Geography Reference
In-Depth Information
2004. As these products are increasingly used in the production of cartographic data, a brief
summary of the results obtained in the case studies carried out by members of the research
group during that period is presented. To facilitate the search, the results were grouped in
three different Classes of scales: 1) scales over 1:25,000; 2) 1:25,000; 3) 1:50,000 or less.
All assessments of the planimetric and/or altimetric accuracies used as reference what is
provided in the PEC; which establishes that 90% of the points tested should present errors
below certain tolerance values and the total set of points cannot surpass a standard
deviation limit. These thresholds are determined for each quality class, which varies from A
(higher accuracy) to C (less accuracy). Table 1 below presents the specific thresholds for the
planimetric and altimetric accuracies.
Planimetry Altimetry
Class
Tol. (mm) SD (mm) Tol. SD
A
0.5 0.3 1/2 Eq. 1/3 Eq.
B
0.8 0.5 3/5 Eq. 2/5 Eq.
C
1.0 0.6 3/4 Eq. 1/2 Eq.
Tol.: Tolerance; SD: Standard Deviation; Eq.: Altimetry difference between contour lines (Contour Interval)
Table 1.
Limits of planimetric and altimetric tolerances indicated by the PEC
It is emphasized that, despite the PEC not being indicated for assessment of digital products
and inputs used in the cartographic production process, this standard was adopted in the
studies here performed for being the only existent official norm in Brazil for assessment of
cartographic products. The use of the PEC serves mainly, to have a national reference for
comparison of the planialtimetric accuracy of cartographic products.
It is stressed also that the assessments refer only to the geometry related accuracy. Most of
the assessed products - if not all of them - present information extraction capacity for lower
scales than those scales for which its geometry is compatible.
All assessments should be treated as case studies, in view that they refer to the assessment
of a unique scene/acquisition for each product, with the possibility of the existence of
different results for other study areas. The assessment of planimetric accuracy was always
carried out using the generated orthoimage, while the altimetric accuracy evaluation was
made directly on the DEM and/or DTM obtained from the sensor images.
The results for each group are presented below, describing the types of evaluated sensors,
their common features, besides the results themselves found in the case studies.
4. Scales greater than 1:25,000
4.1. IKONOS 2: Planialtimetric evaluation of the orthoimage and DEM
Ikonos 2 satellite was launched in 1999, being the first one with high-definition
available on the market, with a spatial resolution of 1 m for the panchromatic band and
