6.1 Basic Surveying, Route Surveying, and Field Geometry

Purpose Drives the Measurement

Surveying is the science of determining the relative positions of points on, above, or below the earth's surface by measuring distances, directions, and elevations. The NCEES Fundamentals of Surveying (FS) exam rarely asks you to operate an instrument; instead it asks whether you understand what a measurement means and how it is reduced. The first habit to build is identifying the purpose of every quantity before reaching for a formula. A distance can be a slope distance, a horizontal distance, an ellipsoid distance, a grid distance, or a ground distance, and confusing them is the single most common FS error.

Field geometry reduces to four primitives: a horizontal angle (or direction), a horizontal distance, a vertical angle (zenith angle), and an elevation difference. From these the surveyor derives bearings, azimuths, coordinates, areas, and volumes. The exam expects fluency converting a measured slope distance to horizontal using the zenith angle: horizontal distance = slope distance times sin(zenith) = slope distance times cos(vertical angle).

Route Surveying and Stationing

Route surveying establishes the alignment of a linear facility (highway, rail, pipeline, transmission line, canal) along a corridor. The corridor is described by a horizontal alignment (a chain of tangents joined by horizontal curves) and a vertical alignment (grades joined by vertical curves), plus cross sections taken perpendicular to the alignment.

Positions along the route are referenced by stationing. In US customary practice, one full station equals 100 feet, written as the whole hundreds plus the remainder: a point 1,275.40 ft from the origin is station 12+75.40. In metric practice a station is commonly 1,000 m (1 km), so 1,275.40 m is 1+275.400. The distance between two stations is found by simple subtraction of their values.

A station equation handles a break in continuous stationing when an alignment is revised. It is written as a back station equal to an ahead station (for example, "Sta 50+00 BK = Sta 48+50 AH"), and it changes the count of stations without moving the physical point.

Building Defensible Field Geometry

A defensible survey treats measurements as quantities with uncertainty, not exact numbers. The FS exam expects you to recognize redundancy, closures, and checks. A closed traverse should return to its origin; a level loop should close on its starting benchmark; angles around a point should sum to 360 degrees and interior angles of a closed polygon should sum to (n - 2) times 180 degrees.

For stationing and alignment problems, remember:

• Tangent length and curve length are measured along the alignment, so stations advance through curves by the curve length, not the chord or the tangent.
• The PT station = PC station + L (curve length), never PC + tangent length.
• Profiles report elevations, while plan views report horizontal positions; do not mix the two when scaling.

FS questions reward the candidate who first classifies the scenario - is this a horizontal-alignment problem, a vertical-profile problem, or an angle-closure problem - and only then selects the equation. That classification habit is the bridge from raw field notes to the reductions and adjustments developed later in this chapter.

Field Notes, Profiles, and Cross Sections

Field notes are the legal and technical record of a survey, and the FS exam treats them as evidence. Good notes are kept in ink, are never erased (a wrong value is struck through and rewritten), and pair sketches with tabulated readings. They record the date, crew, weather, instrument and serial number, station occupied, backsight, foresight, and the reduced quantities. Because a survey may be litigated years later, the notes must let another surveyor reconstruct exactly what was measured and how it was reduced.

For a route survey, two derived products dominate:

• A profile plots ground elevation against stationing along the centerline. Engineers lay the vertical alignment (grades and vertical curves) on the profile to balance cut and fill and to meet drainage and sight-distance requirements. Profiles typically exaggerate the vertical scale (for example 10:1) so grade changes are readable.
• A cross section records ground elevations transverse to the alignment at a station, usually at regular offsets left and right of centerline. Cross sections drive earthwork volume computations (average end area and prismoidal methods) and define the template of the finished facility - travel lanes, shoulders, ditches, and side slopes.

The exam links these back to fundamentals: a profile is just a chain of elevation differences along the line, a cross section is a fan of horizontal distances and elevation differences from the centerline, and stationing is the common spine that ties plan, profile, and cross section to the same physical points. Mastering this vocabulary lets you read any FS route-survey scenario and immediately see which measured primitive - angle, distance, or elevation difference - the question is really testing.