Theory
The cartographic objects of surface type are mainly the water areas.
With SPOT resolution wide rivers can be locally regarded as surface elements.
Water is more easily detectable on channel SPOT-XS3. It appears into
dark and is generally very homogeneous. However, reflections can disturb
its smooth aspect. In the case of seas and oceans, the edge of the coasts
can be strongly textured, because of the waves.
For surface elements, segmentation can be carried out by classification.
Different types of classification can be noted:
- Complete segmentation of the image,
is sometimes long and delicate to process. But the great number of classes,
involved in such a process, generally leads to obtaining very small areas
which should then be amalgamated.
- Partial segmentation consisting
in seeking only some classes reduces the risk of excessive partitioning,
but there remains partial.
- Local segmentation restricts and
constrains the process in a zone of the image, and it is necessary to have
an a priori knowledge to force the process.
The use of some criteria makes a significant improvement of the results:
- Multispectral bands give
the pixels reply in various wavelengths, and make it possible to better
characterize surfaces types.
- Space coherence, taking into account
the homogeneity of the neighbour pixels, has the advantage of using only
one channel to obtain the solution.
- Structural information are
used to locate the objects in a certain context, for example the altitude
or the proximity of other objects favourable or not with their existence.
Of course, the combination of all this information, insofar as they
are available, increases the quality of the results.
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