All images were registered to Zone 18 of the Universal Transverse Mercator
(UTM) coordinate system. Map coordinate units are meters, and the datum de-
fined in the World Geodetic System 1984 (WGS 84).
The final step in preparing the individual mosaics was to clip out a rectangu-
lar image. This subsetting operation removes the majority of the background (i.e.,
black or blank) pixels around the uneven borders of the mosaics. Figure 13
shows the clipped, or subsetted, mosaics for Knapps Narrows and Piney Creek.
Enhanced Levee Inspections
The band-to-band registration procedure worked well for both acquisition
dates. The radiometric correction algorithm worked poorly for the 2001 acquisi-
tion date owing to excessive clouds and cloud shadows. The resulting images
showed significant shifts in the intensity and hue of colors when comparing
overlapping frames. The 2002 images showed excellent color balance along
flightlines after application of the radiometric correction technique.
The frame-to-frame mosaicking step created the initial multi-image mosaics
within their own "pixel space." This process was completed manually using the
ENVI image processing software. Visually interpreted control points are em-
ployed to "stitch" together overlapping images. Therefore, as the mosaics were
built by adding one frame at a time, the X and Y coordinates were assigned to
each pixel as the row and column numbers of the expanding digital file. This
frame-to-frame registration procedure also used the RST geometric transforma-
tion to sequentially combine the overlapping images. The use of the RST trans-
formation is important to maintain the rectangular shape of each individual frame
during the construction of the "pixel space" mosaics. This method of frame-to-
frame mosaicking is extremely labor intensive. However, the image analyst has
complete control over the geometric fidelity of the initial mosaics.
After completing the preliminary mosaic for a section of levee (e.g., flightli-
nes 1 and 2), the next step was to assign real world coordinates to the linear im-
age. The mosaics were geometrically registered to the panchromatic orthophotos.
First, second, and sometimes third order polynomial transformations were em-
ployed to register each mosaic. This second, and final, geometric registration step
was also labor intensive. The 2001 mosaics were poorly registered to the ortho-
photos. The poor output from the geometric registration was attributable to the
linear nature of the pixel space mosaics. Significant warping (i.e., distortion) was
observed at the ends of the flightlines (Fig. 14). Results were improved for the
2002 images by geo-registering smaller clusters of four to five frames to the or-
thophotos (Fig. 14).