® Problems with electrical noise when the 
signal-to-noise ratio is low (i.e., very little den- 
sity variation) as in snow-covered or heavily fo- 
rested areas 
6 Objects as large as the scanning spot size in 
one photo and smaller than the spot size in the 
other photo, thereby showing different density 
patterns. 
(b) The BAI image correlator is being consi- 
dered for mensuration of camera-calibration 
plates. A closed-circuit television system was 
added so that the operator can visually check 
correlation for approximate settings. 
Digitized stereoplotter 
A Kern PG 2 stereoplotter has been modified 
to digitize both point and line data. Three ro- 
tary encoders, a coordinatograph, and an Altek 
digitizer provide X, y, z coordinates on magne- 
tic tape. On-line monitoring of linear data is 
provided on a CRT. The system is being tested 
for several potential uses, including the digiti- 
zing of planimetric data and terrain profiles 
for digital orthophoto systems. 
Digitally driven orthophoto system 
The digitally driven orthophoto system (To- 
pographic Division, 1975) is designed to produ- 
ce digital profile information from a stereosco- 
pic model, store the information on magnetic 
tape, and use the information offline to con- 
trol the photographic unit that produces the 
orhophotograph. The electronic units are desi- 
gned with flexibility for adaption to several ty- 
pes of profiling and exposing units. The system 
includes three subsystems : 
® Profiling device - Attached to a stereoplot- 
ter (direct-projection or optical-train) for trans- 
lating the platen in z and the guiderail carria- 
ge in x, y, under servocontrol. A prototype de- 
vice for a PG 2 stereoplotter has been tested ; 
it contains a set of x, y rails with servomotors 
on each axis, fixed to the draftingtable surface 
and connected to the parallelogram arm for 
translating the tracing stand in x and y. Con- 
trol of the z axis is manual. Servomotors and 
encoders have also been attached to a C 5 Ste- 
reoplanigraph to provide another subsystem 
collecting digital profile data. 
® Electronic subsystem-consists of a profile- 
recording assembly (magnetic tape) and a play- 
back or scanning assembly. The logic for each 
assembly is housed in a separate console. A pro- 
16 
totype recording assembly is being tested. 
® Orthophoto output instrument - A projec- 
tor equipped with a three-axis servosystem for 
off-line exposure of orthophotographs. The 
electronic playback assembly can be adapted to 
various projectors. First tests of the complete 
system will consist in profiling with a Stereo- 
planigraph and exposing with a GZ 1 Ortho- 
projector. To adapt USGS T-64 Orthophoto- 
scope for use in the automated system, several 
changes are needed. A modified model T-64B 
should be ready for testing by the time of the 
Helsinki Congress. 
Simultaneous production of orthophotos, 
contours, and digital terrain model 
USGS is acquiring a Gestalt Photo Mapper 
(GPM2) scheduled for delivery early in 1976. 
With this equipment playing the central role. 
the input to the system will be pairs of aerial 
photographs with model control, and the out 
put, automatically produced, will be orthopho- 
tos, contours, and digital terrain data. 
Digital processing of LANDSAT images 
The feasibility of enhancing the photogra- 
phic quality of LANDSAT (formerly ERTS) 
multispectral-sensor (MSS) images by digitally 
processing and combining successive images of 
the same Earth scene is being investigated. 
Computer-compatible tapes of MSS imagery 
from five satellite passes over the Phoenix area, 
and five passes over the Upper Chesapeake Bay 
area are being digitally processed and correla- 
ted and converted to hard copy for comparison 
with versions produced from single MSS ima- 
ges. If the quality of LANDSAT imagery can 
be improved by digital techniques, the scope of 
cartographic and other applications could be 
expanded. 
Automated mapping from LANDSAT 
USGS is studying the possibility of future 
developments that may provide an automatic 
system for small-scale mapping with LAND- 
SAT materials (Colvocoresses, 1974). The state 
of the art in Earth-sensing systems is changing 
rapidly so that the vidicons or scanners now 
used may soon be replaced by solid-state or 
other devices. Along with the expected conti- 
nuing development and improvement of space- 
craft technology, development of optimum ima- 
ge-processing, display, and dissemination sys- 
tems is needed to provide maximum informa-