Geography Reference
In-Depth Information
the optimum solution may be a survey carried out by radar. Other times we find people
who do not even think of using photographs, because beforehand they state that the
solution is through the use of orbital data. But it may happen that in this type of survey you
may not get the acquired image with the geometry and/or level of detailing most
appropriate for your application. In other words, the choice of the sensor should be made by
an evaluation comparing costs and benefits, focused on the application. Each option
normally presents benefits and drawbacks which should be evaluated case-by-case.
Since the appearance and dissemination of high resolution (spatial) sensors, they became the
source of the most desired images for several applications that work with medium to large
scales. However, not always the spatial resolution is what matters. For several thematic
applications, such as mapping of vegetation or land coverage and land use, the spectral
resolution is a factor equally or more important than the spatial resolution. On the other
hand, one problem which is often overlooked in the planning is the time that it may take to
acquire an image (or pair of images) for a certain location. Even with sensors presenting a
small revisiting period, for meteorological issues or for great data demand, months may
pass until the data is effectively acquired, even if paying a priority rate. For several
applications this may be a limiting factor.
There are other basic issues that are sometimes neglected: when choosing to pay an
expressive amount for high spatial resolution images, it is assumed that the application is
for scales that need greater detailing which, at their turn, demand also greater accuracy in
terms of geometry. However it is not rare to find users who acquire such data and do not
make the appropriate geometric modeling/correction - the orthorectification, with RPCs
supplied by the satellite owner, GCPs (ground control points) with compatible accuracy and
appropriate DEM. Even images with good internal geometry will present relatively high
external errors if they are not well modeled, especially in areas with great attitude
differences. And this is not acceptable in the immense majority of applications that
effectively require data with great detailing.
Users of images that allow a more regional coverage - sensors normally included in the
category of medium spatial resolution - should be feeling more difficulty to update data
since Landsat 5 presented problems in November 2011 in [20], since there is practically no
alternative that offers the same possibilities (especially considering cost and data
availability). Environmental studies have a great demand for data of this category and need
to search for alternatives, as the use of sensors of greater spatial resolution, normally losing
spectral resolution - with highlight for the lack of availability of the SWIR band - in most of
those sensors.
While the producers of thematic data are experiencing a certain period of low availability of
these sensors of medium spatial resolution and highest spectral resolution, the opposite has
been occurring for producers of reference cartographic data. These sensors of medium to
high spatial resolution (between 2.5 and 5 meters), high (between 1 and 2.5 metros) and very
high (below 1 m) can rely on a great offer of inputs. New sensors with higher spatial
resolutions are becoming available almost every year. In Brazil, as mentioned before,
