to September; 3) they should not show defoliation (therefore, for some 
years and some parts of the state, the choice was further limited to 
early or late in the leaf-on season); 4) they should be as recent as 
possible; and 5) they should be from the same year, if possible; but if 
not, the same month in successive years. These criteria were related 
to use of the data in the defoliation assessment project. 
From the data base, a classification of all forested land was to be 
performed and a binary forest-nonforest mask produced. This mask was 
to be used to confine further processing to just the forest type, when 
working with recent data. It has the further effect of reducing confu 
sion with targets in nonforest areas which are spectrally similar to 
defoliated forest. 
Ten scenes were required for complete coverage of Pennsylvania. Using 
the above criteria, personnel of the Earth Resources Branch of NASA's 
Goddard Space Flight Center (GSFC) selected scenes sensed over the 
period 1976-79 (see Williams et al., in manuscript, for scene list). Al 
though a narrower range of years would have been preferable, this was 
not possible within the constraints of the above criteria. 
The Universal Transverse Mercator (UTM) map projection was chosen for 
the mosaic because of its increasing acceptance and ease of translation 
into latitudes/longitudes. This did create a problem however: the 
western half of Pennsylvania is in UTM Zone 17 and the eastern half in 
Zone 18. Since mosaicking these two zones would create major distor 
tions and departures from the UTM projection in the boundary region, it 
was decided to create two mosaics, one of eastern Pennsylvania and one 
of western Pennsylvania. Six scenes were necessary to form each of the 
mosaics, with the center pair being used in both. The data were re 
sampled to 37-meter square pixels, which will be the standard cell size 
for future Landsat products. 
3. CREATION OF THE MOSAIC 
The Mosaic was made at the NASA Jet Propulsion Laboratory (JPL) in 
Pasadena, California. Procedures for mosaicking data from lunar and 
planetary observation missions have been adapted by personnel at JPL 
for use with Landsat data, and several large scale digital data mosaics 
have been developed (Zobrist and Bryant, 1979). These procedures re 
quire the use of the VICAR/IBIS software system developed at JPL, and 
additional mosaicking software which has been incorporated into VICAR. 
The mosaicking process is a complex series of steps which begins with 
the selection of several ground control points on each frame. Seam 
control points are then selected on adjacent frames by automatic corre 
lation analysis and adjusted by a distortion model for each frame, 
based on the ground control points. Seam points are then reconciled by 
averaging their mapped locations in adjacent frames. Finally, the 
processed Landsat data are "cut" at the mapped seam boundary to produce 
the mosaic piece and then the pieces are "sewn" together (Zobrist et 
al., unpublished manuscript). When all the control points have been 
selected for one Landsat band, they can be applied to the other three 
bands and the same geometric correction performed. 
4. REFORMATTING THE DATA BASE 
The data base was supplied to ORSER on magnetic tape in band-sequential 
VICAR format. Each file contained data for a one-degree latitude by 
two-degree longitude quadrangle, eight quadrangles being necessary to