DIGITAL AERIAL TRIANGULATION - THE OPERATIONAL COMPARISON 
Mostafa Madani, Ph.D. 
Intergraph Corporation 
Huntsville, Alabama, USA 
email:msmadani Q ingr.com 
XVIII ISPRS Congress, Commission Ill, Working Group 11/2 
KEY WORDS: Digital Aerial Triangulation, Image Matching and Correlation, Accuracy and Performance 
ABSTRACT: 
Although the block adjustment phase of aerial triangulation has been automated to a great extent, point transfer and 
point mensuration phases have practically remained conventional. The need for near real-time results may be the 
main reason that photogrammetry techniques have not been fully accepted in non-mapping applications, such as 
architecture, industrial engineering, and medical disciplines. 
Recent progress in digital photogrammetry has opened new possibilities. Digital photogrammetry has been accepted 
as economical in medium- and small-scale mapping applications, particularly in digital terrain model (DTM) extraction 
and orthophoto generation. The next advance in digital photogrammetry involves automatic aerial triangulation using 
image matching techniques to automate point transfer and point mensuration phases. Several methods and computer 
programs for digital aerial triangulation are being developed. These methods are based on the automatic selection and 
transfer of tie points using multiple image feature-based and/or least squares matching. 
The object of this paper is to compare the operation of a “conventional” and an automatic digital aerial triangulation in 
terms of accuracy and time. For this purpose, two small blocks of aerial photographs were scanned at 15 um and 30 
um pixel sizes. Photogrammetric measurements were carried out in a semi-automated mode on an Intergraph 
InterMap Digital workstation. Image measurements were also performed in an automatic mode using the INPHO 
MATCH-AT product. 
Adjustment results and performance time of both modes of aerial triangulation were compared. Based on this 
investigation, the estimated precision of the image/object point coordinates obtained by the MATCH-AT program using 
a 30 um pixel size data set were very close to theoretical values given by conventional triangulation procedures. It is 
expected to achieve high accuracy digital triangulation results using 15 um imagery. The performance time of less 
than 5 minutes was reached with the MATCH-AT program using these data sets. 
1. INTRODUCTION 
Aerial triangulation is a complex operation. This 
operation includes planning photo flight; establishing 
ground control points; taking and developing aerial 
photographs based on pre-determined specifications; 
performing interior orientation; measuring and 
transferring all tie, check, and control points appearing 
on all photographs; and performing a least squares block 
adjustment. This process ultimately provides exterior 
orientation parameters for all photographs and three- 
dimensional coordinates for all measured object points. 
In the past 30 years, the main progress in aerial 
triangulation has been in the area of computational 
techniques for block adjustment. This was due to the 
advent of fast and powerful computers and the 
development of sophisticated aerial triangulation 
computer programs. Self-calibrating bundle and 
independent model adjustments are examples of such 
complex computer programs. With the introduction of 
analytical plotters, the aerial triangulation procedures 
have reached the present perfected status. Accuracy 
achieved by these procedures is quite high for 
photogrammetric point densification such as that 
obtained in photo-geodesy (Brown, 1976). In spite of 
achieving such accuracy, point selection, point transfer, 
and image point measurements of the aerial triangulation 
process remained unchanged, i.e., were done manually. 
490 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996 
Recent progress in digital photogrammetry has opened 
new possibilities. The impetus for this evolution has 
been the advancement of computer technologies and 
image processing techniques. While the principles of 
photogrammetry have not changed, the tools have. One 
of the fundamental changes brought on by the digital 
photogrammetry system is a potential for automated 
measurement and image matching that simply did not 
exist in the analytical stereoplotter environment. Today, 
several commercial digital photogrammetry workstations 
are available in the market. These digital 
photogrammetry workstations have been accepted as 
economical in medium- and small-scale mapping 
applications, particularly in digital terrain model (DTM) 
extraction and orthophoto generation. The automatic 
measurement and image matching techniques are the 
great value-added components that the new digital 
technologies bring to photogrammetry (Parker, Madani, 
1996). 
The next advance in digital photogrammetry involves 
automatic aerial triangulation using image matching 
techniques to automate tie point selection and transfer 
operations. Several methods and computer programs for 
automatic aerial triangulation are being developed. 
Basically, there are two strategies for digital 
mensuration and triangulation. The first strategy, which 
is similar to the conventional approach, can be classified 
   
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