Application of Remote sensing and GIS for Road (Asian Highway) Network Database creation and Risk Assessment
3.4 Database for River Crossings
for each of the major roads, the number of bridges exists in rivers and streams crossing have been identifies using topographic maps and field visits. Location of each of the bridges is recorded in the kinematic GPS. A database for the bridges has been created.
3.5 Slope Stability Analysis
The slope stability is analysis is done using fundamental engineering principles. safety factor is calculated room the ration of driving moment to resistance moment. Slopping areas are divided into number f slices by conversing into raster format for ease of calculation
Fellinious method has been used to calculate slope stability. The factor of safety is calculated using he following relationship
T= RW Sina (1)
N=R(CL+tanF W cosa) (2)
Where T= Driving moment, N=Resisting moment, R=Radius of critical circle,
W=Weight of each slice , a=Angle between horizontal axis and the base of slice,
L= Length of base slice, C=Cohesion and j= Angle of shearing resistance.
The factor of safety is determined for the following relationship.
Fs =SN/ST (3)
From the topographic map, the contours are digitized at the interval of 20 meter and a Digital Elevation Model (DEM) is created with grid size of 20m. a slope aspect map is produced from the DEM. Steepest slope direction for each for the pixel is extracted comparing the surrounding pixels and the profile is drawn. Geo-technical data such as angle of shear resistance and cohesion are used as input for an algorithm to generate a road risk map for the study area.
For determining an alternative route for the highways in most vulnerable locations, a weighted distance function has been used. This function calculated the shortest weighted distance or accumulated travel cost from each cell to nearest cell n a grid. from the accumulated travel cost, a least cost path route can be derived to any destination cell in a grid.
The grid where accumulated distance function is applied contains cell value depending o safety factor and slope. Higher value is assigned to the cells with high slope failure risk and high slope. Figure 5 shows the complete methodology for creating the road network database and risk assessment.
Figure 5: Flow chart showing the complete methodology
5. Result and Discussion
Road network database can be updated very easily using satellite images . while creating an updated vector database for road, river or contour lines, errors are unavoidable while digitizing , keeping precisely corrected image in the background this kind of errors can be rectified. The new roads that are not available in the topographic maps cab be detected in the satellite image. The approximate road width can also be determined from the images. The highway considered in the study has 22 km length with wide croo-section , 15 km length with medium cross- section and 11 km in length with narrow cross-section as shown in figure 7. There are 13 numbers of rovers or streams crossings with bridges and culverts.
Figure 6a: Vulnerable (Fright) Areas
Figure 6b: New Alignment
Figure 7: Type of Roads
As far as road stability is concerned, the road network in the study area shows high stability in flat areas. Figure 6a is a slope failure risk map where bright areas represent high risk of slope failure. An alternative route has been determined for the portion of road as shown in the fig 6 b where high risk is involved .
5. conclusions
Information exits in maps become outdated quickly, particularly I countries where the rate of infrastructure development is very high . satellite images provides an opportunity it update maps at an affordable cost. Using satellite images , road information can be updated and approximate with of a road can be determined.
The stability of slope along the road can also be determined using GIS which is very vital form construction point of view. If a particular section of road is very vulnerable , then and alternative route can be suggested.
The stability of slope along the road cab also be determined using GIS which is very vital form construction point of view . if a particular section of road is very vulnerable, then and alternative route cab be suggested.
6. References
- Topographic Maps (1:50,000), Royal Thai Survey Department, Bangkok, Thailand
- UN-ESCAP, 1997 , Asian Highway Format and Development for the Asian Highway Database, United Nations , New York.
