The building boundary extraction and normalization are the key approach for LIDAR data processing and building 3D modeling.In this paper

"Alpha Shapes algorithm" is first applied on the LIDAR data to extract the building boundary.In addition

an enhanced boundary simplifying algorithm

i.e."Pipe Algorithm" and two other developed normalization algorithms

"Circumcircle Regularization Algorithm" & "Cluster and Adjustment Algorithm" are used to improve the extracted boundary.Finally

the normalized building boundary is generated perfectly with these algorithms.A limited accumulated points S has an alpha shape in polygon.This polygon is determined by S and α.We can imagine that a circle with an α radius is rolling around the S.When α value is big enough

the circle will not fall into the area of accumulated points.The rolling track will form the boundary of these discrete points（for example LIDAR data）.Contrarily

when the α value is very small（α→ 0）

every point might be the boundary.When the alpha value is approaching infinity（α→∞）

alpha shape will be the convex hull.When the S contains evenly distributed points and α value approaching optimum value

the alpha shape can extract the inner and outer boundary of convex and concave polygon.The boundary obtained from Alpha Shape Algorithm above is rough which can be defined as raw boundary.In this paper

an enhanced simplifying algorithm

i.e."Pipe Algorithm" is developed to simplify the raw outline which usually is composed of zigzag shape.Pipe Algorithm retrieves the polygon inflexion points based on the changes of angle direction.These inflexion points are retained while the intermediate points are eliminated.The remained inflexion points will establish a basic framework of the polygon.At the same time

two other developed normalization algorithms

Circumcircle Regularization Algorithm & Cluster and Adjustment Algorithm are used to improve the extracted polygon framework.Now

the two normalization algorithms can be applied for four sided and multi-sided polygon（greater than four sides and the number of side must be even number）.Compared with other algorithms

"Alpha Shapes" algorithm can effectively and stably process LIDAR points-cloud data with high precision.At the same time

it can keep fine features of any building shape boundary while filter the footprints of non-building.The experiment results show that these algorithms are excellent in building boundary（convex & concave polygons） extraction and normalization.The error between the extracted building boundary and the actual outline is usually less than 0.5 m.