3D Stereo Data for the Estimation of Pavement Construction Quantities and Costs Mr. Donald L. Reid Department of Public Works Nashville Metropolitan Government GIS-T March 27, 2007

Existing Process Numerous issues with existing pavement data Accurate data collection entails a manual, costly, and time consuming process Alternative method could be to use stereogrammetric feature collection based on stereo imagery

What is Stereo Imagery When you look at an object you see it from different angles with each eye Your brain merges these images into a single image to provide you with depth perception…the ability to perceive distance This enables you in essence to see in “3D”

What is Stereo Imagery (cont.) When aerial imagery is collected it is collected such that each picture overlaps the previous one by 60% This overlap provides an area in the two images of the same place from different angles Through a process called aero triangulation it is possible to accurately determine the exact x,y position of each pixel in the image The images can then be mathematically aligned along the flight line using a process called epi-polar correction to enable a stereo view This enables viewing and measurement in 3D

Stereo Overlap Flight Path Shot 1Shot 2Stereo Overlap

Flight Path Shot 1Shot 2

Stereo Overlap Flight Path Shot 1Shot 2Stereo Overlap Area

Example of Stereo Overlap Area

Two Ways to See Imagery in Stereo In order to see in stereo, we must trick our brain to see stereo images in a way similar to that which we normally use One way to trick the brain is to use a concept called anaglyph –Anaglyph uses one blue sensitive image and one red sensitive image –Using red-blue glasses we trick the brain to see two images at once and let the brain do its’ magic

Anaglyph stereo image of downtown Nashville

Stereo Viewing Using Shuttered Glasses Electronically shuttered glasses alternate putting one image in the left eye and the other image in the right eye If done at a image refresh rate the brain processes the images just as it would if it were viewing in real life Desktop photogrammetric work stations usually use this method This the method being used in our pilot program

Stereo Imagery Database Properly registered stereo imagery is accurate to to +/- 1pixel in the horizontal axis and 1.5 pixel in the vertical axis –The Nashville data set is accurate to +/- 6” in horizontal and +/- 9” in vertical Typical extracted feature accuracies are similar to image accuracy depending on operator and training Accuracy is generally uniform throughout the image –Digital orthoimagery accuracy is subject to accuracy of imagery used to create the ortho

Imagery Access Individual stereo image pairs can be accessed using any georeferenced data layer For Nashville Metro we are currently using a map layer as the access layer Nashville/Davidson County is 112GB and contains approximately 1400 stereo pairs

Stereo Imagery Provides Feature Extraction Source Desktop stereo photogrammetry provides: Improved accuracy over digital orthophotography Ability to measure distance over the ground Ability to extract 3D planimetric features such as roof elevation footprints Ability to extract digital elevation (DEM’s and topographic contours) for specific areas

Nashville Project Status Stereo imagery data has been prepared and project access files prepared Hardware and software are installed and working properly Training has been completed Pavement data collection process has been defined Data collection commenced this week

Questions/Contacts Mr. Don Reid, Project Manager, Nashville Metropolitan Government/Public Works , Mr. Jeff Deason, GIS Management, Collier Engineering , Mr. Lonnie Hearne, Stereo Software, SimWright Inc ,