accuracy of a printed map that includes these compilation 
errors are five to ten times higher, see for instance Merchant 
(1987) and Tobler (1988). Using a line accuracy of 0.25 mm 
and considering that typical photo scales for base maps are 
between 1:12 000 and 1:15 000, the positional accuracy 
required of objects on the ground is 2-5m to meet the high 
accuracy end of cartographic applications from 1:10 000 
upward. 
In the resource sector, accuracy requirements cover a rather 
broad spectrum. At the high accuracy end of these 
applications, requirements are almost as stringent as in the 
engineering and cadastral applications mentioned above. 
Although sample plots with a size of 20 m by 20 m are 
standard in conventional forest inventory, substantially 
higher spatial resolution is required to reflect the internal 
variability of the sample stand. Detailed measurements for 
the tree diameter at breast height, canopy density, height 
etc. make a spatial resolution down to 0.25 m desirable, see 
Till et al (1987) for details. To discern and interpret 
individual trees implies a spatial resolution of less than 1 m. 
For damage assessment, a resolution of 5 m is needed. 
Table 1 shows a summary of the accuracies required for 
different application areas, expressed as root mean square 
errors (rms) for position and attitude. It indicates that, 
except for a small number of high accuracy applications 
which require positions at the decimeter level, an accuracy of 
2-5 m is fully sufficient for the bulk of the applications. 
This result is important for the design of GIS data bases. 
Instead of mixing information from different application 
areas with different spatial resolution requirements, it seems 
advisable to require a uniform spatial resolution of 2-5 m for 
the standard resource data base. High accuracy applications 
which are usually restricted to smaller projects will normally 
not be part of these data bases. 
  
RMS Accuracy for 
  
  
  
  
Application Area 
Position | Attitude 
Engineering, Cadastral | 0.05 - 0.1 | (15" - 30") 
m 
Cartographic Mapping 
1:10 000 2-5 m 10' - 20' 
Resource Applications 2-5m 20' - 30' 
Forestry (Detailed) 0.2 - 10m 1-3 
  
  
  
  
  
Table 1: Accuracy Requirements 
3. ACCURACY OF CURRENT REMOTE SENSORS 
The georeferencing requirements of an airborne positioning 
and attitude system is determined by the spatial resolution of 
the remote sensor. Commonly used sensors such as 
photographic systems, scanning and linear array systems, 
and synthetic aperture radar (SAR) have quantifiable spatial 
resolution limitations. As such the following discussion 
will be focused on these sensor types. 
3.1 Spatial 
Systems 
Resolution of Photographic 
The spatial resolution (R) of an aerial photograph is 
influenced by a number of factors such as the resolving 
power of the camera lens and the film used in a photographic 
system. In addition, the spatial resolution is affected by any 
uncompensated image motion during exposure, the 
atmospheric conditions present at the time of image 
exposure, and the conditions of image processing (Lillesand 
and Kiefer, 1987). Also, the focal length (f) and the distance 
(d) between a target and the camera also determine the spatial 
resolution of a photograph. Among these, only the 
resolving power of the photographic system and the 
uncompensated image motion may be quantifiable. The 
resolving power of a photographic system is expressed in 
number of line-pairs/mm (n) (i.e., black and white line pairs 
of equal thickness (Wolf, 1974). The optical quality of the 
lens, the granularity and the speed of the film all contribute 
to the determination of the resolving power of a 
photographic system. Under a range of contrast of black and 
white between 2:1 to 1000:1, the resolving power of 
photographic systems ranges from 50 line pairs/mm to 100 
line pairs/mm (Lillesand and Kiefer, 1987). Due to a number 
of other factors mentioned above, R is usually poorer than 
d/(fn2000) m. Thus, 
R > d/(fn2000) m. (1) 
The ratio f/d determines the local image scale (s) for the 
photographed target. For example, an aerial photograph 
with a 1:10000 image scale and a resolving power of 50 
linepairs/mm, has a spatial resolution (R) which is 0.1 m or 
less. Consequently, the positional accuracy requirements are 
10 cm while the attitude requirements correspond to 15 
arcseconds for the exterior orientation of an individual 
photograph. 
In general, multiple strips of photographs are used in a 
block adjustment to obtain a favorable error distribution 
making use of the inherent geometrical strength of the 
photographic image. In this case, georeferencing can be 
done by position control only. Precise independent attitude 
is not needed because bundles of interlocking rays will take 
care of this requirement. By accurately fixing the 
perspective centres of these bundles in space, even high 
accuracy requirements can be met. 
3.2 Spatial Resolution of Scanning Systems 
and CCD Frame Imagers 
Compared with a photographic system, the only influencing 
factor that is different in a scanning system or a CCD frame 
imager is that the resolving power of the film has been 
replaced by the size (z) of charge-coupled devices (CCDs). 
Since the resolving power of a camera lens is considerably 
higher than the size of a CCD, the determining factor 
becomes the size of the CCD. Similar to the photographic 
systems, the spatial resolution (R) for a sensor system based 
on CCD technology cannot be better than z/s or dz/f, i.e. 
R » dz/f, (2a) 
when f, d, and z are given. For CCD-based airborne sensors, 
often the physical dimension (p) of the CCD array, the 
number of CCD elements in a line (nc) and the camera field- 
of-view angle (B) are specified. Here, f can approximately be 
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