Lavakas of Madagascar

This GIS database (created with ArcMap v. 10.8) contains location, elevation, and slope aspect data for 61,223 lavakas throughout Madagascar.  These data underpin the analysis presented in Cox et al. (2010) "Shakedown in Madagascar: Occurrence of lavakas (erosional gullies) associated with seismic activity". 

The image base for counting these lavakas was created from orthorectified Landsat ETM+ images, sharpened to 15 m/pixel resolution. We selected images taken in April or May where possible, because vegetation cover differences are most marked at the end of the rainy season in Madagascar, providing maximum colour contrast between active gullies and non-lavaka'd terrain. We used 7-4-2 band composites, imported into ENVI for image processing. The band composites were first linearly enhanced, and then run through Gram-Spectral sharpening using the panchromatic band 8 (to which a median pass filter had first been applied). The sharpened images were projected in ArcMap 9.2, and further enhanced: first by applying a second pan-sharpening (simple mean) using the panchromatic band 8 image, and finally by increasing image brightness by 8-12% and increasing contrast by 45-55% (both adjustments via the “properties” function of the projected image layer).

Lavakas were counted by systematic east-west lawnmowing on 33-km2 panels (5 km x 6.5 km) at a scale of 1:25,000. Exposed laterite or saprolite—indicating active erosion—showed as dramatic pink in the enhanced images. Only active lavakas (those clearly showing exposed earth in their interiors) were included. Lavakas were distinguished from other gully or ravine types by their characteristic tadpole or inverted-teardrop shapes; and from non-gullied areas of bare ground by the combination of shape and shadowing indicating steep amphitheatre walls.  Comparing the data with ≈2 m/pixel imagery indicates that the database is complete for the largest 50% of lavakas, and close to complete for the largest two-thirds. Lavakas <900 m2 (4 pixels), representing about a third of lavakas (based on field measurements of 433 lavakas: Wells et al., 1991, and unpublished data), are underrepresented in the database because of resolution issues.

Works cited:

Cox, Rónadh, et al. "Shakedown in Madagascar: Occurrence of lavakas (erosional gullies) associated with seismic activity." Geology v. 38 (2010): p. 179-182.

Wells, Neil A., Benjamin Andriamihaja, and HF Solo Rakotovololona. "Patterns of development of lavaka, Madagascar's unusual gullies." Earth Surface Processes and Landforms v. 16 (1991): p. 189-206.