Geography Reference
In-Depth Information
m. The DEM was generated with resampling, taking the pixel to 2 m. The altimetric
accuracy was consistent with
Class C for 1:10,000 scale
, since 90% of points used in the
evaluation showed errors of up to 3.2 meters and SD of 2.3 meters.
4.3. LIDAR: Planialtimetric assessment of orthoimage, DTM & DEM
The LIDAR (LIght Detection And Ranging) used in this assessment comprised not only
DTM and DEM generated by LIDAR proper, but also orthophotos generated from photos
acquired during the survey with a non-metric camera. The point cloud was not assessed,
since the ESPAÇO laboratory only received the models generated and the orthophotos,
which presented a 25 cm (10") resolution. The LIDAR sensor used was Optech ALTM 2050
(Airborne Laser Terrain Mapper), configured for a point density equivalent to one point at
each 80 cm. The airplane used, besides outfitted with a double frequency GNSS tracker, also
used an IMU (Inertial Measurement Unit), responsible for recording the airplane altitude
variations. The great interest in LIDAR, besides the good accuracy expected and speed in
data processing, is the possibility of penetration into the vegetation, due to its high point
density.
Thirty-six (36) evaluation points were used, with 29 identified in the orthophotos. All points
had their coordinates determined by relative static positioning, with single frequency GNSS
trackers.
Study area is located near the border between São Sebastião and Caraguatatuba,
municipalities of the northern coastline of the state of São Paulo. This area is inserted in the
Serra do Mar Range, which in this area presents height differences of up to 800 m, with
vegetable cover of the tropical forest type.
Orthoimages
showed excellent planimetric accuracy, presenting a CE90 (Circular Error at
90%) of 0.86 m and SD of 0.3m. With these values it is possible to say that it presented an
accuracy compatible with
class A for 1:2,000 scale
because, besides the SD being below the
limit for this scale, 93.1% of the 29 points used in the assessment were below the limit for
this scale.
The
DEM
presented good altimetric accuracy, with a LE90 (Linear Error at 90%) of 1.33
meters and SD of 0.73 m, presenting itself compatible with the indication by PEC for
class C
for
I:5,000 scale
, assuming that the contour lines present 2 m of contour interval. A total of
33 (91.7%) of the 36 points used in the assessment presented errors below specification
(1.5 m).
The
DTM
presented an even better performance, presenting a LE 90 of 0.58 m and SD of 0.4
m, which would make it compatible with the indication for a
class B for I:2,000 scale,
since
91.6% of the points used in the assessment presented errors up to the specified value (0.6 m)
for this class.
The results obtained in the assessment of the LIDAR data were very good, but it was not
expected that the altimetric accuracy of the DEM would be lower than the DTM accuracy. It
