The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
4.3 Assessment result of the DEM layer 
Assessment against Japan Triangulation Network: There is a 
triangulation network in Japan providing a set of geographically 
accurate points for survey, mapping and so on. Various 
attributes such as longitude, latitude and elevation of these 
points are freely available through a web service from GSI. The 
latitudes, longitudes and elevations of the 2207 points of the 
triangulation network in the assessment area were manually 
extracted from the web service and used as one reference data 
set to assess the two Reference3D tiles (Figure 3). The black 
dots in higher elevation area and white dots in lower elevation 
area (Figure 3) indicate the positions of the points used. 
Figure 3. Points of Japan Triangulation Network over DEM 
layer of the two Reference3D tiles (N43E144, N43E145) 
Because the points of the triangulation network are not aligned 
with the grid vertices of the Reference3D, an interpolation is 
necessary to superimpose the two data sets. The elevation of 
each point of the triangulation network was kept unchanged, 
and the correspondent value of the Reference3D at the point 
location was calculated by bilinear interpolation of four vertices 
of the Reference3D grid enclosing the point. The elevation 
difference was calculated for each point, then from those 
differences, standard deviation and mean value were calculated 
(Table 2a). 
Whole area 
Flat area 
Mean (m) 
-2.2 
-1.7 
STDEV (m) 
5.3 
1.9 
Max (m) 
11.7 
3.3 
Min (m) 
-91.7 
-8.3 
LE90(m) 
6.7 
4.1 
Table 2a. the Reference3D compared against Japanese 
Triangulation Network: general statistics 
-92 -84 -76 -68 -60 -52 -44 -36 -28 -20 -12 -4 
Figure 4. The histogram of the elevation differences between 
Reference3D and the Japan Triangulation Network 
To confirm the specification of the Reference3D, the linear 
error at 90% confidence level was also calculated (Table 2a). 
An agricultural area of low slope Was separately assessed (area 
within the white rectangle in the Figure 3) and the result is also 
shown in table 2a. The histogram of the whole area is shown in 
Figure 4. Difference intervals were summarized in table 2b. 
Difference 
intervals 
Whole area 
2,207 points 
Flat area 
367 points 
-5 / +5m 
85.5% 
98.4% 
-10 / +10m 
96.6% 
100% 
-15 / +15m 
98.3% 
100% 
-20 / +20m 
99.0% 
100% 
Table 2b. the Reference3D compared against Japan 
Triangulation Network: error intervals 
The difference intervals (Table 2b) show that the difference for 
99% of the points over the whole assessed area is less than 20m. 
In the flat area, the difference is less than 5m for more than 
98% of the points. 
Unfortunately there are very few points in the mountain area 
(white part of Figure 3) so we did not assess mountain area 
alone. There is no point of the triangulation network over the 
two islands within the assessment area, so no assessment was 
done for the two islands. 
Cross evaluation between Reference3D DEM layer and GSI 
DEM: To superimpose the two raster data sets of the 
Reference3D (30m mesh, WGS-84) and GSI DEM (50m mesh, 
Tokyo Datum), the common raster cell size, geodetic coordinate 
system and matrix size (number of rows and columns) are 
prerequisites. Therefore, at the beginning of this assessment, the 
Reference3D DEM raster data was transformed to the geodetic 
coordinate system of GSI DEM (Tokyo Datum) through the 
sophisticated software (ENVI), and a bilinear interpolation 
approach was used to resample the Reference3D DEM to the 
same raster cell size and matrix size. Then by superimposition 
of the corresponding samples, the difference between the two 
DEMs was calculated as “Reference3D DEM - GSI DEM” (see 
histogram in Figure 5). 
Figure 5. Histogram of the elevation differences between the 
Reference3D and GSI (whole area) 
From those differences, standard deviation and mean values 
were calculated for tiles N43E144 and N43E145 as shown in 
Table 3 a. The Table also includes LE 90% as well as maximum 
and minimum differences for reference. The cell with elevation 
1315