Surveying

What is Surveying?

Surveyors are the first people to visit a site, in order to locate the site and the project footprint and general features, to start the designs. After the designs are complete, before any construction activity starts in the field, surveyors also go there before any other trade, and layout and establish the benchmarks for horizontal and vertical control, and locate the project components to be built. These surveys are linked to state surveying system.

Surveying is the measurement of distances, angles, areas, grades and elevations, slopes, establishing horizontal and vertical reference control points, setting stakes, marking lines, for the purpose of laying out anything that will be built, or laying out the natural ground, or measuring items on existing structures, or checking the dimensional accuracy of existing work.

As construction progresses, surveyors come to the field repeatedly, (or they never leave if project is large enough) to make sure excavations are done correctly, utilities, culverts, piles, foundations are laid out correctly, or for example the formwork for concrete is progressing with accurate layout and dimensions or the steel base plates for columns are laid out accurately with correct elevations and so on. In short, surveyors make sure that the location of everything is accurate, as per the intended designs. So, the surveyors not only give the general layouts and guides to every trade, but very often help to measure specific items for trades too, and during construction all trades often need their help.

For large projects such as dams or airports, establishing only benchmark is not enough for control. For such projects, a grid system should be set for horizontal control, and must be linked to state or national grid system. All previous survey work, such as topography, location and elevation of soil exploration holes, should be corrected to agree with the new project’s horizontal control system.

Surveying is a highly specialized profession, and although it is not necessary to have engineering diploma to be a surveyor, licensure is required in almost all parts of the world, with difficult exams to pass. Design and construction can be done very well, but if the surveyors make mistake in the beginning, it will be a financial and schedule disaster. Mistakes done by surveyors are among the most costly, both in terms of time and money, as they affect everything that follows.

To give an idea about surveying work, the very general features of a site surveying plan may include the items below, although they differ greatly between different types of structures and projects:

  • Property line
  • Proposed location of new structures
  • Existing conditions inside project property line and its surroundings, such as streets, roads, signs, sidewalks, landscaping areas, fire hydrants, monuments, fences, manholes etc…
  • Elevations of surfaces, with respect to a nearest official benchmark shown by the state/country, shown by elevation marks and contour lines (a contour line is an imaginary line that connects points of equal elevation)
  • Separation of lots
  • Real north and project north direction. Often times, for convenience, project north can be chosen slightly different than real north. In that case, the angle between the two should be shown, for information.
  • Scale
  • Legend of abbreviations of abbreviations used on the drawing
  • Any notes as necessary
  • Surveying firm, owner / project name and contact information, surveyors stamp / signature
  • Date of drawing, drawing sheet number

Site surveying plan is only the beginning and it differs greatly for different project types. For example, a building site surveying plan might be only 1-2 sheets, where a site survey plan for a road to be built will be many sheets. After the site surveying plan to start the project, throughout the project surveying is needed many more times. Here is an example of what a surveyor may draw, to show a cross section of a road, for a fill section of a road (fill means, natural ground was below proposed ground, so a fill is made by importing earth):

Example: construction surveying of a road cross section

In this figure above, which is far from complete but shows only general concepts, the surveyors marked the vertical and horizontal dimensions of a road cross section, that is made by placing fill earth material on natural ground as seen. To mark elevations, they first used a known fixed point of known elevation outside, such as an official state benchmark, or a project benchmark which they carry from that official benchmark to a close location to jobsite, if the official benchmark is far. Knowing that elevation, first they determined the elevation of their line of sight, shown with dashed line, by putting a rod over that point and measuring the difference, which is not show here. Then, using the known elevation of their line of sight, they can determine the elevations of these three points by measuring the vertical distance from the point to the line of sight, with the help of measuring rods. To mark the horizontal distance to outline the cross section width, the surveyors start with the centerline, that they had determined earlier by using benchmark’s location, and from that centerline they can measure and mark the cross section distances. The marking CL indicates the centerline of the cross section, and is shown as in the figure by convention. Many details of such a cross section is skipped here, in order to clearly show the basics.   

In a construction site, surveyors put the (location and grade) stakes at a certain distance off the actual work area, in order to avoid the constant destruction of these stakes by construction activities.

A surveyor constantly checks herself while doing measurements, by using simple geometrical formulas, even if they are doing their actual measurements with the most advanced equipment and techniques. For example, after laying out the rectangular footprint of a building by four corners, the diagonal of the rectangle should be measured, and this distance should be compared to the calculated distance from two perpendicular sides of the triangle, using the famous Pyhtagoras formula, for finding hypothenus for a right angled triangle.

Surveying for large scale projects is often accompanied by GIS, Geographic Information Systems, which is a field that includes many concepts to give different types of relevant information about the area that is being considered. GIS allows users to gather, see and interpret data in much more informed and accurate ways, by combining lot of data from different layers about different characteristics about the area, on top of just the bare surveying data. The examples of layers of information include parcels and ownership of land, utilities, surface terrain, hydrographic points, soil types, existing structures and roads, and whatever will be applicable to that particular project. Each of these aspects are laid on top of each other as layers, and the users can use them altogether or just a few of them at one time or individually as needed. 

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