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PRODUCTION STRATEGY 
The demand for digital orthophotos increased due to the 
maturity of GIS technology, the general lack of current 
base cartographic data, and the dramatic drop in computer 
hardware costs. ^ This sudden demand for digital 
orthophoto data could not be met with existing resources 
and budget levels. Pooling of government resources and 
relying on private industry were practical solutions to the 
budget limitations and the limited production capacity. 
SYSTEM AND DESIGN CONSTRAINTS 
Although the USGS had long abandoned any in-house 
photogrammetric hardware development, the agency 
continued software development to perform system 
integration tasks. For the digital orthophoto development, 
the USGS expanded its prototype software into a digital 
production system because commercial software was not 
available. However, the system design was for a modest 
amount of production work because the production 
strategy was to accomplish the majority of the work by 
contracting to firms in the private sector. 
The basic inputs to the present system are (1) diapositives 
generated from available photography acquired from the 
National Aerial Photography Program (NAPP), (2) 
camera calibration data, (3) existing photoidentified 
control from previous mapping projects, (4) 
aerotriangulation data in different formats, (5) existing 
digital elevation model (DEM) data from the National 
Digital Cartographic Database, and (6) topographic maps 
(fig. 1, box A). 
The National Aerial Photography Program is a 
cooperative program with State and Federal agencies to 
acquire  color-infrared or black-and-white aerial 
photography over the conterminous United States on a 
cyclic basis. Photography from this program is the 
primary source imagery for the digital orthophoto 
program and diapositives are generated from the original 
aerial film. In this scenario, the aerial photography has 
already been inspected using strict standards. However, 
there are many instances where existing photography is 
not suitable for the requirements of the orthophoto user. 
In these situations, the aerial photography is acquired by 
the mapping contractor as part of the digital orthophoto 
contract. In this second scenario, inspection of the newly 
acquired photography is required before production of the 
digital orthophoto begins. The two scenarios presents 
two dataflow paths through the production process. In- 
house production scanning also presented two dataflow 
paths because diapositive scanning was performed on two 
Scanners made by different manufacturers, which required 
slightly different data handling. 
Since the USGS has the responsibility for camera 
calibration in the United States, the necessary camera 
225 
calibration information is readily available for in-house 
production purposes. However, the current USGS camera 
calibration process does not provide the calibration results 
in digital form to the contractors or to the general public. 
In general, users are required to manually enter camera 
calibration data into their orthophoto rectification systems. 
Where feasible, photoidentified control points from 
previous mapping projects are used to reduce the cost for 
additional field control. The use of these data constrains 
this portion of the production system to be analog 
because these data are obtained entirely from analog 
techniques. 
When available, digital elevation data from the National 
Digital Cartographic Database were extracted and used in 
the orthophoto generation. However if digital elevation 
data were not available, they were generated by 
photogrammetric methods or derived from digitized 
hypsographic data. This represented two additional 
dataflow paths. 
ADDITIONAL DESIGN REQUIREMENTS 
In addition to producing the standard 3.75- x 3.75-minute 
digital orthophoto quadrangles (DOQ’s), mosaicking of 
DOQ’s into the traditional 7.5- x 7.5-minute quadrangle 
format and limited hardcopy image output was necessary 
for specific needs. Data delivery requirements include 
archiving in a format for easy use in various GIS systems 
while providing sufficient metadata for data management 
purposes. These requirements also include using a 
compression algorithm that is in the public domain and 
available on various computer platforms, and distributing 
the data on various media. 
STANDARDS DEVELOPMENT 
Since digital orthophoto data are to be generated by other 
agencies and private contractors, there is an obvious need 
for DOQ standards (fig. 1, box B and C). Currently two 
standards are being developed in the United States. The 
USGS has been working with participants of the National 
Digital Orthophoto Program to develop standards for the 
digital orthophoto (U.S. Geological Survey, 1995). 
Meanwhile, each Federal agency also participates on the 
Federal Geographic Data Committee to develop digital 
orthophoto standards for the National Spatial Data 
Infrastructure (Federal Geographic Data Committee, 
1995). Unfortunately, an accepted national standard was 
not available before system development began. 
Furthermore, the long review process and the iterative 
changes to the draft standard complicated the system 
development effort and prolonged the development 
period. 
Digital orthophoto production by other agencies and 
contractors also adds a functional requirement that the 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996