International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004 
  
  
   
       
15°N 
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14°N — 
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Study area 
     
        
Matagalpa 
   
  
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87°W] 86°W| 
Figure 1. Simplified map of Nicaragua with the location of the 
SPOT-5 Hi+Pan images. The white frame represents 
the area of investigation (equal to the Matagalpa 
INETER 3054-1V topographic sheet). 
85°W| 
  
mentioned maps were used for validating the input of SPOT-5 
products for hazard identification and risk assessment. 
2. VALIDATION PARAMETERS AND 
METHODOLOGY 
2.1 GIS and RS database 
CNES and Spot Image provided UNOSAT with two 60 x 60- 
km-wide satellite images centered on Matagalpa (Fig. 1), 
acquired during the dry season (19 April, 2003), with the 
following specification: 
— a level 1A, 2.5 m pixel size SPOT-5 Panchromatic 
image, and 
—  alevel lA 4-band, 10 m pixel size SPOT-5 Hi image. 
Both images were orthorectified to level 3 using a DEM based 
on the 3-arc-second SRTM data and 13 Ground Control Points 
(GCPs) from a Global Positioning System (GPS) field survey. 
The combination of both images gives a high-resolution 2.5 m 
4-band color image. 
In addition to SPOT-5 imagery, the project database, built by 
UNOSAT in cooperation with CIGMAT encompasses the 
following Remote Sensing (RS) and GIS products: 
— a l4-class land cover map, built from an unsupervised 
classification of the previous SPOT-5 Hi image, field- 
validated by CIGMAT, 
—  1:50°000 INETER topographic sheets (mosaic) 
covering the Rio Grande of Matagalpa watershed, 
—  aregional, 90 m pixel size DEM based on the 3-arc- 
second SRTM mission, covering the upper part of the 
Rio Grande of Matagalpa watershed, 
— 8 20 m pixel size DEM rasterized from topographic 
curves of the 1:50'000 INETER Matagalpa sheet, 
— a 100 m pixel size slope map, created from the 20 m 
DEM product, and a corresponding classified polygon 
slope map (7 classes in degrees: «5, 5-10, 10-15, ]5- 
25, 25-40, 40-60, 760), 
—  SAR land deformation maps of the Matagalpa urban 
area, processed by UNOSAT partner Gamma RS from 
a 1997-1999 set of six ERS images, 
— a8 GIS inventory of the water bodies (rivers, lakes, 
springs), extracted from the INETER maps and the 
study of Havlíéek et al. (2002), 
— several 1:50'000 geological vector layers (lateritic 
soils, landslide deposits, scarps, etc), based on the 
work of Havliéek et al. (2002), and 
— vector layers of the initiation points and run out areas 
of 1998 active terrain movements (Cannon et al, 
2001), covering a 10-km-wide square north of the city 
of Matagalpa. 
All these raster and vector products are in a standard map 
projection (NUTM 16, WGS 84). 
2.2 Hazard definition 
In this contribution, we will focus the suitability analysis of 
SPOT-5 imagery to the most common geological hazards that 
have occurred or might occur in the Matagalpa region and city. 
These are landslides, mud flows and debris flows (Carrefio and 
Barreto, 2000; Cannon et al, 2001; Havlièek et al, 2002). 
According to the nomenclature of Lateltin (1997) and Schneider 
(2001), landslides are defined as displacements by shearing of 
compact masses of loose or rock grounds along a failure 
surface. Mud flows are movements of material mass of 
polyphasic nature (solid fragments and water) similar to a fluid 
of variable viscosity. Lastly, debris flows are a mix of solid 
materials (blocks, gravels, etc.) transported by a viscous fluid 
(composed of fine sediments, clays and water) under the gravity 
and which occurs in the drainage network. Whereas mud and 
debris flows are instantaneous phenomena mainly driven by 
huge rainfalls (e.g., Hurricane Mitch), landslides are slow 
movements (from a few mm/y if substabilized to a few cm/y if 
active) that may last for years, with abrupt accelerating phases 
that might create along valleys stream blockage and subsequent 
breaking up (high flood risk, particularly at Matagalpa). Key 
risk factors, indicating a strong predisposition to terrain 
movements are, in addition to geology, the presence of water 
(proximity to river network, soil moisture, and discharge zones) 
and moderate to steep slopes. Second-order factors that 
reinforce this susceptibility include faulted zones, barren soils, 
deforested areas and informal settlements. 
2.3 Methodology 
There are two ways for assessing geological hazards: (a) by 
describing occurred and ongoing phenomena (inventory map 
that reports and represents signs and indicators of terrain 
movements) and (b) by classifying soil areas according to their 
predisposition to documented natural hazards (susceptibility 
map that orders according to three degrees which sectors are 
potentially risk areas for each natural hazard type). 
For the first approach, we tested the visual recognition on 
SPOT-5 data of morphologies or soils typical of terrain 
movements (disturbed vegetation, scarps, disturbed drainage, 
etc.; Dikau, 1999) and their spatial delimitation. The medium 
for this analysis is a combined Hi+Pan 2.5 m image, processed 
in pseudo-natural colors and draped over a DEM for 3D 
visualization. We also evaluated the potential of semi-automatic 
methods, based on image radiometry, and particularly the use of 
vegetation indices (NDVI) for generating low- or non-vegetated 
thematic vector layers (Liu et al., 2002), that are further filtered 
for extracting possible landslide or debris flow deposits. 
For the susceptibility approach, we evaluate the integration of 
SPOT-5 data to the multi-factor risk methodology developed by 
   
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