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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
3. Average No of void pixels/airport: 44 (-5% of pixels in 
RB); 
4. The highest No of voids: 255 (-31% of pixels in RB) - 
(Newark Liberty International Airport). 
The items in the above list should be compared with an average 
number of void pixels in the 3-arc-second SRTM (version 2), 
which is on the level of approximately 0.005%. 
For the subsequent analysis, only non-void pixels were 
considered (51,793). Further, pixels representing the water 
bodies in the vicinity of airports were also excluded from 
analysis. Consequently, 49,604 pixels were available for further 
consideration. 
An initial validation of the SRTM data has been performed by 
comparing the reference airport’s elevation with the SRTM 
elevation within RB. Figure 1 shows a histogram of the 
disparities in elevation between the airport's reference elevation 
and the SRTM elevation. 
  
  
  
   
    
  
  
  
  
  
  
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An 50 0 50 100 
Difference (Reference - SRTM) (m) 
Figure 1. Histogram of differences in elevation between the 
airport’s reference elevation and the SRTM 
elevations within the 1.5 km buffer centred on the 
airport’s reference point for 64 major U.S. airports. 
The mean difference is -6.0 m (the SRTM mean 
elevation is higher than the reference elevation). The 
standard deviation is 49.2 m. 
The mean difference of -6.0 m indicates that the SRTM 
elevations are higher overall. This situation is expected due to 
the presence of the elevated structures within the RB. An 
asymmetry of the histogram is evidence of this fact. However, 
the contribution of the elevated structures is limited to a 
relatively small number of pixels, because, after all, an airport is 
an unobstructed space that comprises much more than the space 
taken up by the terminal building and other structures. In order 
to identify artefacts, the threefold of standard deviation criterion 
was used. However, to address the  above-described 
‘contamination’ of the SRTM pixels with the elevated 
structures, an asymmetric criterion has been adopted and can be 
expressed as follows: 
m—-50; Smsm+3o,; 3) 
where m is the difference between airport’s reference elevation 
and SRTM pixels (-6.0 m), and g, is the instrumental error 
31 
component of the SRTM elevations, which was estimated at + 
1.55 m (see Section 2.1). This error covers the instrumental 
component only, because the target-induced component is 0 due 
to the fact that the terrain within the RB is by assumption flat 
(slope s in Equation 2 is 0). 
The SRTM elevations outside the range defined by Equation 3 
were tagged as anomalies. There were 21,142 outliers, which 
constitute about 42.6% of all non-void SRTM pixels. The 
number of outliers ranged between 1 (Niagara Falls) and 802 
(Pittsburgh International Airport). Examples of histograms of 
the SRTM Elevation Anomalies for Atlanta and Pittsburgh 
airports are shown in Figures 2 and 3. A range of elevation 
differences within the limits defined by Equation 3 is shown 
using darker bars. Clearly, the majority of the SRTM elevations 
within both airports’ areas are anomalies. A similar conclusion 
is valid for the majority of the investigated airports. 
Histogram of the SRTM Elevation Anomalies 
  
0 
[-46.93:-44 122) [-32.892;-30.084) [16.045;-13.238) [-2.007; 0.801) — [12.032; 14.839) 
Elevation Difference (True - SRTM) (m) 
Atlanta International Airport 
Figure 2. Histogram of the SRTM elevation for the Atlanta 
International Airport. The elevation anomalies are 
represented by lighter bars. A range of 'correct 
elevations is represented by darker bars. There were 
two data takes for this airport. 
Histogram of the SRTM Elevation Anomalies 
  
[-48.915; -47.094) [-37.986; -36.165) [-25.236; -23.415) [-12.486;-10.665)  [-1.558; 0.264) 
Elevation Differences (True - SRTM) (m) 
Pittsburgh International Airport