Site Control - Principle 2
2. Surround the site with control points
In our site control principle 1 article we discussed how every project is surveyed and designed on a virtual grid. Site control is what orients your GPS machine control system's 3D model and linework in the real world. It's what takes a virtual design and places it on the that corner lot or just outside the right of way. To orient your site, horizontally and vertically, in the real world it is important that your control points are well distributed.
Let's imagine your project is drawn on a piece of paper and you have to place it on a globe. To get the paper - your project - to conform to the shape of the globe, you would need to pin down all of the edges. If you only pinned down one side or just three corners, the paper would drift away from the surface of the globe on the side opposite the pins. I may have oversimplified a little, but site control places your GPS machine control file in the real world in much the same way. To get your 3D model in the right place horizontally and vertically, you need to "pin" down the edges. A "pin" or two in the middle is good practice, particularly on large sites.
See below for a couple illustrations showing good site control practices. Note that on every example if you drew a line between each of the control points (yellow triangles) on the outside edges, the working area (outlined in red) for the project would be entirely within the line.
This project has three control points on the site and three a few hundred feet outside the site boundary.
Control does not necessarily have to be on or right next to the project site. It might be down the block or across a stream. As long as you have good GPS signal and radio contact with your base station, it will work fine. It is more important to place control where you can get good GPS signal and it is not disturbed during construction than to have it right on the project site. In general, make sure your site control is placed away from trees, power lines, and tall structures.
Here is an example of where control was placed on a round about project with several intersecting streets included in the project. Note that some of the control was placed a block from the project.
This is the finished round about project. The perimeter control points were placed outside the working area, preserving them during the construction process.
Road and highway projects can be tough to surround with control points but still must be fully encircled. They will require more control points for the area covered. Control must extend outside any adjacent streets or driveways that are included in the project. A good rule of thumb is to have control placed at least 250' on either side of the center line, or wider if grading for intersecting streets extends past 250'. On longer projects, make sure you have control points on both sides at least every half-mile; more if you have to work around a curve. The two pictures below are examples of good site control for a city street and interstate highway project.
One of the most common problems we see with control placement is when they are located on just one or two sides of a project. For example, say you are working on a subdivision project and are given three control points on the East property line and one 300' West of them - but the actual project extends 700' further West. Your GPS system does not show any errors when you finish measuring in control and your cut/fill is reading about as expected when you do some quick checks against the existing surface. Your crew finishes grading the site and the surveyor comes in to survey the as-builts for the project. The surveyor reports back that your work is about 0.3' high on the West end of the project but gets closer to grade as you get further East.
There was no way to know, but remember that one control point 300' West of the East property line? It was 0.1' high. Your GPS system did not show any errors because it was the only point out of line with the rest. This is another example of how surrounding the site with control is so important. It allows the GPS system to check each point against the others for error. Had there been control points on the West side of the project your GPS system would have alerted you to the 0.1' error. You could have either not used the bad control point or known to have the surveyor check them before you started grading. This was an actual problem one of our new GPS clients had to deal with. We never asked what it cost to get the site rebuilt to spec.
Another common site control placement blunder occurs when they are all placed in-line down the center of a road. Again, the GPS system did not display any control errors when you measured the points in. When you start checking cut/fills and elevation on the edge of the road or at intersections, the 3D model is high on one side and low on the other, but correct at the center line. The errors appear somewhat random as you move down the alignment. It seems like the model is bad. The problem is actually in the control placement. When control is placed in a straight or nearly straight line, the system cannot orient itself vertically on either side of control. The site model can tip one way or the other. Often, these errors are 0.5' or less, but we have seen errors of 3' high one side and 3' low on the other, just within the width of the road! Using control in the center of a road alignment is not a problem, just be sure anchor it with more points on both sides of the alignment. Remember to aim for at least 250' on both sides of the center line. The illustration below shows good control layout for a highway and a city street.
The third big control error we see is when a contractor blindly uses the control points listed on a table in the plans. The control points (if any) listed on the plans were probably not intended for machine control. They are just what the surveyor used to orient his equipment to the site. Sometimes they will only list northings and eastings with one vertical benchmark elevation. The vertical benchmark could be from another point like a fire hydrant, power pole or manhole cover without a northing/easting location listed with it. They could reference a vague or unmarked location like a manhole cover or buried property corner. Usually the surveyor measured into them with a robotic total station and was not concerned with good GPS reception. How do you fix this issue? You don't. Best practice is to have the surveyor place new control points suitable for machine control around your site. They may use the existing control listed on the plans but will check it when they place the new points surrounding the site. It is not recommended to try and transfer a benchmark elevation or try to add your own control points.
Surrounding your project with control is one of the best ways to ensure your job is located correctly and within grading tolerances.
Principle 2: Surround the site with control points.
Disclaimer: Every GPS manufacturer has their own recommendations for control placement and measurement. It is well worth the time to understand and implement their recommendations. Our suggestions above are great rules of thumb but your GPS supplier has the definitive answers on how to set up your system. Always consult them on very large projects like wind farms or long highway jobs.