3cavi 
  
presented at the 
  
A SOLUTION FOR IMAGE ORIENTATION OF CLOSE RANGE BLOCKS 
M. Scaioni, G. Forlani 
Università degli Studi di Parma, Dipartimento di Ingegneria Civile, viale Parco Area delle Scienze, 43100 Parma, Italy 
(marco.scaioni, gianfranco.forlani)@unipr.it 
Commission V 
KEY WORDS: Cultural Heritage, Algorithms, Automation, Direct. 
ABSTRACT: 
Image attitude in architectural photogrammetry blocks may be very far from the normal and standard configurations. Close range 
triangulation is therefore a non trivial task to be performed also by a skilled operator. This paper describes the solution developed at 
the University of Parma to deal with the orientation of close range blocks without requiring approximate values for the bundle 
adjustment parameters. The system implements algorithms for space resection (which needs at least 4 GCP per image), space 
intersection and bundle block adjustment, integrated in a management module which oversees the sequence of computations and the 
whole orientation procedure. In short, we provide approximate values for the exterior orientation of every image by space resection 
and use a closed formula to compute the approximate ground coordinates of tie points. To provide the resection module with at least 
4 object points for every image, we apply an incremental process where, starting from a minimal set of oriented images, we add new 
images and compute preliminary ground coordinates of tie point as soon as this is feasible. Simulated and real blocks oriented with 
the procedure, which seems to be considerably robust, will be presented discussing the performance of the algorithms. 
INTRODUCTION 
Image orientation is a fundamental step in the workflow of any 
photogrammetric task: it takes time and skill to be completed 
and its accuracy and reliability will affect the remaining phases. 
Efforts to try to make it faster and accurate went along two 
different paths: 
- direct image orientation: systems like the photo-theodolite 
in close range or the solar periscope in aerial 
photogrammetry were applied to directly measure the 
exterior orientation (EO) parameters; 
- indirect image orientation techniques based on tie and 
ground control points, capable of reducing as much as 
possible the amount of topographic work while still 
controlling block deformations were developed. 
Over the years, technological developments in computing 
capabilities seemed to give the prevalence to indirect methods, 
with Aerial Triangulation becoming more and more 
sophisticated with the introduction of additional parameters. 
Digital photogrammetry gave further momentum to this 
approach, by making tie point selection and transfer automatic 
to a large extent with Automatic Aerial Triangulation. 
In aerial photogrammetry the lead may now be possibly 
returning to direct sensor orientation methods, thanks to 
IMU/GPS integrated systems. While such systems promise to 
get rid of any ground control, apart from calibration and check 
purposes, there is nothing like in sight in close range 
applications so indirect methods are not challenged. 
Roughly speaking, we may say that here two different scenarios 
apply. In controlled environments and in industrial applications 
strong network geometry and the use of (possibly coded) retro- 
reflective targets is commonplace and synonymous of high 
accuracy, largely automated measurement techniques. Image 
orientation is achieved by a bundle adjustment and approximate 
values are obtained mostly alternating between resection and 
intersection. In architectural applications of photogrammetry, 
possibly within extensive projects such as building high 
resolution 3D city models or surveying large historical or 
monumental sites, surveying costs must be kept to a minimum 
to be viable. Here targeting may often be impractical or too 
costly to install and remove. The extent and complexity of the 
environment, that can be very high, often demands for still 
significant topographic work and for relatively large blocks. 
Besides, image attitude may be very far from the normal case 
while standard configurations for tie points (i.e. *von Gruber" 
positions) and optimal ground control points distribution are 
difficult to achieve: this makes network design complex. Initial 
approximations for the orientation parameters, though mostly 
not required, may be difficult to guess; besides, outliers in the 
data set, whose number may be high when using automatic 
procedure for image point measurement, may affect the 
estimates for those initial values. Close-range triangulation is 
therefore usually a complex task that has to be performed by a 
skilled operator, so restricting the community of people which 
can make use of photogrammetric techniques. Despite this, 
photogrammetry in architectural applications is in our country 
increasingly the domain of professionals with little or not proper 
background in photogrammetry. In an attempt to reduce costs, 
specialists in monument restoration also execute the survey, 
opting for a “do it yourself" approach. Very often many ground 
control points are measured, so as to register each stereo-model 
individually, thanks also to the wide diffusion of total stations 
capable to measure distances without reflector. In such cases, 
space resection may be used to compute the approximate 
orientation of each image, followed by a bundle adjustment of 
the stereo-pair to refine the parameter values, rather than 
proceeding with the relative orientation and later with the 
absolute orientation. 
Inexpensive monoscopic plotting programs or more expensive 
ones with stereo capabilitites are being used. Despite online 
help and good tutorials, image orientation in large blocks is still 
often a problem and may get stuck trying to get a correct 
solution. In most cases this is the results of poor block design or 
poor block geometry and self-diagnosis may help to overcome 
the problem; sometimes this has to do with the capabilities of 
the algorithm used. 
In the last years several methods have been proposed in order to 
make easier the computation of the orientation of close-range 
blocks and nowadays several software packages are available. 
Now also many low-cost commercial packages feature 
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