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Section 3: Trigonometric Leveling

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Calculating the difference in elevation between points is accomplished with trigonometric leveling. Differences in elevations are determined by measuring vertical angles and slope distances. Trigonometric leveling is often used where accurate elevations are not available or when elevation of inaccessible points must be determined. In Texas, most highway surveying does not require this type of leveling.

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Trigonometric Leveling Use

The establishment of vertical control using the total station or theodolite is not recommended. The slightest variance of vertical angle on the instrument is amplified over the long distances normally associated with trigonometric leveling. In addition, balancing the shots has no effect in compensating for instrument calibration error. Shots of over approximately 1,000 feet must have curvature and refraction calculations factored in. Spirit leveling with a conventional automatic level or a digital level is normally recommended; there will be few instances in Texas where rugged terrain would require trigonometric leveling for highway surveys.

Elevations for topographical work relative to an occupied control point are perfectly acceptable using the total station. The processing of total station data produces DTM’s of adequate quality and in an economic manner. Only topographic surveys and other non-critical work, however, should be performed using data derived from trigonometric calculations.

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