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the technology of cartography and geographic informa 
tion systems (GIS) which make the direct input of digital 
imagery more attractive than has been the case in the 
past. The opportunity to achieve improvements in the 
quality and timeliness of data for remote sensing, carto 
graphic and engineering applications lies in the attributes 
of electro-optical imaging systems and the associated dig 
ital processing systems. Among these are: 
• The increased dynamic range and spectral reso 
lution of electro-optical elements vis-a-vis photo 
graphic film will provide a level of information con 
tent that has not been available in the past. 
• The direct availability of digital data and subsequent 
display on computer graphics workstations will elim 
inate a great deal of manual labour in handling data 
and extracting information. 
• Image manipulations such as scaling, rotating, merg 
ing and enhancing are much easier to perform on dig 
ital data than on photography where special purpose 
optics would be required for even relatively simple 
operations. 
• The digital format and graphics display systems will 
enable many more operations on combined data sets 
such as vector or polygon information from GIS data 
bases overlaid on raster images. 
1.1 Background 
The MEIS has been described in detail in several papers 
during the last seven years (McColl, 1983, Neville, 1989, 
90, Till, 1983, 86, 87). It was developed by CCRS and 
built by Macdonald, Dettwiler and Associates (MDA) of 
Vancouver. It has been in operational use for eight years 
and to date has been flown on approximately two hun 
dred remote sensing missions (Till, 1986). The MEIS is 
based on a linear array containing 1728 charge-coupled 
detectors. One of the advantages of the linear arrays in 
MEIS over older imaging systems which utilized a ro 
tating mirror to scan a single detector element is that 
each element in the array is able to integrate the inci 
dent light during the entire scan interval and thus re 
sults in a greatly improved signal to noise performance 
(Till, 1983). MEIS was built with eight separate optical 
channels which has enabled the creation of very narrow 
spectral bands through the use of individual filters. This 
design has made it possible to add mirrors to two of the 
channels to create a single-pass stereo viewing capabil 
ity. One mirror is directed forward and the other to the 
rear which results in a single-pass stereo data acquisition 
capability (Gibson, 1983). 
1.2 Geometric Effects Only 
This paper deals only with the geometric effects with re 
spect to the resampling of the imagery; it does not address 
the radiometric corrections that would have to be per 
formed in order to permit the proper operation of remote 
sensing classification algorithms. The only radiometric 
adjustment considered at this stage of the processing is a 
simple balancing of intensities across each section of im 
agery to produce uniform average intensity across the im 
ages so that mosaiced products will not have step changes 
in intensity. The MEIS system has been radiometrically 
calibrated using a laboratory facility for the determina 
tion of radiometric gain and offset coefficients to normal 
ize the response of each channel. Periodic geometric cal 
ibrations of the MEIS imager are also performed in the 
laboratory for the co-registration of the 8 optical channels 
and to correct for lens distortions (Neville, 1990). 
1.3 Application Areas 
With the ability to provide precise geometric correction 
of the imagery from MEIS, it is possible to operate in 
several distinct spheres: 
Remote Sensing: With its precisely controlled radio- 
metric capabilities, MEIS can provide data for re 
mote sensing classification analysis and interpreta 
tion work. 
GIS Input: With the precise geometric correction ca 
pability, MEIS can provide data for input to Geo 
graphic Information Systems with subsequent carto 
graphic applications. 
Engineering Projects: MEIS can provide data at dif 
ferent resolutions during the course of an engineering 
project starting with lower resolution data over wide 
areas from initial flights during the planning stages 
and ending with high resolution data over specific 
sites or corridors in subsequent flights (Hart, 1990). 
1.4 Image Geometric Distortion Sources 
Although nominally similar to satellite imagery, MEIS 
imagery is distinctly different from the former with re 
spect to the inherent distortions in the raw data. There 
are two major sources of image geometric distortion: 
Aircraft Motion: The first is due to the irregularities 
of aircraft motion which have much higher dynamics 
than satellites. 
Terrain Relief: The second is due to terrain relief and 
the close proximity of MEIS to the earth during data 
acquisition. This effect is further compounded by the 
wider field of view (40 degrees) of the MEIS imager. 
1.5 Data Volume 
The volume of data required to cover a specific area can 
become very large as a function of the area covered and