The principle of height control measurement
Height control measurement is a precise geodetic method for measuring differences in elevation between two points. By combining it with other methods such as trilateration or triangulation, it can be used to calculate the absolute elevation of a point relative to sea level or to a given reference datum.
In a height control measurement, two survey points are established, one higher than the other. From each point, a set of angles and distances is measured to predetermined known points and the difference in elevation, Δh, between the two points is thereby determined. The process of gathering and processing the data is known as height determination.
The actual data collection is done using either a vertical or a horizontal theodolite and can be done in a variety of ways, depending on the accuracy and complexity of the job. The most common methods are height and angle to deflection point (HAD), height and angle to backsights (HAB), and height and angle to both line of sight and cornersight (HAC).
The first step of the data collection involves establishing a baseline, generally between two height control points, and orienting the theodolite to a known azimuth and an angle of 90°. Once the baseline is established, the angle and distances from each point are measured to a predetermined set of known points. The distances can either be measured directly by a chain or rangefinder or by indirect methods such as trilateration, triangulation and altimetry.
After the data is collected, the accuracy of the measurements is checked and the data is processed to calculate Δh. This is done by subtracting the lower height control points distance from the higher one and then by computing the intercept between the baseline and the point of greatest mathematical expectation. The calculated value of Δh is used to determine the absolute elevation of each point.
Height control measurement is an extremely accurate and cost-effective method for determining elevation differences and is used extensively in fields such as engineering, geodesy, and hydrography. It can also be used to map areas since it provides accurate elevation data which can then be rendered in 3D. Due to the precise nature of the measurements, it is important to use the best quality survey equipment available and to check the results before submitting them for final verification.
