How do I Establish Vertical Control Using a GPS Satellite Survey?

You can perform GPS surveying anywhere there is satellite reception.

You can use vertical control in a global positioning system satellite survey to reference a survey to a point of known elevation or benchmark. Once you have set up the benchmark within a GPS unit's survey controller, you can reference the remainder of the surveyed points to the known point, providing accurate topographic information for the survey site in question. This type of surveying is most common in civil engineering and is necessary for the completion of detailed engineering design drawings.

Things You'll Need

• GPS survey system
• Survey data collector with installed survey controller
• Vertical control point (benchmark)

Instructions

1. Enter the COGO function of the survey controller. Key in the coordinates and the elevation of the point at which you will vertically reference the survey. The horizontal coordinates can be random; however, use a number that is easy to remember.

2. Find the benchmark. It should be relatively close to the site of your survey. If you are surveying for a town, county or municipality, there should be records available of benchmark locations and elevations in the area of your survey.

3. Turn on the GPS unit and set it up directly over the vertical benchmark. In the data collector, enter the survey controller and under the "Survey" tab and select "Take Measurement." Make sure that the measurement records at least 60 static epochs. This can be done by setting the "Measurement Type" to "Control Point" under the options tab.

4. Find the "Site Calibration" or "Localization" menu in the survey controller program within your data-collector. In this menu, select the "Add Points" option. The survey controller will ask you to select the keyed-in point from Step 1 and the GPS-measured point recorded in Step 3. Enter them.

5. Select the "Vertical Only" site-calibration option. Select "Y" (yes) for vertical and "N" (no) for horizontal.

6. Select the "Apply" or "Save" option in the "Site Calibration" or "Localization" menu. Your GPS survey will now reference all your surveyed points to the benchmark. If you have already surveyed points prior to setting up the vertical control, the GPS will recalculate these points based on the new vertical reference point. Any shots taken subsequent to the calibration the GPS will automatically record with the reference vertical control.

Tips & Warnings

• Depending on the location of your survey, the local governing authorities (county, city, municipality or region) should have records of existing vertical benchmarks in your area. Find a benchmark close to your site and use it. The closer the benchmark is to the location of your survey, the smaller the potential for vertical error.

• Avoid adding additional vertical control points to your survey. The addition of vertical control points alters the functions by which the GPS automatically calculates the grade of any location in your site and can lead to error. Unless your site is larger than 500 acres, you generally do not need more than one vertical benchmark. If you are in doubt as to whether one vertical benchmark will suffice, contact your GPS technical support team.

References

• Carlson SurvCE: Reference Manual, 2006
• Can-Net: Support & Documentation -- Site Calibrations