Istanbul 2004
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ial have been
5.1.1. Comparison of Reference DEM Datasets (Laser DEM
& BDTOPO DEM): Both the DEMs have been imported into
native format of Erdas Imagine. The affine transformation
option of Erdas Imagine has been used to transform the
BDTOPO DEM at 5m for comparing both the DEMs on same
pixel resolution. A common area on both the DEMs have been
extracted using the subset option of Erdas and the two sets of Z
values compared.
Table 1: Difference Statistics of Laser DEM and BDTOPO
DEM for Montmirail
Range of Height Difference: -20 to 27 m
Difference (in | Percentage Cumulative
Meters) of Pixels Percentage
of Pixels
Up to | m 60.53 60.53
Up to 3m 31.61 92.14
Upto5m 05.88 98.02
Up to 7m 01.42 99.44
Above7 m 00.56 100.00
As per the specifications of both the DEMs the accuracy is 1 m
rms. The statistical result shown above indicates that the DEMs
differ by more than ] m. Only 60.5 percent of the pixels are
under | m difference. Since the input sources for DEM
generation are different, there may be some difference in the
results. The other cause of difference may be due to digital
surface model (DEM + Canopy) output in case of Laser DEM. :
5.1.2 Comparison of Saphire DEM (SACDEM) with
Reference DEMs: The Saphire DEM and the reference DEMs
have been imported into the native format of Erdas Imagine.
Since the Saphire DEM has been generated in Geographic
projection system, it has been re-projected in Lambert-2 NTF
format with Clarke 1880 IGN ellipsoid. A common area on the
three DEMs have been extracted and Z values compared.
Results are tabulated in Tables 2 and 3.
Table-2: Difference Statistics of Saphire DEM with
BDTOPO DEM (10 m resolution) after compensating a
vertical bias of 30 m
Difference (in Percentage | Cumulative
Meters) of Pixels Percentage
of Pixels
Up to 10m 67.34 67.34
>10 mand <= 20 m 17.05 84.39
>20 m and < = 30 m n I 91.50
230 m and < = 40 m 3.33 94.83
>40 m and < = 50 m 1.80 96.63
>50 m and <= 60 m 1.01 97.04
Above 60 m 2.96 100.00
5.1.3 Comparison of PCIDEM and Reference BDTOPO
DEM: Both the DEMs have been imported into native format
of Erdas Imagine. A common area on both the DEMs have been
extracted using the subset option of Erdas. The PCIDEM was
generated for the maximum overlap area but has been extracted
according to the available BDTOPO DEM. Initially the DEM
was generated in UTM projection with WGS 84 spheroid and
datum. For comparison the DEM was re-projected into LCC
Clarke1880 IGN projection with NTF datum using Erdas
Imagine standard software re-projection tool. The comparison
results are much poorer (up to 20 m accuracy for 54% of points
and up to 10 m for only 30% of points) and hence not included.
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004
457
Table-3: Difference Statistics of Saphire DEM with Laser
DEM (5 m resolution)
Height Percentage Cumulative
Difference of Pixels Percentage
(in Meters)
Up to 1 m 24.26 24.26
Up to 2 m 15.77 40.03
Upto3m 6.02 46.96
Up to 4 m 6.11 53.07
Up to 5 m 5.20 58.26
Up to l0 m 17.11 75.37
Up to 20m 13.80 89.17
Up to 30m 5.10 94.27
Above 30m 373 100.00
Figure-1 gives a comparison of profiles from Saphire DEM
(SACDEM), PCIDEM and BDTOPO DEM in 10 m resolution.
Figure-2 gives a comparison of profiles from Saphire DEM
(SACDEM ) and LaserDEM in 5 m resolution.
Figure-1: A Diagonal Profile of BDTOPO DEM, Saphire
DEM (SACDEM) and PCIDEM (10 m resolution)
Comparison Graph Without Compensating Bias
300
450
400
350
E30 — TOPODEM
5 250 -——— —SACOEN |
$ 200 RD |
I
130
100
30
0
0 1000 2000 3000 4000 5000 6000 7000 8000
Distance (m)
Figure-2: A Diagonal Profile of LaserDEM and Saphire
DEM (5 m resolution)
Comparison Graph (LaserDEM Vs SaphireDEM)
600
500 Pry
450 X
L
7 4
; &. A EN. ur am
en f / Ä 77 v uti SEN TI
: y. N N
d —LaserDEM ——
Ar =
Saphire DEM.
4 ..Süpilre UEM.
0 1000 2000 3000 4000 3000 6000 7000 8000 3000 10000
Distance (m)
5.2 Melbourne Data Set: The DEM for this data set could not
be generated using PCI Geomatica software possibly due to the
