There has been a rapid movement for GIS analysts to create life-like, three-dimensional city models for purposes such as homeland security. When it comes to 3D mapping, LIDAR data is advantageous because it enables analysts to collect Z-value data (depth), in addition to X- and Y-values (width and height). The VLS software, LIDAR Analyst®, simplifies the process of extracting 3D features from LIDAR data through automation.
Once the bare earth is identified, buildings are easily recognized by LIDAR Analyst. From the LIDAR Analyst drop menu, select Extraction Tool, and then select Buildings. The LIDAR Building Extractor dialog box opens, providing the following parameters:
OPTION
FUNCTION
Remove Buildings with area less than __ square meters
Enter the minimum building size to extract in map units.
Remove Buildings with height less than ___ meters
Enter the minimum building height to extract in map units.
Maximum Slope for building roofs __ degrees
Enter the maximum building roof slope to extract in degrees. The maximum slope for building roof values specifies the cut off for roof area. Above this maximum the slope is classified as a building edge. Once identified as a potential building region, further processing occurs to complete the building extraction.
Texture Variance
Use this option to allow LIDAR Analyst to use texture to differentiate between trees and buildings. In general, higher texture variance means that more buildings will be extracted, but also more trees. Lower texture variance means that fewer buildings will be extracted, but also fewer trees.
Smoothing Tolerance
Enter the number of pixels to move a vertex to smooth the line.
Z-Enabled using
It is sometimes beneficial to have a 3D view of your buildings. This option applies Z-value for the polygon vertices, applying elevation values from the input DEM. Apply 3D visualization by choosing a Z-Value from the drop list. Options include:
No Z-Value - Select this option if you do not want to apply z-values to your building polygons.
Actual Z-Values - Attaches actual z-values to each vertex. This may result in lopsided polygons, if the elevation varies greatly from vertex to vertex.
Average Z-Values - Takes the average of the pixel z-values underlying the polygon and attaches that value to each vertex.
Max Z-Value - Takes the maximum of the pixel z-values underlying the polygon and attaches that value to each vertex.
Standard Deviation - Takes the standard deviation z-value to each pixel underlying the polygon and attaches it to each vertex.
Square Up Polygons
Check this option to apply the LIDAR Analyst software ’s Square Up Shapes algorithm to square the building polygons in the result layer automatically. This feature can also be run as a separate post-processing function.
Extract as multi-component buildings
Check this option to split roof tops into multiple sections. For example, a pitched roof would have two polygons separated by the roof’s ridge line. A multi-level flat roof would appear as separate polygons for each level.
Below is a feature extraction project from the city of Anchorage, Alaska, illustrating a typical LIDAR data image, the bare earth surface, and buildings.
To view the 3D attributes for your building polygons, simply right-click on the building layer in the Table of Contents, and then select Open Attribute Table from the popup menu. The Attribute Table displays the Z-value for each polygon; notice the amount of invormation provided with your LIDAR data extraction returns.
In order to visualize your model in 3D, use ArcScene, ArcGlobe or a similar map software that is customized for displaying 3D data. If you are using ArcScene, create a new map and add your data. Right-click your data layer (DEM, for example), and choose Properties. Click on the Base Heights tab and select the file path for your bare earth surface data in the "Obtain heights for layer from surface" space. Continue to select your bare earth layer in the Base Heights tab for each of your additional data layers (buildings, roads, etc.) and all of the layers will share the same base elevation and display proportionally-correct height. Below is a three-dimensional fly-through animation viewed in ArcScene.
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