coaltech upper olifants river catchment wetland inventory ...

6. DATABASE CONSTRAINTS
The greatest problem was the lack of georeferenced aerial photographs for the area. Google
Earth was investigated to determine if it would provide value to the project, but the quality of
the images was too poor for the purpose of this project. The 1:50 000 aerial photographs, as
obtained from the Chief Directorate: Surveys and Mapping at the Department of Land
Affairs, Mowbray, Cape Town, was the only complete set of aerial photographs available
for the site. The georeferencing of these aerial photographs was based on the 1:50 000
topographical map and accuracy is therefore dependant on the accuracy of the topographical
layers. In addition, all aerial photographs are skewed towards the edges due to the concave
structure of the camera lens. We attempted to correct this feature during the georeferencing
process, but the effect could not be removed completely. The wetlands located to the edges
of the aerial photographs may therefore be slightly removed from their exact position.
Where areas under cultivation were obviously located on a seepage area the wetland was
indicated and the status of the wetland was given as E or F (destroyed). Some additional
seepage areas covered by cultivation may occur, but have not been identified. This is
because wetlands under cultivation are not easily distinguished. Patchiness in a cultivated
area can be caused by a number of influences, including differences in soil and patchy
distribution of fertiliser, in addition to the presence of seepage wetlands. Other highly
disturbed wetlands may also not be identifiable on the aerial photographs as wetlands and
these wetlands will be absent from the wetland layer. In addition, small wetlands are not
visible on the 1:50 000 aerial photographs and are therefore not included.
Tonal variations between the aerial photographs complicated the mapping of the wetlands on
the aerial photographs. The changes from one strip of aerial photographs to another may be
due to the time of day the photographs were taken and the light quality. Even without cloud
cover the light quality may differ from day to day.
7. COMPARISON TO THE NATIONAL LANDCOVER WETLAND LAYER
The final Landcover wetland layer, completed in June 2006, was compared to the completed
UORC wetland database. The Landcover wetland layer compared very well to the UORC
wetland database, since all of the Landcover wetlands were already identified in the UORC
database (Figure 20).
Wetland Database for UORC - 89 -