The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
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Figure 10. Reference3D Reliability Mask over the maximum 
error areas (black areas) 
MQu masks (black zone in Figure 10) shows that artefacts were 
suspected by IGN over the minimum and maximum error areas. 
The gray zones in Figure 8 and 9 represent MQu masks over the 
two profile lines and suggest that the elevation values within the 
gray zones of Figure 8 and 9 have less reliability. Therefore the 
most probable causes should be #2 or #5 in the above list. 
4.5 Horizontal accuracy assessment of the orthoimage 
To assess the horizontal accuracy of the orthoimage, a 
preliminary check was done for the two Reference3D tiles. 
Because the small land coverage and snow over the islands in 
tile N43El45, the horizontal accuracy of the orthoimage was 
only assessed for the tile N43E144 (Figure 11). 
The 1:25,000 digital map of GSI, which can be freely accessed 
online, was used as the reference. The first step of the 
assessment was to find checking points which could be 
identified clearly in both the orthoimage and the digital map. 
Then the longitudes and latitudes in the orthoimage and in the 
digital map were recorded. The differences between the two 
latitudes and longitudes of each checking point were calculated 
and converted from degrees to meters (AX for longitude and AY 
for latitude). Finally the horizontal circular errors were 
calculated from AX and AY of all checking points and the 
statistics were made. 120 checking points were manually found 
(white points in Figure 11). Because the Reference3D data was 
acquired in winter, snow in the orthoimage limited the number 
and location distribution of the checking points. Most of the 120 
points were centres of road junctions which were easy to 
identify in the orthoimage and the digital map. 
Figure 12 shows the circular errors of the 120 checking points. 
All points are within a circle of 18.2m radius. 
Figure 12. Circular errors for the 120 checking points 
Figure 9. Profile of the place with maximum negative 
difference: 142m at the top of the volcano, 
gray zone : MQu mask 
These errors can be interpreted in several ways: 
1. either an effective modification of the relief since the 
GSI data were acquired , 
2. or an artefact within Reference3D, 
3. or an artefact within the GSI DEM, 
4. or an interpolation or smoothing effect in either DEM, 
5. or any combination of the above. 
The best way to know the exact reason for these errors would 
be to compute the DEM from a 3 rd source, or to perform field 
works. However, as mentioned above, Reference3D includes a 
set of Quality Masks which give information about the quality 
of the DEM. One of the masks, named MQu, shows the 
reliability of the elevation given by the Reference3D DEM, as 
seen by the producer himself at the end of the production chain. 
Figure 11. Checking points (White dots) in tile N43E144 
16 
12 
8 
4 
0 
-4 
-8 
■12 
•16 
20 
800 
'i 700 
I 600 
I 500 
LU 
400 
300 
' I 1 1 1 ■ r ,- r . r - r .,.| T-| ill I i I I ' ' I ' ■ I I I ■ ■ ’-I '''I 
-16 -12 -8 -4 0 4 8 12 16 20 
A longitude (m) (X) 
500 1000 1500 2000 
Distance (m) 
GSI 
— Reference3D