Small format aerial photography (SFAP) is a low cost, do-it-yourself technique to obtain actual data that can be used for a wide range of applications. For example to detect recent changes caused by disasters, like mudflows, flooding and earthquakes, but also to analyze urban changes, land degradation or land use changes over a longer period, when recent images are not yet available.

This exercise focuses on creating a photo mosaic of seven small format aerial photographs of the Longonot-Kijabe Hill area just southeast of Lake Naivasha in Kenya, which is currently suffering from severe wind erosion. An important constraint to the proper analysis of the wind erosion problems is the general lack of up-to-date information about the area. This constraint includes the total absence of recent aerial photographs from the area. The most recent aerial photographs are from 1991, but the land degradation problems as a result of wind erosion started only after 1995. Landsat TM satellite images are available of 1989, 1995 and 2000 but they generally have a too low resolution for the analysis of wind erosion features. Under such circumstances there appears to be a clear need for the application of small format aerial photography (SFAP) - partly in complementary use with conventional aerial photography - to analyze and assess wind erosion problems.

Before the seven photographs can be assembled into a single mosaic they have to be georeferenced (and rectified). As the imaged terrain is almost perfectly flat (all points are coplanar), the aerial photographs do not contain relief displacement. In such cases the photographs can be rectified by a simple projective transformation, using a perspective projection (from plane to plane).

After this process the maps are resampled to new output maps and glued together to form a mosaic of the area.

Overlaying the photo mosaic with the land use maps of 1991 and 2000 gives interesting information about causes of recent land degradation in the Longonot area. Between 1991 and 2000 the arable fields area was reduced from 145 ha in 1991 to only 57 ha in 2000. Almost 50% of the deflation trenches are found in areas where agriculture was practiced in 1991.

References

• Warner, W. S., R. W. Graham, et al. (1996). Small Format Aerial Photography. Caithess: Whittles, 1996. 348 pp.; 25 cm. ISBN 1-870325-56-7. (ITC Adlib 528.7)
• Hofstee, P. (1984). Small format aerial photography: simple and cheap do-it-yourself technique. In: Cities vol 1 no.3, Feb 1984, pp.243-247.
• http://www.gartrip.de/ An article in the GARtrip FAQ, subject Precision on the accuracy of the Garmin 12 (after disabling the selective availability in May 2000).

For information on this case study, contact:

A. Nagelhout and P. Hofstee
Department of Urban and Regional Planning and Geo-information Management,
International Institute for Geo-Information Science and Earth Observation (ITC),
P.O. Box 6, 7500 AA Enschede, The Netherlands.
Tel: +31 53 4874237, Fax: +31 53 4874399, e-mail: hofstee@itc.nl