Building an Infrastructure Database for a North Texas Utility
One of the biggest obstacles developers face is knowing the true location of every utility pipe concealed within or beneath a construction site—particularly if maps from one agency disagree with those of another. To prevent such problems, a north Texas consortium of ten government agencies and other groups built a common geographic information system (GIS) so that its members could share boundary, infrastructure and demographic data for the Dallas-Ft. Worth area.
At the heart of the GIS are 2,400 aerial photographs which were converted to "digital orthophotos". (An orthophoto is a raster image of ground features in true map positions.) The digital orthophotos comprise the base map of the GIS; thus all participating local governments, agencies and other groups can use the same coordinate system for planning and mapping projects.
Accuracy of a horizontal and vertical aerial survey was essential to the production of the orthophotos. To obtain this accuracy, ground control crews used Trimble 4000 receivers. Workers surveyed horizontal and vertical coordinates for 183 control points. Using relative positioning techniques, crews established an accuracy of 1:100,000 or better. National Geodetic Survey's HARN (Highly Accurate Regional Network) system served as the control for the new positions. After the ground points were surveyed, the contractor created a digital file of latitude, longitude, and ellipsoid values and converted them to geographical values.
Contractors were Woolpert LLP, a national engineering firm with headquarters in Dayton, Ohio that specializes in infrastructure GIS services; and Halff Associates, a civil engineering firm in Dallas, Texas. Consortium members included the cities of Cedar Hill, Dallas, Ft. Worth and North Richland Hills; Dallas County; Dallas County Appraisal District; Dallas Area Rapid Transit; Lone Star Gas; Tarrant County Appraisal District; and Tarrant County 911.