Drawing shows graphically how the contours are converted to a three dimensional surface.  First the input vertices from the digitized contours are resampled; not every point is necessary to make a Triangular Irregular Network (TIN) that accurately reflects the topography represented in the topographic map.  Two parameters are used to perform this data reduction; weedtolerance, which eliminates points along the arcs at a specified interval and proximal tolerance which eliminates, or includes, point from adjacent contours in the triangulation.  Determining the optimum point reduction parameters (weedtolerance and proximal tolerance) was made easy by AML.  The main problem when constructing a TIN from contours is when the closest three points used to make a triangle are sample from the same contour, since all three have the same elevation the triangle has zero slope and is perfectly flat.  This constitutes an error in the surface, correcting these errors by adding supplementary points between contours and along breaklines (ridges and valley bottoms) is effective but errors remain.  Setting the proximal tolerance and weedtolerance that resulted in the least flat triangles was done using a iterative program; AML varied these two parameters between a reasonable range of values and determined the settings that resulted in the fewest errors.  There are two graphics that follow this one in the main table, both how weedtolerance and proximal tolerance reduce the number of TIN input points.  The last step is to interpolated a continuous surface of points, called a lattice, from the TIN.  This can be done using Quintic Interpoaltion of Linear Interpolation, the difference between these two methods is depicted at the top of this graphic.