Connecting the Dots: Using Global Positioning to Lay Out a Parking Lot

How does he decide what locations to mark with GPS? Partly he selects specific locations he can use to verify that the engineering design he received is accurate.

"We sometimes run into situations where the design on the paper is not consistent with the exact site conditions, so we set up control points to verify the accuracy of the design," he says. "We'll mark the outer limits of the parking area, for example, the corner of a building, and light poles within the lot itself."

These "known" points serve as a control network. Once Stewart has established "known" points on the parking lot, he returns to the office, uploads the points and their coordinates into his computer, and then overlays the map of coordinates onto the engineering design to make sure they are working off the same dimensions.

"The engineering design often uses a digital plan based on aerial mapping to show the outer limits of the lot, but sometimes the aerial mapping is not up do date," he says. "Maybe the parking lot was extended 5 or 10 feet in on area for some reason, so we locate the perimeter of what the intended limits are. None of that happened on this job. It went okay."

Then, using the established GPS coordinates, Stewart assigns new coordinates to specific, important points within the engineering design's striping layout.

"Using the computer we point to the end of a centerline, for example, and tell the computer to give us the coordinates of the end of that line. It does and we mark that point and the coordinates in the layout," Stewart says. "Now we know exactly where that centerline needs to start to execute that layout."

He works his way through the entire layout that way, using the computer and GPS points to determine coordinates of other important layout points. Important points include the beginning and end of a bank of stalls, the beginning and end of a stall stripe, or any point in the layout where a centerline might bend.

"Basically we determine coordinates for key locations where a striper might run into a problem or any place in the layout where the people striping need to know something different is going to happen," Stewart says. "We also will provide some markers along long lines so the striper can check himself as he goes."

Once he has determined the coordinates of the key locations, he loads that information into the Leica Geosystems GX 1230 GG GPS units and returns to the site to mark them on the pavement for the striper.

Back on the site

Once on the site Stewart essentially reverses the process, again using triangulation with the satellite, the base, and the rover.

So assume Stewart assigns "100" as the number of the end of one centerline and "101" as the opposite end of the same line. At the site he tells the GPS unit he wants to go to Point 100 and the unit tells him how to walk there.

"We tell the GPS unit what point we want to go to and it actually tells us where to go to get to that point—maybe 50 feet north and 20 feet west or whatever it is," Stewart says. "We walk in those directions and as we get closer to the point the unit begins to beep. The closer we get the more rapid the beeps become, and once we reach the point it's one solid signal, no beeps."

Stewart then paints the point number on the pavement. He continues the process until all the point numbers essential to the layout are painted on the parking lot. Once the points are marked he gives the striper a printout of the layout with all GPS points marked, and the striper can start chalking the layout.

"Vinny is not working blind out there," Stewart says. "He has a copy of the layout with the numbers indicating essential points, and he can match those up with the point numbers we painted on the ground for him. There's no question about what he's doing and the work goes smoothly. From that point on it becomes a game of connecting the dots."

Striping the job

On this job Stewart divided the project into six smaller sections so V&V could work while Advanced Pavement Technologies was repairing and sealcoating other areas. Once the GPS points were determined for the layout, V&V's crews start chalking the lines. On the first day all V&V's crews worked the morning snapping chalk lines. That afternoon one crew began applying paint.