LOW-COST FEATURE EXTRACTION FROM AERIAL PHOTOGRAPHS FOR DATABASE REVISION 
T. Bouloucos, R. Kunarak, K. Tempfli 
ABSTRACT: 
With the advent of geographic information systems, 
important means for acquiring spacial data. 
photogrammetric methods have become once more an 
paper describes a comparative study of digital 
monoplotting, using standard PC equipment, and stereoplotting using a low-cost analytical plotter, both 
in the context of GIS. The functionality of the methods for different application fields is evaluated. 
KEY WORDS: Monoplotting, Database 
Interpretability. 
INTRODUCTION 
Digital spatial data handling systems are widely 
used. Map making, map updating and the collection 
and production of geoinformation is no longer the 
domain of specialists of a certain discipline but 
of all professionals who need spatial data for 
their particular applications. 
Adequate data collection remains critical. 
Up-to-date spatial data acquired from recent 
imagery can provide appropriate geoinformation. 
Several types of sensors and platforms are used 
for this purpose. Satellite imagery and near 
vertical aerial photographs are used mainly for 
making and updating maps. Satellite data are 
usually gathered in digital form and thus can be 
directly processed digitally. Standard aerial 
survey cameras record images on photographic film; 
digital | cameras are not yet widely used in 
airborne surveys. 
Database revision using aerial photographs implies 
several operations, including interpretation, 
detection of changes, digitization and 
transformation to a reference coordinate system 
before the data can be integrated into an existing 
database. The process of interpretation can be 
performed prior to or simultaneously with 
digitization. Feature extraction can be realized 
by either stereoplotting or digital monoplotting; 
both methods yield digital output and can be used 
with any terrain type. 
Revision processes are required by users such as 
natural resource scientists, urban planners, etc, 
who are not professional map producers. The low 
utilization rates of photogrammetric systems make 
low-cost solutions appealing, but even for 
professional map makers low-cost systems may be of 
interest since revision requires interpretation, 
and thus rather long idle times for equipment. 
Being a low-cost system, digital monoplotting has 
become very popular, especially in "thematic 
disciplines". On the other hand, analytical 
plotters have become cheaper and easier to use, 
and may offer an attractive alternative to 
monoplotting, also for non-photogrammetrists. 
In this paper, we will discuss the advantages and 
disadvantages with respect to accuracy, 
interpretability and time-efficiency of digital 
‘onoplotting in a PC environment, compared with 
low-cost analytical stereoplotting for updating 
and/or upgrading an existing database. 
DATABASE REVISION BY PHOTOGRAMMETRIC MEANS 
Aerial photographs, are one of the important 
sources of up-to-date geoinformation for digital 
map and database updating and/or upgrading. 
Photogrammetric techniques are used to extract, 
process and integrate in to existing databases the 
information from photos. 
revision, stereoplotting, 
Analytical plotter, Accuracy, 
Stereoplotting method 
Stereoplotting is manual digitization of features 
in a well-oriented stereomodel formed by two 
overlapping photographs. It is commonly used to 
extract accurate spatial geoinformation in 3-D. 
The method requires dedicated equipment, 
preferably analytical plotters, and experienced 
operators. 
Digital photogrammetric workstations, also 
referred to as "soft copy systems", offer the same 
possibilities as analytical plotters, but without 
dedicated hardware except for the stereoviewing 
provisions. Although providing image enhancement 
tools and superimposition, image interpretation 
and feature extraction still have to rely largely 
on "manual" operations. 
The operators should be able to interpret, perform 
the necessary orientations, i.e.,image-to-model 
and model-to-map (or ground) transformation and 
digitization of the required information, and also 
edit and condition the collected information for 
integration in the database. 
A variety of plotters have been produced in the 
past. Nowadays, photogrammetric manufacturers are 
producing only analytical plotters, but, because 
of their durability, analogue stereoplotters with 
microcomputer support are still much in use. 
Analytical ‘plotters increase accuracy and 
production rates. They are ergonomically designed 
and equipped with highly interactive software that 
makes operation more convenient, thus decreasing 
the time needed for operator familiarization. 
Presently introduced low-cost analytical 
stereoplotters supported by microcomputers further 
reduce equipment costs. 
Changes can be mapped inexpensively using a 
transparent overlay of the photo indicating the 
areas of change (prepared before digitizing) which 
is then positioned with the photo in the 
instrument. More . sophisticated but rather 
expensive superimposition techniques can also be 
used. The digital data from the database to be 
revised are mapped to the image space, and one or 
two synthetic images (mono or stereo 
superimposition) are injected in the optical 
stream of the instrument and viewed with the 
stereomodel. This technique also provides on-line 
quality control of the data, but is not yet 
implemented on the low-cost range of analytical 
plotters. 
Digital Monoplotting Method 
  
Digital monoplotting is a feature extraction 
method that used two-dimensional digitization of 
single photographs supported by relief information 
in the form of digital terrain models (DTM). 
Monoplotting was developed as a simple, 
inexpensive alternative method for feature 
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