COORDINATE SYSTEMS 101: WHY THEY CAN MAKE OR BREAK A PROJECT
A construction project’s success factor begins with accurate survey data and the coordinate system it is based on. The coordinate system is the source for obtaining the precise survey data. Coordinates are necessary when laying projects out, whether it’s a highway, bridge, site work, utilities, or another type of construction project. A coordinate system is a method for identifying the location of a point on Earth. Even though Earth is spherical, the assumption is that the area is flat because coordinate systems are based on a plane (2D). Unfortunately, even one wrong number in a coordinate system will throw off an entire project. Therefore, surveyors need to be well-versed in coordinate geometry to do their job well and share accurate information with project managers and engineers. Commonly, local systems are created and used on a project-by-project basis.
Rectangular Coordinate System where X-axis is Easting and Y-axis is Northing
Rectangular Coordinate System
The rectangular coordinate system has been around for a long time. Its design is two intersecting lines where the horizontal axis is the ‘Easting’ component, and the vertical axis is the ‘Northing’ component. Where they cross (like the lower-case letter t) forms a 90-degree angle and is called the origin (0,0). All points located directly on either axis will be a zero value. As you move along the horizontal axis to the right of the intersection, you will be in the positive Easting quadrant, and as you move to the left, you will be in the negative Easting quadrant. As you move vertically up from the intersection, you will be in the positive Northing quadrant, and as you move down, you will be in the negative Northing quadrant. For example, if you move to the right nine and up three, your coordinate would be (9,3), which signifies Easting is nine and Northing is 3. This system is a simple way to figure out where you are on the job site. Once the coordinate points are known, geometry and trigonometry are applied to determine the distance and direction between the points.
State Planar and Local Coordinate Systems
There are two major types of coordinate systems used for construction: state planar coordinate systems and arbitrary (Local) coordinate systems. State planar coordinate systems are unique to the state you are working in. Larger states can have up to ten individual state planar coordinate systems as a maximum size is deemed accurate. The origin (0,0) for these coordinate systems is usually set so that the area covered by the coordinate system is always a positive number. If you had a project in the southwest corner of a state, you would be much closer to the origin of (0,0). Almost all state and municipal projects will use the local state planar coordinate system for their projects. Arbitrary or local coordinate systems are a bit different. The origin point for these coordinate systems is either set in the field when doing the survey or in the office when an engineer picks a spot on his screen. These coordinate systems are convenient to work with as an engineer and are still highly accurate when used with a good control point network. The control points accurately place an arbitrary coordinate system in the right spot in the real world.
GPS plays a major role in construction projects
GPS and Coordinate Systems
Another critical reason for coordinate systems is that every CAD file is in a rectangular coordinate system (2D). The coordinate survey and CAD coordinate systems must match so that the GPS works properly in heavy equipment. If the two coordinate systems do not align, then the machine will not be able to pin the two together, and therefore it won’t work correctly. Coordinates are used on projects of varying sizes and levels of complexity.
Mistakes in the office can have significant consequences in the field. Because a project usually involves several different engineering disciplines, multiple coordinate systems may be used on the same project. Civil engineers typically use a state planar coordinate system, while structural will usually use an arbitrary coordinate system. This difference in systems can become an issue when merging the data to create GPS files. If the engineers don’t align the two data sets correctly, buildings can be shifted slightly and not align with the site plan. Sometimes the layout files can be shifted somewhat from the grading files, meaning that roads and sidewalks are not quite in the right spot. For this reason, you need someone who knows what to look for to create your GPS files for your project. ECI’s team of virtual construction engineers is ready to help get your project going so give us a call.