The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
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SRTM DEM and points/profiles from Geoscience Laser 
Altimeter System (GLAS) onboard ICESat. 
The aim of this paper is to compare the difference between the 
Canadian Digital Elevation Data (CDED) level 1 and SRTM 
data over Mount Carleton in New-Brunswick. We will contrast 
comparisons between CDED and SRTM DEM in the first step 
of the examination of the difference. In a second step, the 
comparison will be done between CDED with ICESat’s 
points/profiles. The land cover and the morphology of the relief 
will be used as additional elements for the comparisons. Three 
slope classes and eight geographic directions will enable us to 
perform this vertical assessment. Concerning CDED level 1, we 
want to reveal as stated by Aguilar Fernando J. and al. (2005), 
that the slope (morphology) and the land cover (vegetation) 
have a relative influence impact on the accuracy of a DEM. 
2. DATA AND METHODOLOGY 
2.1 Study area 
Figure 1: The location of the study area in the New Brunswick 
province (Natural Resources Canada) 
The study area is located in the province of New Brunswick, 
centered at the coordinates 47°23’ N and 66°53’ W which 
corresponds to the position of Mount Carleton. The area covers 
an area of approximately 36 x 24 km. This mountain is the 
highest elevation in the Canadian province of New Brunswick, 
and is also the highest peak in the Canadian Maritime Provinces. 
With a maximum elevation of 817 meters, it is also one of the 
highlights of the Canadian portion of the International 
Appalachian Trail (Figure 1). This region was chosen because 
of his land cover dominated by various species and his location 
in the east part of Canada. 
2.2 The reference Canadian Digital Elevation Data 
The Canadian Digital Elevation Data (CDED) is today available 
at no cost on http://www.geobase.ca/. The reference CDED 
used is of level 1. The region of Mount Carleton is included in 
the 021007 of the National Topographic Data Base (NTDB) at 
the scale of 1:50 000. This is a digital terrain model depicting 
ground elevation in geographic coordinates with spacing of 
0.75" x 0.75". For the purpose of future comparison with SRTM 
model and ICESat points/profiles, it was essential to well 
prepare the data so that they should be compatible and subject 
of comparisons. The first step was to convert these DEM 
from .dem format to .tiff format. The two part of the DEM (East 
and West) were merged. All studies employing DEM make use 
of planar coordinates to have the same measurement units for 
both (x, y) and elevation. The CDED level 1 was provided in 
geographic coordinates (longitude (X) and latitude ($)); 
therefore it was necessary to reproject the CDED level 1 to 
NAD83 UTM zone 19, because the reference of the other data 
of the study will respect that grid. The software ESRI® 
ArcMap™ 9.2 was used to conduct the reprojection with the 
bilinear interpolation resampling method. This option, which 
performs a bilinear interpolation, determines the new value of a 
cell based on a weighted distance average of the four nearest 
input cell centers. When dealing with different datasets 
resolution, there is often a need of data sampled at one scale to 
be generalized to other scales. Our aim in the study is to 
compare three elevation datasets sampled at different scales 
(spatial resolutions). The pixel size of the CDED level 1 was 
19.56 m after the reprojection. Finally, from the 1201 x 1201 
grid, we obtained a new one with 455 columns and 304 rows. 
The elevation range point is from 242 to 808 m. These 
elevations are orthometric and expressed in reference to mean 
sea level (Canadian Vertical Geodetic Datum 1928 (CVGD28)). 
In order to compare the three datasets, the reference CDED 
level 1 will be subject of an aggregated pixel size, matching the 
dimensions of the SRTM grid for example. 
2.3 SRTM data 
The Shuttle Radar Topographic Mission (SRTM) successfully 
collected Interferometric Synthetic Aperture Radar (IFSAR) 
data over 80% of the landmass of the Earth between 60°N and 
56°S latitudes during an 11-day Space Shuttle mission in 
February 2000. This mission has created an unparalleled data 
set of global elevations that is freely available for modeling and 
environmental applications. There are two SRTM products in 
raster format: The 30 m (1") spatial resolution and 90 m (3") 
data which is available globally (80% of the Earth surface). The 
30 m data is available only for the USA territory. 
Many homepages provided these data for example at 
ftp://e0srp01u.ecs.nasa.gov and http://seamless.usgs.gov/. The 
SRTM data used in our study provided from 
http://srtm.csi.cgiar.org.They are derived from the 
USGS/NASA SRTM data and distributed in decimal degrees 
and datum WGS84 (Jarvis A., H.I. Reuter, Nelson, E. Guevara, 
2006). Since these data were provided in geographical WGS84 
system, therefore it was necessarily to reproject them in NAD83 
UTM 19, to respect the datum of the reference CDED level 1. 
For this operation bilinear interpolation method was used. The 
spatial resolution was maintained. The vertical datum is mean 
sea level as determined by the same WGS84 Earth Gravitational 
Model (EGM 96) geoid. The Elevation range point here is 
between 233 and 794 m. 
2.4 ICESat data 
ICESat (Ice, Cloud, and Land Elevation Satellite) launched 12 
January 2003, as part of NASA's Earth Observing System, is a 
satellite mission for measuring ice sheet mass balance, cloud 
and aerosol heights, as well as land topography and vegetation 
characteristics (Zwally et al. 2002 and Schütz et al. 2005). 
These measures are accomplished using the Geoscience Laser 
Altimeter System (GLAS) combined with precise orbit