SINGLE-POINT CALIBRATION: WHEN IS ONE POINT ENOUGH?
It’s essential to get accurate site calibrations when it comes to earthmoving projects. A single-point calibration is the simplest type of calibration performed. Site calibrations are necessary because they enable multiple GNSS-based rover systems to work in your local site's coordinate system. Specifically, it establishes the relationship between WGS84 (an Earth-centered, Earth-fixed terrestrial reference system and geodetic datum) latitude, longitude, and ellipsoidal height with the local northing, easting, and elevation that is needed. How do you know when a single-point calibration is sufficient or not? We'll share some information and advice to help you figure it out!
CONTROL POINTS
When it comes to control points, the minimum you need is three to get a good network geometry around your site. Having more than three points is even better as it will yield better site calibration results. Encompassing the entire area will not only give you more accurate calibration but will allow you to identify potential issues early on.
SINGLE-POINT CALIBRATION
As we mentioned earlier, site calibration is a series of mathematical computations that transform WGS84 coordinates into local grid coordinates. These site calibrations make both horizontal and vertical adjustments. Within the horizontal adjustments, it rotates, translates, and scales. The vertical adjustments block shifts, tilt planes, and model the effects of the local geoid. In some instances, a single-point calibration of the area is sufficient. To do one, it requires a single 3D control point. This control point can be either known or arbitrary. It is this point that will define the coordinate system orientation. When the sensor output is scaled to valuable measurement units, you can use a single-point calibration to correct sensor offset errors. This method can be used when only one measurement point is needed. Other examples where a single-point calibration may be all you need are when you're doing an initial site topo or conducting a quick stockpile or volume topo. They are not recommended for long, linear projects.
It’s essential to get accurate site calibrations when it comes to earthmoving projects
IN OUR EXPERIENCE
In our experience, the most common use for a single-point site calibration is when you are performing your own site survey. Pro Tip - if you are in a hurry and don't have time to set up a control network, you can just set one control point near the center of your project with a northing of 1,000, easting of 1,000, and a height of 100. On larger sites, you would increase the values to 10,000, 10,000, and 1,000, respectively. The goal is to keep the entire area in a positive location for ease of data manipulation. Again, the single-point site calibration sets the location within your local coordinate system. Suppose you have another contractor who plans to use your survey data because they want to localize on your project. In that case, you will need to set and measure additional control points surrounding your project (this won't change your site calibration).
The second most common reason we've come across for using a single-point calibration is when all but one of the control points for an engineer-designed project is either missing or not suitable for GPS. How many times have you gone to a new site to localize and found that the control point hubs have been removed by a dozer, overgrown in vegetation, or not findable due to kids stealing the stakes? If this happens to you, do not dismay. If you have one good control point, you can do a single-point site calibration to get you going. Doing this involves some risk because you rely on one measurement to place the local coordinate system into the WGS coordinate system. On a typical site calibration, if one measurement is bad or the control point has been disturbed, you can tell because there will be a large residual. This is not the case with a single-point site calibration. If your control point measurement is off by 5', your project will be off by 5'. Due to this, we strongly recommend that you ask the engineer to set additional survey control points as soon as possible to verify that your single-point calibration is good.
Our expertise with setting control runs deep. Check out our numerous blogs on the topic and contact us with any questions. Precision is critical at every step in the process. ECI Technologies was founded based on that principle. Call us to experience it for yourself!