3. IMAGE INTERPRETATION 
The satellite data, used in this paper, are SPOT Panchromatic 
image, dated 2007-10-05, preprocessing level 2A, resolution 
10m. The study — image interpretation is done with ERDAS 
Imagine 2011 software and it particularly concerns Gallikos and 
Axios river delta areas. 
  
Figure 2. SPOT image of the wider study area 
Two portions of the above image were selected with the 
software, subsetting delta area of Gallikos (figure 2, number 3) 
and Axios (figure 2, number 4) rivers. 
Visual image interpretation of these subsetting images followed, 
on the base of geometric (shape, size, etc.) and radiometric 
(tone, texture) characteristics. 
Enhancement techniques (digital image processing) were 
applied, next, to make the image more interpretable for the 
application (Erdas Field GuideTM, 2010). 
Particularly, we applied: a) point based operations (operations 
dealing with a single pixel) and b) local operations (the output 
value at a specific coordinate is dependent on the input values in 
the neighbourhood of that same coordinate) (ASPRS, 2004). 
In this way, the following were done respectively: a) histogram 
equalization (non-linear contrast stretch) and brightness 
inversion (opposite contrast of the original image) and b) high- 
pass filtering and edge detection with Sobel operator (zero-sum 
kernels convolved separately with the original image and then 
combined) (Erdas Field Guide™, 2010). Below, we refer to 
relevant observations and comments. 
Gallikos (figure 2, number 3) is a small river discharging in 
Thermaikos Gulf, figure 3. The differentiations of tone at the 
mouth of the river, indicates that the water of the sea has come 
in the riverbed, or in other words the riverbed at the mouth has 
formed a lagoon (figure 3, L). Orthogonal pattern is identified 
on the left and right of the riverbed, corresponding to cultivated 
land. Tone and texture on both sides differentiate the overall 
appearance. Road network is also detected in the area. 
After applying point operations (figure 3a, figure 3b), we 
observe that the boundaries of the cultivations in relation with 
the original image (figure 3) appear more clear. The boundaries 
      
   
of the lagoon also, as well road network are more discernible, 
while in the mouth of the river areas of low vegetation are 
detected. 
  
  
  
Figure 3b. Gallikos river - brightness inversion 
After applying local operations (figure 3c, figure 3d) linear 
features are highlighted, as 3x3 high pass filtering serves as 
edge enhancer without eliminating other features and the 
application of Sobel operator shows points at which the 
     
   
  
  
   
   
   
   
   
   
   
   
   
  
   
   
     
  
  
  
   
  
  
  
  
   
    
   
   
  
intensity 
boundarie 
Axios (1 
in north 
large bii 
4).