Once control points have been established, surveyors use their instrumentation to measure the positions of existing site features. As we move around the site, we place a prism pole at each feature for the observation. A survey prism is a retroreflector designed to precisely refract laser beams such that they reflect backwards and return directly to the survey instrument’s sensors. It is fair and reasonable to think of the rod and prism as a surrogate for the feature itself. In practice, it is usually not possible to collocate the prism with the feature, so we typically offset the location of the feature straight up along the longitudinal axis of the rod.
The specific list of features to be measured for a given survey are to be determined through discussion with each client based on their needs and circumstances. However, it is typical to measure buildings, structures, and pavement outlines at a minimum. Depending on the survey goals, we may also include observations of the relief of the ground surface and/or utility features. Features are classified as either point, line or area features. Relative to each point or vertex of each feature, surveyors measure and record a slope distance, horizontal angle, and zenith angle.
