Elevation data are collected and plotted at discrete locations. The data do not necessarily form an organized grid. Start by determining the assumed elevation between any two points. For example, between the 203 and 255 data points all of the elevations higher than 203 and lower than 255 exist. If you were to draw a contour line with the elevation of 225, it would be placed between these two points. An elevation c 225 would not exist between the points 255 and 235. So do not draw the elevation line representing 225 between them. Using the block diagram below draw contour interval at 25-foot intervals. The "300-feet" elevation lines are already drawn. Lines will disappear off the grid edge (figure 11)