) estimation of the positional accuracy of the mathematical transforma-