GPS is Underused - Part 2

Article and Movie instruction


When a new user of GPS gets training, it is usually on an active job site. The dealer will go to the site and help the contractor place the base station and localize the job. With the site all set up, the contractor goes to work. They are now trained in the use of the rover and machines they will use on the project. Every day, the user turns on the equipment and checks in. The machines are verified and work continues. It will be months until the crews go to a new site and need to setup a site from scratch again. Usually the process has been forgotten by then.

It is no fault of the contractor, even with notes and written instructions; the localization process is not done often enough to get the “muscle memory” required for a fluid operation. In addition to the actual steps required to get things going, I feel a complete understanding why you are doing this is critical. Before I get into the details, I want to get some definitions out of the way.

·       Localization The terms localization and calibration are synonymous. This refers to the association of grid coordinates to GPS Latitude, Longitude and Height. These can be legacy points, such as section corners or property pins. The surveyors will set control on the job. Whatever the situation, be sure to set up on the same points as the surveyor. Avoid the urge to convert the surveyor’s localization to your system. All systems need to set up and localize using their native firmware.  When localized, a site will give you the northings and eastings associated with your site. GPS actually is working with latitude and longitude in the background and converting those coordinates on the screen.

·       Base Station Base station setup is critical. The two major factors to consider are coverage and security. In a perfect world, you will be able to set a steel pole in concrete that will remain untouched for the life of the project. It will be accessible so you can remove the antenna daily.

With the advent of networks, contractors are able to do layout and even machine control without a base station on site. Besides saving cost, it reduces daily setup time.

·       Residuals Any time you localize a job, the GPS firmware does some complex math. The points on the ground are generated from a flat sheet of paper, the Lat/Long information received by the GPS receiver has to be adjusted to match your site. Refer to the Calibration Screens video for additional information. The end result of a good calibration is residuals with four nines after the decimal point or four zeros after a one and the decimal point.

·       Coordinate System There is a growing desire to Geo-Reference information. In other words, all projects will be performed in State Plane Coordinates and that information can be distributed to municipalities and other interested parties. With the coordinates being correctly referenced these jobs can provide information in the future. It’s easy to know if your job is Geo-Referenced, the coordinates are in the millions or hundreds of thousands, instead of the southwest corner of the job being 5000,5000.

With an understanding of the terms, we will now discuss why localization is so important. The overall accuracy of any job is only as good as the localization. The first thing that must be addressed is the location of control.

For illustration purposes, consider the control points to be fence posts. With the control points shown on your screen, visually connect the control points with lines from point to point. You need to insure the control points used in your calibration completely surround the job. If any part of the job is outside the enclosed control area it has the chance of reporting incorrect values. There is no need to discuss the details of why, just be sure your control is placed correctly. If you need to have the surveyor place a few more points to accomplish your goal it will go a long way to a better job. The Control Points Video explores this further.

Roadways present another challenge. Oftentimes roadway control follows the right-of-way. When localizing on these points, the control is narrow because the points are close together. Think of the roadway control points as table legs. A long narrow table is unstable. Get some points outside the right-of-way area to provide better control. The Control Points Video looks into this.

Regarding Base Stations It is not necessary to place your base station in the middle of the job. Here is the best scenario for the placement of a GPS base station;

A sturdy (3 or 4 inch) pipe cemented into the ground in a location that will not be disturbed for the duration of the project. The pipe should be high enough to see the entire job, if you can’t see an area of the job, reception can suffer. The pipe should have a cross member welded to it and have a fixed height for the daily removal of the base antenna. It is critical the GPS antenna can be reliably placed at the same location and height each day. The other end of the outrigger can hold the radio antenna.

This is not always possible, here are the priorities of the desired setup from most to least important.

1.     Base location is accessible so antennas can be removed daily. I have had a base station stolen while it was 500 feet away and in view.

2.     Permanent location. If you can place the antenna for 95% of the job, then move it to finish. It is still better than daily setup on a tripod in varied locations.

3.     View of the job. By following the localization guidelines provided, base location can be adjusted for the job. Radio repeaters can also fill in dead spots on larger sites.

Many jobs have been incorrectly setup. This results in unexplainable errors and this equipment not being utilized. Good base location and calibration are the foundation of a job. Done incorrectly everything else is subject to error.


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