Geography Reference
In-Depth Information
all surveyed with GNSS, through relative static positioning, with single frequency trackers -
in function of the variation of the pixel size and the coastline. The study area corresponds to
part of the municipality of São Sebastião on the north coast of the state of São Paulo, in
southeast Brazil.
Table 2 presents a summary of the assessment results from the SRTM DEM in the four
versions. In it, the number of assessment points used in each product are shown, as well as
the percentages of points with errors up to the limit for a
class C for scale 1:50,000
(15 m)
and
class A for scale 1:100,000
(25 m) as well as the Standard Deviation (SD) of each set of
points used in the assessment for each DEM SRTM version.
DEM
SRTM
Assessment
Points
Class C
1:50,000
Class A
1:100,000
SD
(m)
Version 1
90
91.10%
100%
6.82
Version 2
91
90.50%
100%
6.85
Version 4
91
90.10%
100%
6.84
TOPODATA
92
83.30%
93.40%
10.38
Table 2.
Summary of assessments of the 4 versions of DEM SRTM
From the results presented in table 2 one perceives that versions 1, 2 and 4 have very similar
performances and present altimetric accuracy very near to the limits indicated for
class C of
scale 1:50,000
. Only Topodata is indicated only for
class A of scale 1:100,000
. As the main
objective of Topodata is to obtain a better detailing of the morphometric information
derived from SRTM DEM, a small loss of accuracy is tolerated in favor to gain better
detailing of the morphometric representation. Additionally, the goal and SD observed
should be emphasized, mainly in versions 1, 2 and 4, showing that the DEM SRTM presents
homogeneity and consistency in results.
6.2. ASTER/VNIR: Planialtimetric assessment of the orthoimage and DEM
ASTER images (Advanced Spaceborn Emission and Reflection Radiometer), onboard Earth
satellite, are very interesting for being of low-cost and providing good spectral resolution,
besides allowing the generation of DEMs from the stereoscopic pair of the VNIR (Visible
and Near Infrared) sensor. This sensor provides a spatial resolution of 15 m, counting with
the green, red and near infrared bands (the latter being acquired with two different
incidence angles in a same satellite passage). Due to these features, those images can be
useful both for thematic as for reference cartography.
Study area again was the vicinities of the municipality of São Sebastião. An assessment was
carried out based on the results made from the images and DEMs generated according to
two different approaches:
With Orthoengine 10.2 by PCI Geomatics based on 2 scenes (same date) acquired
September 19, 2008, with processing level L1A, making use of 12 control points and 9
Tie Points per scene.
