Volume 42, March 2001: Global Positioning on Roofs

Perspectives
A Quarterly Information Source from Benchmark, Inc.
Volume 42 March 2001

Global Positioning on Roofs

by Mike Bassett

Global Positioning Systems, or GPS, have been used for a number of years, primarily for those needing general location. During its inception, the systems were quite expensive and the accuracy not very precise, but its uses have grown over the years.

Developed by the Department of Defense, GPS are utilized today for a variety of uses. Rental cars are now equipped with GPS, giving the driver his locations and destination directions. And small hand held units are economical enough for the weekend hiker.

GPS is based on a constellation of satellites orbiting the earth at a very high altitude. Currently, there are 24 operational satellites in orbit. The idea is simple; send a signal from one position to another position and record the travel time. Since we know the speed of light, we can therefore calculate the distance from point A to point B.

So it seems that recording the time it takes a signal to go from your location to an orbiting satellite will give you your location, but it doesn't work that way. In reality, we need four readings to give us our location. If a satellite is directly overhead it would only take the signal .06 seconds to complete its journey, illustrating the accuracy of the equipment required. Each satellite is equipped with three atomic clocks to assure that the system is working properly and to achieve accuracy. At a price of $100,000 each, it is not feasible for the ground unit to be equipped with an atomic clock.

A series of codes have been developed to determine the radio signal travel time, alleviating the need for an atomic clock on the ground unit. The ground unit consists of an antenna to send and receive the signals, a receiver to monitor the signals, and a data collector to store readings taken.

At Benchmark, a good share of the time taken to perform a roof survey is involved in collecting the data to produce a detailed roof plan. Roof plans are an invaluable tool in aiding roof management. When we perform roof surveys, roof measurements are typically taken with the aid of a nylon measuring tape, where perimeter and penetration measurements are recorded to the nearest six inches. Field sketches are then used to produce a roof plan.

A tape is no longer needed to measure roofs since we've begun utilizing the GPS. Building features are entered into the data collector as either a point, line or area feature. A building's perimeter is generally recorded as an area feature with entries recorded at each transition.

Knowing the location of these transitions, the data collector's software treats the feature as an area connecting the points, thus creating the building's perimeter outline.

Other typical building features, such as drains and pipe penetrations, are entered as point locations. Readings are taken at a feature's center point, and flagged with a feature tag, such as a drain. Crickets or valley/ridge locations are an example of a line feature. At each feature transition, readings are taken and the points are connected with a line and stored in the data collector. Upon completion of the survey, the data is downloaded and transferred to a DXF file, or data exchange format. This format allows a CAD program to import the data. Once complete, a program is run that searches the data tags. The drain entered is now replaced with a standard symbol of a roof drain.

Since we purchased the GPS last year, we've used it on several projects. Actual time spent on the roof during a survey has not decreased significantly, since a roof with numerous projections still needs to have all points located. But drafting time has been reduced somewhat, and we feel that the accuracy has improved.

The graphic below shows a rough roof plan created through the raw data the data collector output. An initial output plan, such as this, will need to be enhanced through the use of AutoCad programs, to produce a detailed, complete plan.