International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B-YF. Istanbul 2004 
  
Figure 3. The panchromatic orthoimage using control 
points measured with GPS. 
4. ACCURACY ESTIMATION OF THE TWO 
ORTHOIMAGES 
4.1. Required accuracy from the Hellenic Cadastre Project 
As it is known the technical specifications of the Hellenic 
Cadastre fixes bearable limits for horizontal and vertical 
accuracy rise up to two (Technical specifications of Hellenic 
Cadastre Project, 1999) (Arvanitis A., 2000). 
In order to achieve accuracy control the technical specifications 
demand the coordinates’ comparison of certain characteristic 
points in the orthoimage with the coordinates that were received 
with any method (digitalization, GPS). 
The map accuracy is expressed by variance o^, or the Root 
Mean Square error of the points' coordinates, that was 
calculated (Patias P., 2001): 
e From the n points' horizontal coordinates 
  
5 ] 2 ~ 5 ~ ” 
e e o tns 
2n t7 (1) 
1 $ > / > 
2 Ó C 
Gr = & = x, ) T (v; "xdi ¥ 
2n ^7 
e From the n points heights 
Î n 2 
2 Ó 
f —- = 2 
o ver = S. (z i z i ) (2) 
n ja 
Where 
X ; y ? s : = the values of control points’ 
coordinates in the orthoimage 
X. Vi ZZ, Me corresponding topographic control 
1 
values. 
  
O0 2 : A : 
Ox; ; Óy ; * the coordinates" residuals 
N= the number of points for the accuracy control 
2 2 
Q noc — the variances' estimation for the horizontal 
> ver 
and vertical control 
Before the value o becomes acceptable, the values dxi, dyi are 
checked for probably gross errors applying data snooping 
control: 
ÓX;, y; « 1.96 Ohor (3) 
02; $2.16 0. (4) 
The percentage of the rejected values resultant from the 
application of this control should not exceed the 5% of total 
values. After the rejection of the off limits deviations the root 
mean square error of the remaining points is calculated and the 
global test takes place. The value o is checked with the help of 
the reference variance. The prices o,, which come in this case 
from the technical specifications of the Hellenic Cadastre (oy, 
Kat 607), depending on the scale, are given for the coordinates x, 
yz. 
e For horizontal accuracy: 
Gun 0 9264 22 096,0 (5) 
x 
e For vertical accuracy: 
Lnd 
Over £0980, — 0.96, 1 (6) 
Accordingly to the statistics if the inequalities are valid, the 
estimation of accuracy that was calculated is oxy-2m and 
o07-2m by tacking into consideration the technical specifications 
of the Hellenic Cadastre about o, as well. 
4.2. Orthoimage's accuracy estimation that was produced 
with the method of digitization 
Nine points were used for the accuracy control, which were 
distributed in the total of region. The points' coordinates were 
received from ortoimage and they were compared with the 
coordinates that were received by the digitization of surveying 
diagram in scale 1:5000. After calculations it was calculated 
that 6,,,=1,245m and 6,.,=0.412m for the nine points. 
The data snooping control for gross errors in the nine control 
points accomplished successfully. There was no necessity to 
reject any point because of the type: 
0X;,0y;<1.960},,,=2.44m (7) 
As well as the type: 
02;<2.160,,=0.89m (8) 
Continuously a total accuracy control was applied, where the 
following type should be verified: 
Gu 50.92 exy cx 0.9 oy (9) 
As well as the type 
0.20.9980, 2 090, (10)