DEFORESTATION AROUND THE WORLD - India Environment Portal

Geospatial Analysis of Deforestation
and Land Use Dynamics in a Region of Southwestern Nigeria
Fig. 4. SPOT Satellite XS 1986
With constraints such as spatial, spectral, temporal and radiometric resolution, relatively
simple remote sensing devices cannot record adequately the complexity of the Earth’s land
and water surfaces. Consequently, error creeps into the data acquisition process and can
degrade the quality of the remotely sensed data collected. Therefore, it is necessary to
preprocess the remotely sensed data before the actual analysis. Radiometric and geometric
errors are the most common types of errors encountered in remotely sensed imagery. The
commercial data provider has removed the radiometric and systematic errors of the data
used. However, while the unsystematic geometric distortion remains in the image. The
geometric errors were corrected using ground control points (GCP).
The process of georeferencing in this study started with the identification of features on
the image data, which can be clearly recognized on the topographical map of the study
area and whose geographical locations were clearly defined. Stream intersections and the
intersection of the highways were used as ground control points (GCP). The latitude and
longitude of the GCPs of clearly seen features obtained in the base map were used to
register the coordinates of the image data used for the study. All the images were
georeferenced to Universal Transverse Mercator projection of WGS84 coordinate system,
zone 31N with Clarke 1880 Spheroid. Nearest-neighbor re-sampling method was used to
correct the data geometrically. A correlation threshold was used to accept or discard
points. The correlation range was within limits that is, 1 pixel size. The x and y corrections
were below 0.5 pixel.
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