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ArcNews Online
 

Summer 2002
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Fast Track to Accuracy

The powerful combination of imagery, GPS, surveying, and GIS data produces the mapping precision required in many engineering and surveying applications. It also saves valuable time, as shown by the CSA Group (see main article, "Rescuing a Sewer Pipeline Avoids Catastrophe in San Juan, Puerto Rico"), when it had just four months to develop several options for cost-effectively moving one of the City's primary sewer lines before it collapsed under the weight of the San Juan Sanitary Landfill and flooded the adjacent wetlands, other environmentally sensitive areas, and possibly the City. The Puerto Rico Aqueduct and Sewer Authority (PRASA), preferring gravity as the less expensive method of water flow, also wanted to avoid constructing new pumping stations to support the new route.

The existing route of the San Jose Trunk Sewer (SJTS) changes every 150 to 300 feet. Locating the exact points of horizontal alignment change was crucial to developing alternative solutions. The line was identified with stations spaced 10 meters (33 feet) apart. Waypoints were staked out in the pipe alignment and 15 to 20 ground control points were collected using Leica Geosystems' SR530 GPS unit, pinpointing the locations where soil samples had been taken to evaluate the viability of running the pipeline through the landfill. Existing topography was collected within 25 meters (82 feet) of both axis sides on each station. Leica Geosystems' TC305 total station was used to position and measure distances and footprints of the various utility poles, buildings, manholes, fences, and roads on the site. Offsets were verified using DISTO pro, a handheld laser rangefinder from Leica Geosystems.

Leica Geosystems' real-time kinematic (RTK) method of GPS data collection, which transfers collected field data directly as shapefile or .txt formats and speeds integration with other GIS data and imagery, avoided errors and saved time transferring data.

"Directly inputting field data in shapefile format into our systems using SKI-pro [Leica Geosystems' GPS software for processing field data] saved us a lot of time in postprocessing," says Jorge E. Rodriguez, GIS analyst for the CSA Group. "We were able to use this data immediately in ERDAS IMAGINE for warping and rectifying the aerial photos and then bringing it together in ArcView."

The ability to use data from the nearby USCOE Puerto Nuevo Canalization Project was important to steer the SJTS project clear of right-of-way and jurisdictional issues. According to Rodriguez, the file format compatibility enabled the CSA Group to transfer the USCOE's control points from CAD data, fast and with precision.

Once assembled in ArcView for analysis, the data provided a clear and accurate picture of the site and the impact of each alternative, assisting the CSA Group in presenting PRASA with its best options for saving the San Jose Trunk Sewer from collapse.

For more information about spatial solutions, contact Leica Geosystems (toll free: 1-877-463-7327), or visit Leica Geosystems at (www.gis.leica-geosystems.com).

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