3-6 Rasterization

Conversion between raster and vector data is very useful in practical applications of GIS. Rasterization refers to conversion from vector to raster data. Raster format is more convennient to produce color coded polygon maps such as color coded land use map, while map digitizing in vector format is more easier to trace only the boundary. Rasterization is also useful to integrate GIS with remote sensing becaues remote sensing images are in raster format.

A simple algorithm for calculation of trapezoid area can be applied to convert vectorized polygon to rasterized polygon with grid cells as shown in Figure 3.7. If vertical lines are drpped to the x axis from two adjacent vertices, a trapezoid will be formed as shown in Figure 3.7.

The area of trapezoid is given by

Ai = (xi+1- xi) (yi + yi+1) / 2

The sum of all trapezoids will give the area of the original polygon as shown in Figure 3.7. Using this algorithm, the grid cells in the polygon are easily identified as shown in the upper right of Figure 3.7.