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GIS

Bringing GPS Data Quickly into CAD

4 Aug, 2008 By: Andrew G. Roe,P.E.

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Software lets users collect and upload field data into AutoCAD using handheld consumer GPS units.

Land professionals seeking to quickly obtain field data often encounter a common dilemma: When public domain mapping lacks detail and an accurate field survey is not feasible, how do you roughly locate selected field points without burning up valuable budget? For example, when a grove of trees is not properly depicted on outdated aerial mapping, but a detailed tree survey is not needed, how can you generally define the outline of the trees and bring that data into a CAD drawing?

Short of sending out a survey crew, engineers and planners have historically used various techniques employing tape measures, electronic distance meters, and old-fashioned pacing to establish distances and determine rough locations of key points. But to use the data in a CAD environment, the information needs to be reduced somehow to points or other objects with coordinate values.

With GPS capabilities growing in recent years, many have turned to simple recreational GPS equipment to obtain field data. And a new software release from a small Arizona company is helping smooth the exchange of GPS and CAD data. GPS2CAD, developed by Phoenix-based AMC, allows users to import data from a variety of handheld GPS units into AutoCAD, as well as export AutoCAD points back to the GPS units. A new feature also enables plotting of points on custom images, such as those obtained from Google Earth.

Environmental Benefits
The simple GPS2CAD interface has saved significant time and effort in recent years for Boylan Environmental Consultants of Ft. Meyers, Florida. "It helps us locate things like wetlands and protected species habitats," such as gopher tortoise burrows, said Brian Marino, Boylan's information manager. Boylan's preliminary field work is often later supplemented by detailed surveys to more accurately determine easements and legal boundaries, but for preliminary planning and site map preparation, GPS2CAD has proven invaluable, he said. The company has used other GPS transfer programs, but settled on GPS2CAD for its simplicity, Marino added.

Boylan often uses the product to identify features for environmental permitting purposes. The company's environmental scientists typically locate wetlands and other features in the field using handheld GPS units and then import the data into AutoCAD. The data can then be used to prepare AutoCAD-based maps and prepare point files that can be sent to surveyors to establish general locations. The surveyors can quickly locate the area of interest and conduct more accurate surveys to pin down wetland boundaries and other features.

Other GPS2CAD users have field-located power lines, parks, and wildlife management areas. AMC counts approximately 250 users in several countries, said Mark Crigler, the company's founder and president.

How It Works
GPS2CAD works with a variety of Garmin and Magellan GPS units equipped with either a serial or USB interface. The product is compatible with AutoCAD versions ranging from 2000 through 2009. Data can be imported to AutoCAD as points, polylines, or blocks, with attributes showing point data.

To import data, users connect the GPS unit to a PC, then select waypoints, routes, or tracks stored in the GPS unit. Imported data can be converted to numerous different coordinate datums and plotted in an active AutoCAD drawing. Data can also be overlaid on aerial photos, topographic maps, or Tiger census maps retrieved from TerraServer-USA. In addition to AutoCAD DWG compatibility, data can be exported to DXF, MDB (Access database), or simple text files. Data can also be uploaded back to a GPS unit for later use, a feature Boylan has used to reassess past projects. "Sometimes two or three years later you may want to put points back into the unit and see where something has changed," Marino said.

Google Earth Images
The latest release of GPS2CAD also includes a feature called Earth2CAD, which uses a wizard to import and calibrate Google Earth images. The user selects an area in Google Earth, selects two points with known coordinates, saves the image as a raster file, then imports the image into GPS2CAD, using the two known points as calibration points so the image can be merged with GPS-collected data. The combined data can then be imported into AutoCAD for further processing.

AMC's Crigler, a civil engineer, conceived the need for GPS2CAD while struggling with how to efficiently locate field data such as cacti and dry streambeds on preliminary site plans. He found that "plotting cacti to the nearest centimeter is overkill," he said. After developing early versions of GPS2CAD in Visual Basic, he rebuilt the latest release "from the ground up" using the .NET platform.

Obviously GPS2CAD and competing products are not suited for all projects, such as detailed engineering designs, property surveys, and other legal-sensitive tasks that require more accurate field surveys. But the products can fill a crucial niche when data is needed quickly and affordably. Accuracy is generally limited to the accuracy of the GPS unit, so with modern handheld units often capable of locating points within 10 feet horizontally, users exercising a dose of caution can find numerous applications for GPS2CAD.